Haruna Sugahara

Cornelia Meinert

Laurent Nahon

Nykola C Jones

Søren V Hoffmann

Kenji Hamase

Yoshinori Takano

Uwe J Meierhenrich

D-Amino acids in molecular evolution in space -Absolute asymmetric photolysis and synthesis of amino acids by circularly polarized light

Keywords: Homochirality, Asymmetric Photolysis, Asymmetric Synthesis, Circularly Polarized Light, Amino acids, Extraterrestrial delivery Ala, alanine, Glu, glutamic acid, Met, methionine, Leu, leucine, Tyr, tyrosine, Phe, phenylalanine UV, ultraviolet, VUV, Vacuum Ultra-Violet, NIR, near infrared, CP, circular polarization, VUV, vacuum ultraviolet, CD, circular dichroism, ORD, optical rotatory dispersion, PEM, photoelastic modulator, GC×GC-TOFMS, multidimensional gas chromatography coupled to time-of-flight mass spectrometry, ISM/CSM, interstellar/circumstellar medium, HOMO, highest occupied molecular orbital, 2D-HPLC, two-dimensional chiral high-performance liquid chromatography, GC/MS, Gas Chromatography/Mass Spectrometry, ADS, Active Descent System, SD 2 , Sample Drilling and Distribution, COSAC, Cometary Sampling and Composition, ROSINA, Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, IDPs, interplanetary dust particles, OSIRIS-REx, Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer, MMX, Martian Moons eXploration, C-type, carbonaceous-type, MOMA, Mars Organic Molecule Analyzer, LDMS, Laser Desorption Mass Spectrometry, NBD-F, 4-fluoro-7-nitro-2,1,3benzoxadiazole, MTBSTFA, N, N-methyl-test-butyl-dimethylsilyltrifluoroacetamide

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction 1.The Emergence of Biomolecular Homochirality

Homochirality is an essential molecular characteristic of terrestrial life. Living organisms on Earth almost exclusively use only one enantiomer for the molecular architecture of their biopolymers: L-amino acids (left-handed) for proteins and D-sugars (right-handed) for nucleic acids. There are no differences between D-and L-enantiomers with regard to chemical and physical properties in an achiral environment, except for tiny energy differences between enantiomers due to parity violation in the weak interaction [START_REF] Lee | Question of parity conservation in weak interactions[END_REF]. Although there are a number of works that have attempted to connect parity violation with the origin of biomolecular homochirality, a clear relationship has not yet been revealed [e.g., [START_REF] Quack | How Important is Parity Violation for Molecular and Biomolecular Chirality?[END_REF][START_REF] Bonner | Parity violation and the evolution of biomolecular homochirality[END_REF][START_REF] Laerdahl | D-or L-Alanine: That is the question[END_REF][START_REF] Macdermott | The effect of parity violation on kinetic models of enantioselective autocatalysis[END_REF]. The selection of one enantiomer is an important requirement when building up biopolymers such as proteins. The existence of a variety of diastereoisomers would interfere with the proper folding of proteins. Thus, the selection of enantiomers with only a single handedness must have occurred at an early stage during the evolution of life. However, the preference of L-amino acids, as well as D-sugars, over the optical antipodes is a longstanding question hitherto unanswered.

D-Amino Acids in Organisms

The investigation of the existence of enantiomers in the form of D-amino acids in living organisms is an increasingly developing and fascinating research field. It has been known for some time that bacteria use D-amino acids in their cell walls, especially D-αalanine (D-α-Ala) and D-glutamic acid (D-Glu), which are characteristic components of the peptide moiety of the peptidoglycan polymer [e.g. , 6]. Peptidoglycan is a major constituent of the cell wall of gram-positive bacteria and to a lesser extent gram-negative bacteria [START_REF] Schleifer | Peptidoglycan types of bacterial cell walls and their taxonomic implications[END_REF]. Recent advances in analytical chemistry have enabled the chiral separation, identification, and quantification of minor D-amino acids and thereby many more 'unusual' D-amino acids have been discovered [e.g. , 7]. It has become apparent that bacteria produce various D-amino acids, such as D-methionine (D-Met), D-leucine (D-Leu), D-tyrosine (D-Tyr), and D-phenylalanine (D-Phe), to regulate the synthesis of peptidoglycan due to the changes in environmental conditions [START_REF] Lam | Damino acids govern stationary phase cell wall remodeling in bacteria[END_REF].

The discovery of D-amino acids has also expanded to include animals [e.g., [START_REF] Corrigan | D-Amino acids in animals[END_REF][START_REF]D-Aspartic acid: An endogenous amino acid with an important neuroendocrine role[END_REF], in particular, D-aspartic acid (D-Asp) which is the most widely distributed amino acid in various animal phyla [START_REF]D-Aspartic acid: An endogenous amino acid with an important neuroendocrine role[END_REF]. It is commonly found in the nervous and endocrine systems [START_REF]D-Aspartic acid: An endogenous amino acid with an important neuroendocrine role[END_REF][START_REF] Ota | D-aspartate acts as a signaling molecule in nervous and neuroendocrine systems[END_REF] of octopuses and squids [START_REF] D'aniello | Identification of D-aspartic acid in the brain of Octopus vulgaris Lam[END_REF][START_REF] D'aniello | Presence of D-aspartate in squid axoplasm and in other regions of the cephalopod nervous system[END_REF], lobsters [START_REF] Okuma | Simultaneous determination of D-and L-amino acids in the nervous tissues of crustaceans using precolumn derivatization with (+)-1-(9 fuorenyl)ethyl chloroformate and reversed-phase ion-pair high-performance liquid chromatography[END_REF], marine molluscs [e.g., 15], fish [START_REF] Kera | D-Aspartate oxidase and free acidic D-amino acids in fish tissues[END_REF], frogs [START_REF] Raucci | D-Aspartate modulates transcriptional activity in Harderian gland of frog, Rana Esculenta: Morphological and molecular evidence[END_REF], lizards [START_REF] Assisi | Enhancement of aromatase activity by D-aspartic acid in the ovary of the lizard Podarcis s. sicula[END_REF], and birds [START_REF] Neidle | Developmental changes in free D-aspartic acid in the chicken embryo and in the neonatal rat[END_REF]. D-Asp has been also identified in mammals, such as rats, mice, and even humans [e.g., [START_REF] Han | Changes in D-aspartic acid and d-glutamic acid levels in the tissues and physiological fluids of mice with various d-aspartate oxidase activities[END_REF][START_REF] Katane | D-Aspartate-An important bioactive substance in mammals: A review from an analytical and biological point of view[END_REF]. Another well-known D-amino acid in cellular life is D-serine (D-Ser). It widely exists in various classes of organisms and especially accumulates in brains of mammals in high concentration [START_REF] Nishikawa | Analysis of free D-serine in mammals and its biological relevance[END_REF], where it plays a key role in the central nervous system as a neuromodulator [START_REF] Nishikawa | Analysis of free D-serine in mammals and its biological relevance[END_REF]. Several other D-amino acids have also been identified in various species (e.g., D-Glu, D-arginine (D-Arg), D-Phe, D-Met, and D-histidine (D-His)). It is noteworthy that D-α-Ala is the predominant D-amino acid in some aquatic invertebrates, such as marine crustaceans [START_REF] Abe | Role of free D-and L-alanine in the Japanese mitten crab Eriocheir japonicus to intracellular osmoregulation during downstream spawning migration[END_REF][START_REF] Okuma | Distribution of free D-amino acids in the tissues of crustaceans[END_REF][START_REF] D'aniello | Presence of D-alanine in crustacean muscle and hepatopancreas[END_REF] and several species of bivalves [START_REF] Okuma | Distribution of free D-amino acids in bivalve mollusks and the effects of physiological conditions on the levels of D and L-alanine in the tissues of the hard clam, Meretrix lusoria[END_REF]. The percentage of D-α-Ala compared to the total α-Ala abundance in some crustacean species ranges from 30 to up to 60% [START_REF] Okuma | Simultaneous determination of D-and L-amino acids in the nervous tissues of crustaceans using precolumn derivatization with (+)-1-(9 fuorenyl)ethyl chloroformate and reversed-phase ion-pair high-performance liquid chromatography[END_REF][START_REF] Okuma | Distribution of free D-amino acids in the tissues of crustaceans[END_REF] and was found to be involved in the osmoregulation against salinity changes [START_REF] Abe | Role of free D-and L-alanine in the Japanese mitten crab Eriocheir japonicus to intracellular osmoregulation during downstream spawning migration[END_REF].

The physiological functions and the origins of D-amino acids in mammals have been extensively investigated in recent years. D-Asp in endocrine and neuroendocrine tissues [e.g., 21] and D-Ser in brains [e.g., [START_REF] Nishikawa | Metabolism and functional roles of endogenous D-serine in mammalian brains[END_REF][START_REF] Mori | Serine racemase knockout mice[END_REF] are the most studied. Several other D-amino acid residues that have been discovered due to recent analytical developments [e.g., [START_REF] Weatherly | D-Amino acid levels in perfused mouse brain tissue and blood: A comparative study[END_REF][START_REF] Miyoshi | Enantioselective two-dimensional highperformance liquid chromatographic determination of amino acids; Analysis and physiological significance of D-amino acids in mammals[END_REF][START_REF] Brückner | Determination of amino acid enantiomers in human urine and blood serum by gas chromatography-mass spectrometry[END_REF] include D-Glu, D-Ala, and D-Pro. D-Glu was recently found to be accumulated in heart tissue [START_REF] Ariyoshi | D-Glutamate is metabolized in the heart mitochondria[END_REF]. It has been recognized that D-amino acids could be used as biomarkers for aging and various diseases such as D-Ser for Alzheimer's disease [START_REF] Kaneko | Suppression of mitochondrial succinate dehydrogenase, a primary target of ß-Amyloid, and its derivative racemized at ser residue[END_REF] and chronic kidney disease [START_REF] Kimura | Chiral amino acid metabolomics for novel biomarker screening in the prognosis of chronic kidney disease[END_REF], although the mechanisms are not fully revealed yet [e.g., [START_REF] Kaneko | Suppression of mitochondrial succinate dehydrogenase, a primary target of ß-Amyloid, and its derivative racemized at ser residue[END_REF][START_REF] Kimura | Chiral amino acid metabolomics for novel biomarker screening in the prognosis of chronic kidney disease[END_REF][START_REF] Fujii | D-Amino acids in aged proteins: Analysis and biological relevance[END_REF][START_REF] Ishii | Two-dimensional HPLC-MS/MS determination of multiple D-amino acids residues in the proteins stored under various pH conditions[END_REF]. In this case, D-amino acids are the result of racemization of proteinogenic amino acids, D-Asp, for example, has been detected in various tissues of elderly individuals due to the susceptibility of Asp to racemization [START_REF] Fujii | D-Amino acids in aged proteins: Analysis and biological relevance[END_REF].

The existence of D-amino acids in almost all living organisms raises a question about their origin. D-amino acids have most likely been incorporated into the organisms at several different stages in evolutional history and have developed specific physiological roles. The fact that the fundamental structure of terrestrial life is almost all of the same 'hand' could indicate that the selection of L-amino acids finished during an early stage in the emergence of life on early Earth.

Origin of Homochirality

When, how, and why L-amino acids were selected for terrestrial life are important and far-reaching questions in origins of life research. Many researchers have proposed physicochemical pathways for the symmetry breaking of chiral biomolecules. For example, Soai et al. demonstrated that chiral quartz promotes enantioselective synthesis of organic molecules and subsequent enhancement of the enantiomeric excess (ee) by autocatalytic reactions and yielded significantly high ee (93-97%) [START_REF] Soai | d-and l-Quartz-Promoted Highly Enantioselective Synthesis of a Chiral Organic Compound[END_REF]. Hazen et al. and Orme et al. successfully used calcite as a mineral surface for enantioselective adsorption of amino acids [START_REF] Hazen | Selective adsorption of L-and D-amino acids on calcite: Implications for biochemical homochirality[END_REF][START_REF] Orme | Formation of chiral morphologies through selective binding of amino acids to calcite surface steps[END_REF]. Viedma reported on a unique phenomenon to obtain homochiral crystal aggregates from a racemic mixture known as "Viedma ripening" [START_REF] Viedma | Chiral symmetry breaking during crystallization: Complete chiral purity induced by nonlinear autocatalysis and recycling[END_REF]. In his first experiment, he used a saturated aqueous solution of racemic sodium chlorate (NaClO3), which was prepared by dissolving an equal mixture of d-and l-crystals with an excess of undissolved crystals, and employed mechanical abrasion-grinding by stirring with glass balls under isothermal conditions [START_REF] Viedma | Chiral symmetry breaking during crystallization: Complete chiral purity induced by nonlinear autocatalysis and recycling[END_REF]. As a result, single-chiral crystals of complete chiral purity were formed. In spite of the practical simplicity, the underlying mechanism of Viedma ripening is quite complex and has not yet been fully understood. Enantiomer-specific oriented attachment is currently considered to be one of the key mechanisms [START_REF] Viedma | Enantiomer-specific oriented attachment: Formation of macroscopic homochiral crystal aggregates from a racemic system[END_REF]. In addition, there are strict requirements for Viedma ripening to be able to proceed, conglomerate crystallization, and racemization in solution [START_REF] Mclaughlin | Viedma ripening of conglomerate crystals of achiral molecules monitored using solid-state circular dichroism[END_REF], and thus only two amino acids (Asp and Glu) have been successfully subjected to Viedma ripening [START_REF] Viedma | Evolution of solid phase homochirality for a proteinogenic amino acid[END_REF][START_REF] Spix | Complete deracemization of proteinogenic glutamic acid using Viedma ripening on a metastable conglomerate[END_REF].

The enantiomeric enrichment of naturally occurring L-amino acids [e.g., [START_REF] Cronin | Enantiomeric excesses in meteoritic amino acids[END_REF][START_REF] Glavin | Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies[END_REF] and D-sugar derivatives [START_REF] Cooper | Enantiomer excesses of rare and common sugar derivatives in carbonaceous meteorites[END_REF] found in carbonaceous meteorites has raised the hypothesis of a possible link between astrochemical evolution and the origin of biomolecular homochirality. Over the past decades, significant progress in the understanding of the formation of such complex molecules in the interstellar medium (ISM) and their incorporation into comets and other celestial bodies such as the parent bodies of primitive carbonaceous chondrites have been made. More than 160 different molecular species have been identified in the ISM by astronomical observations [e.g., [START_REF] Irvine | The composition of interstellar molecular clouds[END_REF][START_REF] Herbst | Complex organic interstellar molecules[END_REF][START_REF] Tielens | The molecular universe[END_REF]. Recently, the first chiral molecule, propylene oxide (CH3CHCH2O), was detected in the ISM [START_REF] Mcguire | Discovery of the interstellar chiral molecule propylene oxide (CH3CHCH2O)[END_REF], although with a non-enantio-selective detection scheme. Laboratory experiments simulating interstellar ice chemistries further suggest the existence of even more complex organic molecules [e.g., [START_REF] Greenberg | Ultraviolet photoprocessing of interstellar dust mantles as a source of polycyclic aromatic hydrocarbons and other conjugated molecules[END_REF][START_REF] Schutte | Production of organic molecules in interstellar ices[END_REF][START_REF] Bernstein | Complex organics in laboratory simulation of interstellar/cometary ices[END_REF], such as amino acids, which will be discussed in Chapter 3. Recent analytical improvements also revealed the synthesis of sugar and sugar related molecules in interstellar ice analogues [START_REF] Meinert | Ribose and related sugars from ultraviolet irradiation of interstellar ice analogs[END_REF][START_REF] Marcellus | Aldehydes and sugars from evolved precometary ice analogs: Importance of ices in astrochemical and prebiotic evolution[END_REF]. These interstellar organic molecules are considered to be inherited by comets. Approximately 30 different molecular species including radicals and ions have been detected by astronomical observations [e.g., [START_REF] Crovisier | Remote observation of the composition of cometary volatiles[END_REF][START_REF] Bockelée-Morvan | New molecules found in comet C/1995 O1 (Hale-Bopp). Investigating the link between cometary and interstellar material[END_REF]. Moreover, complex molecules, such as ethanol (CH3CH2OH) and the simplest sugar glycolaldehyde (CH2OHCHO), have been found in comet C/2014 Q2 (Lovejoy) [START_REF] Biver | Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy)[END_REF].

The discovery of chiral organic molecules, including amino acids in carbonaceous chondrites, is still a controversial issue due to analytical difficulties and potential terrestrial contaminations. Chapter 4 will briefly highlight the analytical history and recent progress of amino acid detection in meteorite samples.

How amino acids can be asymmetrically formed in extraterrestrial environments will be the focus of Chapters 2 and 3. Circularly polarized light (CPL) is considered as a plausible candidate to cause chiral asymmetry in organic biomolecules. We will therefore discuss the two main photochemical processes by CPL, asymmetric photolysis (Chapter 2) and asymmetric synthesis (Chapter 3). We will further summarize the effects of impact shock on the chirality of amino acids after their delivery to early Earth by asteroids and comets in Chapter 5. Finally, several perspectives on the ongoing space missions aiming for a samplereturn from asteroids (Hayabusa2, Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), and Martian Moons eXploration (MMX) missions) and in situ analysis on Mars (ExoMars mission) will be provided in Chapter 6 with regard to the importance of pristine samples.

Absolute Asymmetric Photolysis of Racemic Amino Acids 2.1. Driving Force for Symmetry Breaking in the Universe: Circularly Polarized Light (CPL)

CPL represents an electromagnetic wave, whose electric field vector traces a spiral along with the direction of propagation (Figure 1). CPL exhibits true chirality and has the capability to induce chiral asymmetry into racemic organic molecules [START_REF] Barron | True and false chirality and absolute asymmetric synthesis[END_REF]. Thus, astronomical sources of CPL have been searched for in order to explain the detected Lenantiomeric excess (eeL) values of meteoritic amino acids. Synchrotron radiation from neutron stars, the remnants of supernova explosions, were first suggested as the possible source of CPL [START_REF] Rubenstein | Supernovae and life[END_REF][START_REF] Bonner | The origin and amplification of biomolecular chirality[END_REF], but they turned out to be insignificant at visible and ultraviolet (UV) wavelengths [START_REF] Bailey | Astronomical sources of circularly polarized light and the origin of homochirality[END_REF]. Three other sources have been suggested to produce high degrees of circularly polarization and include: 1) high field magnetic white dwarfs, 2) Polars (magnetic white dwarf binaries), and 3) reflection nebulae in star-forming regions [START_REF] Bailey | Astronomical sources of circularly polarized light and the origin of homochirality[END_REF]. However, the chance of encountering single magnetic white dwarfs or Polars in molecular clouds is extremely low, and therefore represent very unlikely scenarios [START_REF] Bailey | Astronomical sources of circularly polarized light and the origin of homochirality[END_REF]. The most plausible source of CPL is found in star-forming regions, these represent an ideal place for the exposure of organic molecules to CPL in molecular clouds and in the outer part of protostellar discs of a newly forming solar system.

The Orion nebula is the nearest star-forming region where both high-mass and lowmass stars are being formed and has been extensively investigated in the search for CPL. Bailey et al. discovered high levels of CPL of up to +17% in the near infrared (NIR) in the high mass star-forming regions of Orion OMC-1 [START_REF] Bailey | Circular polarization in star-formation regions: Implications for biomolecular homochirality[END_REF]. The circular polarization (CP) image of the Orion nebula showed a quadrupolar distribution of R-and L-CPL and the CP area extends far larger than the size of a typical protostellar disc. Fukue et al. showed the asymmetric quadrupolar pattern of CP (+17% to -5%) in the massive star-forming region of the Orion BN/KL region, which is centered around the young star IRc2 [START_REF] Fukue | Near-Infrared Circular Polarimetry and Correlation Diagrams in the Orion Becklin-Neugebauer/Kleinman-Low Region: Contribution of Dichroic Extinction[END_REF]. This finding indicates that this massive young star IRc2 is the dominant source of the large CP [START_REF] Fukue | Near-Infrared Circular Polarimetry and Correlation Diagrams in the Orion Becklin-Neugebauer/Kleinman-Low Region: Contribution of Dichroic Extinction[END_REF]. The spatial distribution of the CP in this region extends to ~0.4 pc, which is ~400 times larger than the size of our solar system [START_REF] Fukue | Extended high circular polarization in the Orion massive star forming region: Implicationsfor the origin of homochirality in the solar system[END_REF]. Extensive observation of CP in other star-forming regions, especially the massive star-forming region NGC 6334-V revealed the existence of even higher CP than in the Orion nebula (22%), and at 0.65 pc is even larger [START_REF] Kwon | Near-infrared circular polarization images of NGC 6334-V[END_REF]. One of the mechanisms proposed to explain the observed CP in the star-forming regions is scattering from grains aligned by a magnetic field [e.g., 68]. Meanwhile, Kwon et al. argue that dichroic extinction of linearly polarized scattered light should be the main mechanism to produce the observed higher CP (>20%) [START_REF] Kwon | Near-infrared circular polarization images of NGC 6334-V[END_REF].

CPL has only been observed in the NIR wavelength region. The higher energies required for asymmetric photochemical reactions of organic molecules, such as vacuum UV (VUV)/UV photons, are not directly observable due to the high dust obscuration in the massive star-forming regions. However, calculations by Bailey et al. showed that substantial degrees of UV-CPL could be produced by the scattered light from aligned grains, based on the observational results in the NIR [START_REF] Bailey | Circular polarization in star-formation regions: Implications for biomolecular homochirality[END_REF]. All of the observed star-forming regions that have high CP are associated with massive stars. According to the evidence of short-lived radionuclides in meteorites (e.g., 60 Fe), which originate from a supernova explosion [e.g., [START_REF] Hester | The Cradle of the Solar System[END_REF], 70], our solar system is considered to have been formed in a massive star-forming region. Assuming that the size of our solar systems is about 200 AU = 0.001 pc, the observed size of 0.65 pc of CP in NGC 6334-V is extremely large. Thus, it is possible to presume that our Solar System formed in a region where it was exposed to UV-CPL of one handedness only and this triggered the formation of L-enriched amino acids .

Theoretical Background for Absolute Asymmetric Photolysis by CPL

Photochemical reactions induced by CPL are often referred to as absolute asymmetric and distinguished from photochemical reactions of chiral molecules induced by non-polarized light [e.g., 71]. For CPL-induced asymmetric photochemical reactions, the following requirements need to be met: 1) the irradiated light must be absorbed by the reactants (Grotthus-Draper law), 2) the circular dichroism (CD) of the reactants at the irradiation wavelength should not be zero, 3) the reactants must undergo irreversible photochemical transformation, 4) the interconversion of each enantiomer of the reactants should not occur thermally or photochemically, and 5) products should preferably be transparent at the irradiation wavelength [START_REF] Buchardt | Photochemistry with circularly polarized light[END_REF][START_REF] Inoue | Asymmetric photochemical reactions in solution[END_REF].

The study of the CD of chiral molecules allows for the understanding of their interaction with CPL. CD measures the difference of the molar absorption coefficients (or extinction coefficients) between R-and L-CPL by enantiopure molecules and is expressed as following [e.g., 71]: Δε = εL -εR (1) The measurement of the differential absorption is wavelength dependent and recorded by CD spectroscopy. The reaction rates of each enantiomer irradiated by either L-or R-CPL are different. As a consequence, the chirality of CPL can be transferred into chiral or achiral molecules in favour of one enantiomer [START_REF] Rau | Chiral Photochemistry[END_REF][START_REF] Inoue | Asymmetric photochemical reactions in solution[END_REF].

Another chiroptical feature of chiral molecules is optical rotatory dispersion (ORD), which is the variation in the optical rotation of a molecule depending on the wavelength of light. ORD is closely related to CD. In the vicinity of a wavelength where CD shows a maximum peak, ORD changes from a maximum or minimum value to the opposite sign with equal magnitude by crossing zero. This phenomenon is known as the Cotton effect.

The first CD spectra of amino acids were recorded in the visible to the UV region only [e.g., 74], due to the limited photon intensity below 200 nm of commercial CD photospectrometers [START_REF] Evans | Anisotropy spectra for enantiomeric differentiation of biomolecular building blocks[END_REF]. The use of intense and highly collimated synchrotron radiation has enabled CD measurements in the VUV region (λ < 200 nm), where amino acids exhibit intense absorption bands [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF][START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF][START_REF] Kaneko | Natural circular dichroism spectra of alanine and valine films in vacuum ultraviolet region[END_REF]. At most synchrotron radiation facilities, the polarization of the light beam is converted to R-and L-CPL using a photoelastic modulator (PEM) [START_REF] Evans | Anisotropy spectra for enantiomeric differentiation of biomolecular building blocks[END_REF]. Recently, the use of an Onuki-type crossed undulator has further extended the accessible CD range from 140 to 120 nm [START_REF] Yamada | Vacuum ultraviolet circular dichroism spectroscopy using an ac-modulated polarizing undulator[END_REF][START_REF] Onuki | Elliptically polarized synchrotron radiation source with crossed and retarded magnetic fields[END_REF] and even to the soft x-ray region by utilizing a sophisticated helicity switching system [START_REF] Hara | Helicity switching of circularly polarized undulator radiation by local orbit bumps[END_REF][START_REF] Izumi | Characteristic oxygen K-edge circular dichroism spectra of amino acid films by improved measurement technique[END_REF].

The CD spectra of several α-H amino acids (α-Ala, Val, norvaline (NVal), Leu, norleucine (NLeu), isoleucine (Ile), Ser, α-aminobutyric acid (α-ABA), Pro, Phe) and αmethyl amino acids (isovaline (iVal), α-methylvaline (α-methyl-Val)), which are commonly found in meteorites (Figure 2a), have been successfully recorded [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF][START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF][START_REF] Kaneko | Natural circular dichroism spectra of alanine and valine films in vacuum ultraviolet region[END_REF]. To overcome the water absorption below 190 nm, the CD spectra were measured in the solid amorphous state, usually by sublimating individual amino acid enantiomers on a MgF2 window [START_REF] Evans | Anisotropy spectra for enantiomeric differentiation of biomolecular building blocks[END_REF]. The CD spectra of all measured α-H amino acids showed characteristic CD bands between 140 and 190 nm and each pair of enantiomers show opposite signs with equal magnitudes [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF][START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF]. Some features were commonly observed among all α-H amino acids, for example, the distinct CD peak between 165 and 185 nm was assigned to π → π* transitions in the carboxyl group by theoretical calculations [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF][START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF]. On the other hand, amino acids with a branched alkyl group in the side chain (Val, Leu, Ile) displayed a specific peak between 170 and 175 nm that is absent in straight-chain amino acids [START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF]. Furthermore, α-methyl amino acids (iVal and αmethyl-Val) showed an opposite CD sign compared to α-H amino acids [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF]. This could be related to the strong steric hindrance of α-methyl amino acids at the stereogenic center, which may affect the molecular orbitals involved in the CD transition. It is noteworthy that the solid state CD spectra of amino acids clearly differ from their solution spectra [START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF][START_REF] Bredehöft | Understanding photochirogenesis: Solvent effects on circular dichroism and anisotropy spectroscopy[END_REF], especially at around 200 nm. This can be explained by the conformational change between the solid and the liquid state e.g., change in the dihedral angles of the COO -and NH3 + groups [START_REF] Tanaka | Chiroptical study of r-aliphatic amino acid films in the vacuum ultraviolet region[END_REF].

Although CD serves as a helpful measure for understanding the crucial wavelengthdependence of asymmetric photochemistry, another quantitative factor is necessary for the evaluation of the chirality transfer from chiral photons to racemic molecules. Kuhn combined the CD with the magnitude of absorption and introduced the anisotropy factor g, which is defined as following [START_REF] Kuhn | The physical significance of optical rotary power[END_REF]:

𝑔 = 𝜀 𝑙 -𝜀 𝑟 𝜀 = ∆𝜀 𝜀 = ∆𝜀 (𝜀 𝑟 + 𝜀 𝑙 ) 2 ⁄ (3)
The anisotropy factor g is a wavelength-dependent and dimensionless quantity that ranges from 0 to 2. In the case of asymmetric photolysis of racemic mixtures, the anisotropy factor g can be represented by two competitive pseudo-first order reactions with unequal rate constants, kS and kR, for the R and S enantiomers, respectively [START_REF] Kuhn | The physical significance of optical rotary power[END_REF][START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]:

𝑔 = 2 𝜀 𝑅 -𝜀 𝑆 𝜀 𝑅 + 𝜀 𝑆 = 2 𝑘 𝑅 -𝑘 𝑆 𝑘 𝑅 + 𝑘 𝑆 (4)
The rate constants of the two enantiomers are proportional to the molar absorption coefficients (εR and εS). This equation is also applicable to non-first-order kinetics [START_REF] Nakamura | Synchronous enantiomeric enrichment of both reactant and product by absolute asymmetric synthesis using circularly polarized light. Part 2. Verification of the validity of assuming first-order kinetics upon deriving the equation for the relationship between conversion and enantiomeric excess[END_REF]. The anisotropy factor g further gives the quantitative prediction of the inducible ee value by asymmetric photolysis [START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]:

𝑒𝑒 ≥ (1 -(1 -𝜉) 𝑔 2 ) × 100% (5)
In this approximation, ee is dependent on the anisotropy factor g and the extent of the reaction ξ.

The anisotropy spectra of several amorphous solid state amino acids have been recorded between 130 and 350 nm using synchrotron radiation (Figure 2b) [START_REF] Bredehöft | Understanding photochirogenesis: Solvent effects on circular dichroism and anisotropy spectroscopy[END_REF][START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]. The Lenantiomers of α-H amino acids (α-Ala, Val, Leu, Ser, Pro) displayed a minimum between 190-210 nm and a rather small maximum between 170-190 nm [START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]. The peak at 190-210 nm is assigned to a n → π* transition in the carboxylate anion mixed with the n(COO -) → σ*(N-H) transition and the peak at 170-190 nm can be assigned to the first π → π* transition of the carboxylate anion [START_REF] Kaneko | Natural circular dichroism spectra of alanine and valine films in vacuum ultraviolet region[END_REF][START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]. On the contrary, the L-enantiomers of α-methyl amino acids (iVal and α-methyl-Val) did not show any prominent peaks, but a negative plateau between 200 and 220 nm [START_REF] Meinert | Anisotropy spectra of amino acids[END_REF]. The quantitative prediction of the inducible ee by CPL irradiation at 200 nm using equation ( 5) results in an ee of 5% (Leu) when 99.99% are photolyzed [START_REF] Meinert | Anisotropy spectra of amino acids[END_REF].

Laboratory Experiments of Absolute Asymmetric Photolysis of Racemic Amino Acids

The first asymmetric photolysis experiment of organic molecules using CPL was conducted by Kuhn & Braun by irradiating a racemic solution of α-bromopropanoic acid ethyl ester and examining the induced enantioenrichment [START_REF] Kuhn | Photochemische Erzeugung optisch aktiver Stoffe[END_REF]. After this pioneering work, several other molecules have been investigated [e.g., [START_REF] Balavoine | Preparation of Chiral Compounds with High Optical Purity by Irradiation with Circularly Polarized Light, a ModelReaction for the Prebiotic Generation of Optical Activity[END_REF][START_REF] Inoue | First absolute asymmetric synthesis with circularly polarized synchrotron radiation in the vacuum ultraviolet region: direct photoderacemization of (E)-cyclooctene[END_REF], including amino acids [e.g., [START_REF] Bonner | Enantiomeric markers in the quantitative gas chromatographic analysis of optical isomers. Application to the estimationof amino acid degradation[END_REF][START_REF] Norden | Was photoresolution of amino acids the origin of optical activity in life?[END_REF][START_REF] Flores | Asymmetric photolysis of (RS)-leucine with circularly polarized ultraviolet light[END_REF]. The asymmetric photolysis of amino acids was first tested in aqueous solution above 190 nm. Flores et al. irradiated racemic (rac)-Leu, which was dissolved in 0.1 M HCl, individually with R-and L-CPL at 212.8 nm, and obtained +1.98% ee for R-CPL and -2.5% ee for L-CPL after a decomposition of 59 % and 75%, respectively [START_REF] Flores | Asymmetric photolysis of (RS)-leucine with circularly polarized ultraviolet light[END_REF]. Nishino et al. later focused on the pH dependence of the anisotropy factor g and the inducible ee into amino acids [START_REF] Nishino | Mechanism of pH-dependent photolysis of aliphatic amino acids and enantiomeric enrichment of racemic leucine by circularly polarized light[END_REF]. They showed that the intensity of the g factor of aqueous D-and L-Leu increased with the decrease of the pH value of the solution, particularly in the wavelength range of 215-220 nm. The highest ee obtained in this study was +1.3% at pH 1 for a decomposition rate of 55%. The authors argued that the preference of a lower pH for the high g value was due to the n → π* transition of the carboxyl group of Leu. The mechanism of the asymmetric photolysis of Leu was revealed as follows: CPL induced the enantioselective excitation of the carboxyl group of Leu (D-enantiomer by R-CPL, and vice versa). For this excitation, a hydrogen atom from the γ-position was abstracted in a Norrish Type II reaction [START_REF] Mittal | Photo-induced decarboxylation of aliphatic acids and esters in solution. Dependence upon state of protonation of the carboxyl group[END_REF]. This abstraction lead to the cleavage of the C2-C3 bond and Gly was formed by the decomposition. This mechanism was predicted to be identical to other aliphatic amino acids that possess a γ-hydrogen (e.g., Val and Ile) [START_REF] Nishino | Mechanism of pH-dependent photolysis of aliphatic amino acids and enantiomeric enrichment of racemic leucine by circularly polarized light[END_REF]. In the subsequent report, the authors conducted a comprehensive study for all 20 proteinogenic amino acids, investigating the pH dependence of the CD and the anisotropy factor g in the 190-260 nm wavelength domain [START_REF] Nishino | The pH dependence of the anisotropy factors of essential amino acids[END_REF]. Among the 20 amino acids, aliphatic amino acids (α-Ala, Leu, Val, Ile, Pro) and those that have hydroxyl groups (Ser, Thr) showed the highest g values of 0.025-0.035 at 216-226 nm and at pH 1, which was caused by the carboxylic n → π* transition. Amino acids that have aromatic constituents (Phe, Try, His, tryptophan (Trp)) and sulfur (cysteine (Cys), Met) showed lower maximum g values of 0.0007-0.014 at 226-241 nm. The former could be explained by the major contribution of the aromatic π → π* transition and the latter due to the contribution of n → σ* transition of the thiol group (Cys) and the sulfide group (Met).

The use of solid thin films of amino acids in combination with synchrotron radiation allowed for asymmetric photochemical investigations in the VUV region (< 200 nm). Meierhenrich et al. succeeded in performing asymmetric photolysis of rac-Leu in the solid state at 182 and 170 nm [START_REF] Meierhenrich | Asymmetric vacuum UV photolysis of the amino acid leucine in the solid state[END_REF]. The irradiation of solid rac-Leu films with L-CPL at 182 nm resulted in the production of +2.6% ee, while the irradiation at 170 nm induced only a minor ee value. These results were consistent with the recorded CD spectra, in which the wavelength 183 nm matches with the positive π*→ π1 electronic transitions, while at 170 nm almost no CD contribution is observed. In subsequent experiments, the authors showed even higher ee values of +5.2% (±0.5%) by the irradiation of rac-Leu films with R-CPL at 187 nm at a total photolysis rate of 99.23% [START_REF] Meierhenrich | Photolysis of rac-leucine with circularly polarized synchrotron radiation[END_REF]. Although there was no counterpart experiment with the opposite-handed CPL irradiation [START_REF] Meierhenrich | Asymmetric vacuum UV photolysis of the amino acid leucine in the solid state[END_REF][START_REF] Meierhenrich | Photolysis of rac-leucine with circularly polarized synchrotron radiation[END_REF], Meinert et al. confirmed the experiments using a rac-Ala film (Figure 3). In the experiments, they prepared an amorphous rac-Ala film, which was labelled with 13 C to avoid any biological contamination, and applied an improved analytical technique by using multidimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS) for the analysis of the irradiated samples [START_REF] Meinert | Photonenergy-controlled symmetry breaking with circularly polarized light[END_REF][START_REF] Meinert | Anisotropy-guided enantiomeric enhancement in alanine using far-UV circularly polarized light[END_REF]. The rac-13 C -Ala film was irradiated with L-and R-CPL at 200 and 184 nm, wavelengths at which the electronic n → π* transitions in the carboxylate anion mixed with the n(COO -) → σ*(N-H) transition and the first π → π* transition, occur respectively. The photolysis results showed the remarkable mirror image behavior predicted by the anisotropy spectra: irradiation by L-CPL at 200 nm yielded an eeL of +4.19% (±0.14%) and irradiation by R-CPL at the same wavelength yielded an eeL of -4.22% (±0.21%) reaching similar photolysis rates of 99.97% and 99.95%, respectively. The irradiation at 184 nm resulted in the induction of an ee in the opposite direction: L-CPL yielded an eeL of -3.12% (± 0.34 %) and R-CPL yielded an eeL of +3.15% (±0.30%) with photolysis rates of 99.93% and 99.96%, respectively. These complete mirror results demonstrate that sign and magnitude of the induced ee could be fully controlled by CPL, based on the anisotropy factor g.

The experiments to induce an enantiomeric enrichment into amino acids were further extended to the gas-phase. Tia et al. irradiated racemic gas-phase α-Ala with circularly polarized synchrotron radiation [START_REF] Tia | Chiral asymmetry in the photoionization of gas-phase amino-acid alanine at Lyman-α radiation wavelength[END_REF]. The authors selected Lyman-α (121.6 nm) for their irradiation experiments, which is the dominant photon energy in the interstellar/circumstellar medium (ISM/CSM) and has sufficient energy for the photoionization of α-Ala by ejecting photoelectrons from the highest occupied molecular orbital (HOMO) [START_REF] Tia | VUV photodynamics and chiral asymmetry in the photoionization of gas phase alanine enantiomers[END_REF]. The authors demonstrated that CPL at Lyman-α could generate an asymmetric flux of gas-phase alanine cations of up to 4% ee in a given direction via so-called Photoelectron Circular Dichroism (PECD) [START_REF] Nahon | Valence shell one-photon photoelectron circular dichroism in chiral systems[END_REF]. This leads to intense enantio-specific forward/ backward asymmetries with respect to the photon axis in the electron emission direction, and therefore of the corresponding recoiling ion.. Such enantio-enriched cations would have then been embedded into asteroids and/or comets, which seeded early Earth with organic material presenting some ee. Such a PECD-based photophysical scenario would complement the asymmetric photochemical origin of biomolecular asymmetry, as gas phase chirality transfer from light to matter via PECD is allowed in the strong electric dipole approximation, unlike the much weaker magnetic dipole transitions needed for conventional CD.

Absolute Asymmetric Synthesis of Amino Acids in Simulated Interstellar Ices

In addition to the photolysis of racemic mixtures of amino acids, the possible absolute asymmetric synthesis of amino acids in ISM environments, especially in molecular clouds, has been discussed. Molecular clouds, the birthplace of stars, are mainly composed of interstellar gases and a minor contribution of dust grains, which comprises 1% of the total mass of the gases. The temperatures in the molecular clouds are very low (T < 20 K) and most of the volatile molecules (e.g., H2O, CO, CO2, CH3OH, and NH3) are frozen onto the silicate core by forming ice mantles [START_REF] Greenberg | Cosmic dust and our origins[END_REF][START_REF] Dartois | The ice survey opportunity of ISO[END_REF], where UV radiation from stars and energetic cosmic ray particles trigger the formation of more complex organic molecules [START_REF] Greenberg | Cosmic dust and our origins[END_REF].

Synthesis of rac-Amino Acids by Laboratory Simulation of Interstellar Environments

There are a number of experimental studies simulating ISM environments and investigating the formation of organic molecules therein. Most of these laboratory experiments have been conducted in an ultra-high vacuum chamber to simulate the evolution of interstellar ice analogues, which were prepared on a substrate by condensing representative ISM gases at 10-80 K and irradiated with different energy sources [START_REF] Greenberg | Cosmic dust and our origins[END_REF]. In 2002, Bernstein et al. reported on the formation of three amino acids (Gly, α-Ala, and Ser) from a UV/VUV irradiated ice mixture composed of H2O with various concentrations of CH3OH (5-10%, relative to H2O), NH3 (0.5-5%), and HCN (0.5-5%) at 15 K [START_REF] Bernstein | Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues[END_REF]. The two chiral amino acids Ala and Ser were racemically produced, which confirmed their indigenous formation. Muñoz Caro et al. demonstrated the formation of 16 amino acids in Lyman-α -irradiated interstellar ice analogues at 12 K [106]. The authors used a gas mixture that contained higher concentrations of other molecules relative to H2O, compared to Bernstein's work [START_REF] Bernstein | Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues[END_REF], i.e.,H2O:CH3OH:NH3:CO:CO2 = 2:1:1:1:1 (in mol). The most abundant amino acid formed in both experimental studies was Gly and the simplest chiral amino acids α-Ala and Ser. In addition, Muñoz Caro et al. reported that several diamino acids were also formed in relatively high abundances [106]. In later works, Nuevo et al. showed the formation of other biologically relevant proteinogenic amino acids [START_REF] Nuevo | A detailed study of the amino acids produced from the vacuum UV irradiation of interstellar ice analogs[END_REF]. Some of these amino acids synthesized in the experiments are commonly found in carbonaceous chondrites, including some of the detected diamino acids [START_REF] Meierhenrich | Identification of diamino acids in the Murchison meteorite[END_REF].

High-energy protons are the major component of cosmic rays and Kobayashi et al. irradiated ice mixtures of H2O, NH3, and carbon compounds (CO or CH4 or propane) with various mixing ratios with 3 MeV protons at 10 K and 77 K [START_REF] Kobayashi | Formation of amino acid precursors in cometary ice environments by cosmic radiation[END_REF]. The most abundant amino acid in the products was found to be Gly (C2), followed by a series of C3 and C4 amino acids: α-Ala, β-Ala, α-aminobutyric acid (α-ABA), β-ABA, γ-ABA, and β-aminoisobutyric acid (β-AIB). The authors also conducted proton irradiation experiments of gas mixtures at 295 K and obtained a similar amino acid distribution. Takano et al. conducted further detailed analysis of the amino acid composition of the products of the gas-phase experiments and reported the quantitative yields of 9 proteinogenic and 5 non-proteinogenic amino acids [START_REF] Takano | Abiotic synthesis of high-molecularweight organics from an inorganic gas mixture of carbon monoxide, ammonia, and water by 3 MeV proton irradiation[END_REF]. Related studies used γ-rays [START_REF] Takano | Formation of amino acids from possible interstellar media by γ-rays and UV Irradiation[END_REF][START_REF] Kobayashi | Possible cometary organic compounds as sources of planetary biospheres[END_REF] and electrons [START_REF] Holtom | A combined experimental and theoretical study on the formation of the amino acid glycine (NH2CH2COOH) and its isomer (CH3NHCOOH) in extraterrestrial ices[END_REF] as alternative energy sources relevant to cosmic rays and reported the formation of several amino acids, too.

It should be noted here that most of the amino acids were obtained after acidhydrolysis of the irradiated samples, which indicates that the amino acids might be part of higher molecular weight structures [START_REF] Nuevo | A detailed study of the amino acids produced from the vacuum UV irradiation of interstellar ice analogs[END_REF][START_REF] Takano | Abiotic synthesis of high-molecularweight organics from an inorganic gas mixture of carbon monoxide, ammonia, and water by 3 MeV proton irradiation[END_REF]. Recently, Oba et al. performed unique experiments exploiting the D/H stable isotopic ratio to understand the formation mechanism of the amino acids and their precursor molecules [START_REF] Oba | Deuterium Fractionation during amino acid formation by photolysis of interstellar ice analogs containing deuterated methanol[END_REF]. The authors used single deuterated methanol CH2DOH together with H2O, CO, and NH3, with a mixing ratio of 2:5:2:2, to form interstellar ice analogues by irradiating with UV/VUV photons from deuterium lamps in the range 115-170 nm, with two peaks around Lyman-α and 160 nm at T = 10 K. Samples were analyzed before and after acid-hydrolysis. Five amino acids (Gly, α-Ala, β-Ala, sarcosine (Sar), and Ser) with their deuterated isotopologues whose D atoms bond to carbon atoms were detected. The acid-hydrolysis resulted in a significant increase in the abundances of these 5 amino acids. However, their deuteration level (D/H) was different. The Ala-group amino acids (α-Ala and β-Ala) showed significant decrease in D/H, while the Gly-group (Gly, Sar, and Ser) showed no change. The authors concluded that two different formation mechanisms are involved in the photochemical formation of interstellar amino acids. In their subsequent report, Oba et al. reported significant amount of hexamethylenetetramine (C6H12N4, HMT) in simulated interstellar ice analogues [e.g., [START_REF] Bernstein | Complex organics in laboratory simulation of interstellar/cometary ices[END_REF]106], could be a source of D-rich alkyl-amines and amino acids [START_REF] Oba | Deuterium fractionation upon the formation of hexamethylenetetramines through photochemical reactions of interstellar ice analogs containing deuterated methanol isotopologues[END_REF].

Absolute Asymmetric Synthesis of Amino Acids in Interstellar Space

Investigations on the absolute asymmetric synthesis of amino acids are scarce. Pioneering work was conducted by Nuevo et al. [START_REF] Nuevo | The effects of circularly polarized lighton amino acid enantiomers producedby the UV irradiation of interstellar ice analogs[END_REF][START_REF] Nuevo | Enantiomeric separation of complex organic molecules produced from irradiation of interstellar/circumstellar ice analogs[END_REF]. The authors prepared interstellar ice analogues by depositing a gas mixture (H2O: 13 CH3OH:NH3 = 1:1:1) onto a cooled substrate at 80 K and irradiated either with R-or L-CPL at 167 nm. The authors quantified the ee of two chiral amino acids (α-Ala and 2,3-diaminopropanoic acid (2,3-DAP)), but found it to be very low (<1%). In a follow-up experiment (Figure 4), a more realistic interstellar ice composition (H2O: 13 CH3OH:NH3 = 2:1:1) was irradiated with CPL at 186.7 nm [START_REF] Marcellus | Non-racemic amino acid production by ultraviolet irradiation of achiral interstellar ice analogs with circularly polarized light[END_REF], a wavelength where most of the α-H amino acids exhibit a maximum CD intensity related to the n → π* transition [START_REF] Meierhenrich | Circular dichroism of amino acids in the vacuum-ultraviolet region[END_REF]. The analysis of the products by enantioselective GC×GC-TOFMS revealed the production of an ee = -1.34% (±0.40%), for α-Ala with R-CPL irradiation. In subsequent work, Modica et al. produced increased quantities of asymmetrically formed interstellar ice analogues using CPL at 187.9 and 121.6 nm (Lyman-α) [START_REF] Modica | Enantiomeric excesses induced in amino acids by ultraviolet circularly polarized light irradiation of extraterrestrial ice analogues: A possible source of asymmetry for prebiotic chemistry[END_REF]. The authors identified 16 amino acids and the ee of 5 chiral amino acids (α-Ala, 2,3-DAP, α-ABA, Val, and NVal) were determined to range from -0.20% (±0.14 %) to -2.54% (±0.28%) for L-CPL and from +0.90% (±0.33 %) to +2.06% (±0.34%) for R-CPL. The sign of the induced ees was found to be identical for all 5 chiral amino acids for a given CPL helicity and wavelength. In addition, the sign of the ee was opposite between 187.0 and 121.6 nm, which is in agreement with the CD and anisotropy spectra of these amino acids [START_REF] Meinert | Photonenergy-controlled symmetry breaking with circularly polarized light[END_REF][START_REF] Tanaka | First observation of natural circular dichroism spectra in the extreme ultraviolet region using a polarizing undulatorbased optical system and its polarization characteristics[END_REF]. The authors also showed that the CPL irradiation of the room temperature residues of the interstellar ice analogues resulted in the induction of ees. These results imply that the photochemical induction of an ee by CPL could occur at any stage of the evolution of organic molecules in interstellar ices.

Another asymmetric synthesis experiment of amino acids was conducted by Takano et al. [START_REF] Takano | Asymmetric synthesis of amino acid precursors in interstellar complex organics by circularly polarized light[END_REF]. The authors irradiated first a gaseous mixture of CO, NH3, and H2O with 3.0 MeV protons and obtained complex macromolecular organic molecules. These products of protonirradiation were subsequently irradiated with continuum UV-CPL (λ > 200 nm) from a synchrotron radiation source. After the acid-hydrolysis of the products, the authors detected 11 amino acids. The induced ee of alanine was + 0.44% (±0.31%) and -0.65% (±0.23%) for R-and L-CPL irradiation, respectively, compared to the non-CPL-irradiated sample as the racemic reference (ee = 0.00 ± 0.35%).

These laboratory simulation experiments demonstrated that CPL could indeed trigger the induction of a chiral bias into amino acids and their precursors during the evolution of organic molecules in interstellar ices.

Chirality of Amino Acids in Meteorites and Comets 4.1. Amino Acids in Meteorites

Meteorites are the most accessible extraterrestrial samples on the Earth. They can be divided into two major categories, chondrites and non-chondritic meteorites, based on their bulk compositions and textures [START_REF] Krot | 1.1 Classification of meteorites and their genetic relationships[END_REF]. Chondrites are remnants of the early solar system and have not experienced melting and planetary differentiation. They are therefore rich in volatile components and exhibit special textures such as chondrules. On the contrary, non-chondritic meteorites have experienced partial or complete melting and planetary differentiation, and chondritic textures erased by these processes [START_REF] Krot | 1.1 Classification of meteorites and their genetic relationships[END_REF]. The majority of meteorites are chondrites and originated in asteroids. Carbonaceous chondrites are the most studied class of meteorites due to their significantly high organic carbon content of up to 4 wt.%, most of which exists as insoluble macromolecular material [START_REF] O'd. Alexander | The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets[END_REF].

Chemical analyses of amino acids in carbonaceous chondrites gained momentum after the fall of the Murchison meteorite in 1969. The Murchison meteorite was classified as a CM2 (Mighei-type) carbonaceous chondrite based on the bulk chemical composition, mineralogical and petrographic type, and oxygen isotopic compositions [START_REF] O'd. Alexander | The nature, origin and modification of insoluble organic matter in chondrites, the major source of Earth's C and N[END_REF]. Right after its fall, Kvenvolden et al. reported the existence of 7 amino acids (Gly, α-Ala, Methyl-Ala, Sar, Val, Pro, Glu) in Murchison [START_REF] Kvenvolden | Evidence for Extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite[END_REF]. The research group of Cronin and Pizzarello performed a series of extensive analyses of the Murchison meteorite and detected a diversity of amino acids [START_REF] Cronin | Amino acids of the Murchison meteorite:I. Six carbon acyclic primary α-amino alkanoic acids[END_REF][START_REF] Cronin | Amino acids of the Murchison meteorite:II. Five carbon acyclic primary β-, γand δ-amino alkanoic acids[END_REF][START_REF] Cronin | Amino acids of the Murchison meteorite.III. Seven carbon acyclic primary a-amino alkanoic acids[END_REF]. The two most abundant amino acids are Gly (C2) and α-AIB (C4) and the abundances of other amino acids decreased exponentially with the increase of the carbon number. The amino acids show a large structural diversity and almost all of the possible isomeric forms were confirmed for the C2 to C7 α-amino acids. Among them, α-branched isomers were the most abundant, with the abundances decreasing in the order α > γ > β. The branched carbon chain isomers were predominant and the straight chain isomers were the least abundant. The amount of free amino acids is rather low and most amino acids are released from precursor structures by acid-hydrolysis. These amino acids can be grouped into two categories: monoamino monocarboxyl acids (e.g., Ala) and monoamino dicarboxyl acids (e.g., Asp), including related N-alkyl derivatives and cyclic amino acids. Later on, the discovery of diamino acids (e.g., 2,3-DAP) by Meierhenrich et al. added a new class to the Murchison amino acids [START_REF] Meierhenrich | Identification of diamino acids in the Murchison meteorite[END_REF]. Recently, Koga and Naraoka discovered racemic mixtures of 10 new hydroxyl amino acids [START_REF] Koga | A new family of extraterrestrial amino acids in the Murchison meteorite[END_REF]. Up to now, 96 species of amino acids, which comprise α, β, γ and δ isomers of the carbon numbers between C2 and C9 including dicarboxyl and diamino functional groups, were identified in the Murchison meteorite [START_REF] Koga | A new family of extraterrestrial amino acids in the Murchison meteorite[END_REF] (Figure 5).

The analysis of other carbonaceous chondrites, including many Antarctic meteorites, revealed that the amino acid composition of the Murchison meteorite cannot be considered to be universal but that there is a certain variation among different carbonaceous chondrites [START_REF] Burton | Understanding prebiotic chemistry through the analysis of extraterrestrial amino acids and nucleobases in meteorites[END_REF]. For example, the most abundant amino acid in CI chondrites, such as Ivuna and Orgueil, is β-Ala, compared to the α-amino acids that are predominant in the Murchison meteorite [START_REF] Glavin | The effects of parent body processes on amino acids in carbonaceous chondrites[END_REF]. The thermally altered chondrites such as CV and CO are rich in straight chain, amino-terminal acids (n-ω-amino acids) [START_REF] Burton | A propensity for n-ω-amino acids in thermally altered Antarctic meteorites[END_REF][START_REF] Chan | Amino acid analyses of type 3 chondrites Colony, Ornans, Chainpur, and Bishunpur[END_REF][START_REF] Burton | Amino acid analyses of R and CK chondrites[END_REF]. Furthermore, the aqueously altered CR2 chondrites were found to contain the highest total amino acid concentrations, which are 250 ppm for the Graves Nunataks (GRA) 95229 meteorite and 180 ppm for the Elephant Moraine (EET) 92042 meteorite [START_REF] Martins | Indigenous amino acids in primitive CR meteorites[END_REF]. For comparison, the total amino acid concentration of the Murchison meteorite is 14-21 ppm [START_REF] Glavin | The effects of parent body processes on amino acids in carbonaceous chondrites[END_REF]. The major amino acids in the two CR2 chondrites are the α-amino acids α-AIB, Gly, α-Ala, and iVal [START_REF] Martins | Indigenous amino acids in primitive CR meteorites[END_REF]. Aqueous alteration on the meteoritic parent body has also been proposed to influence the abundances of amino acids, as well as their relative distribution [START_REF] Glavin | The effects of parent body processes on amino acids in carbonaceous chondrites[END_REF][START_REF] Botta | Amino acids in Antarctic CM1 meteorites and their relationship to other carbonaceous chondrites[END_REF][START_REF] Martins | The amino acid and hydrocarbon contents of the Paris meteorite: Insights into the most primitive CM chondrite[END_REF]. The variations in the amino acid composition among carbonaceous chondrites imply different formation and/or degradation pathways of meteoritic amino acids in the meteorite parent bodies and/or before their formation in the presolar nebula. The extraterrestrial isotopic signatures of meteoritic amino acids (nitrogen, carbon, and hydrogen) may provide potential clues to their molecular evolution [START_REF] Chan | Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios[END_REF][START_REF] Martins | Extraterrestrial amino acids[END_REF][START_REF] Elsila | Compound-specific carbon, nitrogen, and hydrogen isotopic ratios for amino acids in CM and CR chondrites and their use in evaluating potential formation pathways[END_REF].

Amino Acids in Comets

The synthesis of a variety of amino acids in simulated interstellar ice analogues, as discussed in the previous chapter, suggests amino acids and other organic compounds to be present in comets. However, the in situ detection of amino acids in comets is difficult due to the limited sampling feasibility. Astronomical observations in search of the amino acid Gly of comet Hale-Bopp and Hyakutake was unsuccessful [START_REF] Crovisier | the Team for Target-of-Opportunity Radio Observations of Comets. The composition of ices in comet C/1995 O1 (Hale-Bopp) from radio spectroscopy: Further results and upper limits on undetected species[END_REF].

The National Aeronautics and Space Administration (NASA)'s Stardust mission was the first mission that succeeded in returning pristine cometary dust samples from comet 81P/Wild 2 to Earth [START_REF] Brownlee | The Stardust Mission: Analyzing samples from the edge of the Solar System[END_REF]. The amino acid analysis of the returned sample reported on the possible detection of Gly [START_REF] Elsila | Cometary glycine detected in samples returned by Stardust[END_REF]. The authenticity of this detection is still under discussion due to the terrestrial contamination of the collector aerogel. The European Space Agency (ESA)'s Rosetta Mission was the first to achieve landing on the nucleus of a comet and provided pioneering insights on the morphology and chemical composition of its target 67P/Churyumov-Gerasimenko [START_REF] Goesmann | Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry[END_REF]. After the touchdown of Philae on the cometary surface on November 12 th , 2014, both the gas release system ADS (Active Descent System) and the two anchoring harpoons did not operate as expected. Philae bounced in the low gravity environment and eventually landed after a two hour 'hop' in an unforeseen area of the cometary nucleus [START_REF] Ulamec | Rosetta Lander -Landing and operations on comet 67P/Churyumov-Gerasimenko[END_REF]. As a consequence of the unexpected 'vertical' landing of Philae it was not possible to collect a sufficient amount of a cometary surface sample by the instrument SD 2 (Sample Drilling and Distribution) for the Philae's enantioselective gas chromatograph COSAC (Cometary Sampling and Composition). However, the in situ analysis of the cometary volatiles by Rosetta's on-board mass spectrometer ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) detected a molecule with a mass number of 75 Dalton, which indicates the presence of Gly (C2H5NO2) and/or other structural isomers having the same chemical formula and mass numbers (cf. [START_REF] Oba | Deuterium Fractionation during amino acid formation by photolysis of interstellar ice analogs containing deuterated methanol[END_REF]) [START_REF] Altwegg | Prebiotic chemicals-amino acid and phosphorus-in the coma of comet 67P/Churyumov-Gerasimenko[END_REF].

Chirality of Amino Acids in Meteorites

The enantiomeric excesses in amino acids found in carbonaceous chondrites is still a controversial issue due to the analytical challenges to resolve meteoritic amino acid enantiomers in highly complex samples from the potential terrestrial contamination of meteorites. The first enantioselective analysis of the Murchison amino acids was performed shortly after its fall and revealed that the chiral amino acids were racemic [START_REF] Kvenvolden | Evidence for Extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite[END_REF][START_REF] Kvenvolden | Nonprotein amino acids in the Murchison meteorite[END_REF]. The non-proteinogenic α-methyl amino acids that are distinctive to meteorites (e.g., iVal) were also reported to be racemic [START_REF] Pollock | Stereoisomers of isovaline in the Murchison meteorites[END_REF]. The analysis of the Murray meteorite (CM2), which fell in 1950, also revealed racemic amino acids [START_REF] Lawless | Amino acids indigenous to the Murray meteorite[END_REF]. The racemic distribution of meteoritic amino acids was argued to be an indicator of the indigeneity of the results and the negligible level of terrestrial contamination.

In subsequent studies, various degrees of ees in amino acids from different meteorite groups have been reported. The significant L-excess (eeL) in the Murchison proteinogenic amino acids (e.g., [START_REF] Assisi | Enhancement of aromatase activity by D-aspartic acid in the ovary of the lizard Podarcis s. sicula[END_REF].9% (±4.2%) for α-Ala, 51.3% (±4.9%) for Glu) reported by Engel and Nagy [e.g.,[START_REF] Engel | Distribution and enantiomeric composition of amino acids in the Murchison meteorite[END_REF]] turned out to be terrestrial contribution and analytical artefacts [START_REF] Pizzarello | Non-racemic amino acids in the Murray and Murchison meteorites[END_REF][START_REF] Bada | On the reported optical activity of amino acids in the Murchison meteorite[END_REF]. Cronin and Pizzarello later reported on slight to moderate degrees of eeL in several nonproteinogenic α-methyl-α-amino alkanoic acids that are rare or not existing in terrestrial life: in the Murchison values of 2.8-9.2% (±0.3-1.3%), and in Murray values of 1.0-6.0% (± 0.3-0.7%), were reported for α-methyl Ile, α-methyl alloisoleucine (α-methyl-allo-Ile), 2-amino-2,3-dimethylpentanoic acid (2-a-2,3-DMPA), iVal, α-methyl-NVal, α-methyl-Val, and αmethyl-NLeu [START_REF] Pizzarello | Non-racemic amino acids in the Murray and Murchison meteorites[END_REF][START_REF] Cronin | Enantiomeric excesses in meteoritic amino acids[END_REF]. The reported eeL of proteinogenic amino acids (α-Ala and Val) were relatively small (1.2-2.2% (±0.3-0.6%)) in this study. The authors argued that the eeL in meteoritic α-methyl amino acids compared to the racemic α-H amino acids could be explained by the resistance to racemization of the α, α-dialkyl amino acids. In addition, Pizzarello et al. showed a heterogeneous distribution of eeL of iVal in several fragments and even among one fragment of the Murchison and Murray meteorites [START_REF] Pizzarello | Nonracemic isovaline in the Murchison meteorite: Chiral distribution and mineral association[END_REF]. The relative abundances of iVal to Ala (iVal/Ala) varied between 0.5 and 6.5. The authors discussed that the heterogeneity of iVal correlates well with the distribution of hydrous silicates and thus, the eeL of meteoritic amino acids may have been produced by a secondary process in the meteoritic parent bodies. Glavin and Dworkin further analyzed several different types of carbonaceous chondrites (CI, CM, and CR) targeting the C5 Val series of amino acids (Val, iVal, NVal) and reported a large eeL in iVal of Orgueil (CI1, 15.2% (±4.0 %)) and Murchison (CM2, 18.5% (±2.6 %)) meteorites, while iVal was found to be racemic in QUE 99177 and EET 92042 CR2 chondrites [START_REF] Glavin | Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies[END_REF]. The large eeL in iVal is proposed to be the result of chiral amplification by aqueous alteration on the parent bodies. Aqueous alteration was also argued to cause the observed heterogeneity of the relative abundances of amino acids among different carbonaceous chondrites [START_REF] Glavin | The effects of parent body processes on amino acids in carbonaceous chondrites[END_REF].

The difficulty of the enantioselective analysis of meteoritic amino acids arises from the high sample diversity but low concentration of each molecule present in meteorites. The enantioselective analysis of meteoritic amino acids requires several steps: 1) extraction from a powdered meteorite sample, 2) acid-hydrolysis, 3) fractionation by ion exchange column chromatography, 4) adequate derivatization, 5) separation on chiral GC or LC columns and 6) their final detection and quantification. Baseline separation of each enantiomeric pair is critical for the precise determination of any ee. Moreover, the derivatization method has to be carefully chosen to avoid the formation of artefacts as discussed by Ohkouchi et al. [START_REF] Ohkouchi | Advances in the application of amino acid nitrogen isotopic analysis in ecological and biogeochemical studies[END_REF]. The conventional mono-dimensional GC or LC techniques show limitations for the quantitative separation of molecules with similar chromatographic properties, and the development of comprehensive multidimensional GC and/or LC systems coupled to very sensitive detectors has enabled some of these limitations to be overcome. Myrgorodska et al. applied enantioselective GC×GC-TOFMS to the analysis of amino acids extracted from the Murchison meteorite [START_REF] Myrgorodska | Quantitative enantioseparation of amino acids by comprehensive twodimensional gas chromatography applied to non-terrestrial samples[END_REF]. Using the improved resolving power of enantioselective GC×GC [START_REF] Meinert | A new dimension in separation science: comprehensive two-dimensional gas chromatography[END_REF], racemic to small eeL, up to 5.95 % (±0.62%) for non-proteinogenic and proteinogenic amino acids with exceptions of one negative eeL value -9.55% (±0.70%) for allo-Ile, were reported. High values of two proteinogenic amino acids were found to be considerably enriched in the naturally occurring L-enantiomer, namely Ile and Leu with eeL values of 9.49% (±1.16%) and 26.33% (± 0.76%), respectively. The eeL of α-Ala was 3.16% (±0.80%), Val 4.88% (±0.73%), and iVal 4.61% (±0.83%). No significant difference in the eeL values between α-H and αmethyl amino acids have been observed, which is in contrast to previous reports. An innovative analytical technique by Hamase et al. has cast a new light on the controversy of the meteoritic eeL [160]. The authors developed a two-dimensional chiral highperformance liquid chromatographic (2D-HPLC) system, initially aiming for the analysis of trace levels of D-amino acids in mammals [START_REF] Miyoshi | Enantioselective two-dimensional highperformance liquid chromatographic determination of amino acids; Analysis and physiological significance of D-amino acids in mammals[END_REF][START_REF] Ariyoshi | D-Glutamate is metabolized in the heart mitochondria[END_REF][START_REF] Kimura | Chiral amino acid metabolomics for novel biomarker screening in the prognosis of chronic kidney disease[END_REF][START_REF] Hamase | Comprehensive analysis of branched aliphatic d-amino acids in mammals using an integrated multi-loop twodimensional column-switching high-performance liquid chromatographic system combining reversed-phase and enantioselective columns[END_REF]. In the 2D-HPLC system, analytes are separated on an achiral reversed-phase column in the first dimension where targeted amino acids are collected on-line in a multi-loop device. Afterwards, they are automatically injected onto a chiral column in the second dimension and detected by a fluorescence detector. Prior to the 2D-HPLC analysis, the amino acids were derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The authors applied this technique to extraterrestrial amino acids from Yamato 791191, an Antarctic CM2 meteorite. The enantioselective analysis of a total of 8 chiral amino acids, including 2 proteinogenic (α-Ala, Val) and 6 non-proteinogenic (α-ABA, NVal, N-methyl-Ala, iVal, β-ABA and β-AIB) amino acids, revealed an overall racemic distribution. These new data on the eeL of meteoritic amino acids will stimulate further investigations on the origin and formation processes of chiral amino acids in extraterrestrial samples.

As summarized above, the chirality of amino acids in meteorites is still a highly debated subject, with controversy among the reported ee values. Complementary data, such as the in situ and ex situ analyses of pristine asteroidal or cometary samples by sample return missions, are necessary to ultimately prove the origin of left-handed amino acids in extraterrestrial environments. This topic is further discussed in Chapter 6.

Delivery of Organic Molecules by Comets and Asteroids and the Amplification of ee

For the delivery of chiral amino acids from extraterrestrial bodies, such as comets and asteroids, the impact shock at the entry to the early Earth is an inevitable step. The shock wave generated at the impact creates extremely high pressure and temperature conditions, which are assumed to influence the organic molecules inside the meteorite. Therefore, the effects of impact shocks on the distribution of amino acids have been intensively studied in order to examine the feasibility of an extraterrestrial delivery of amino acids towards Earth. Since organic molecules are generally labile to heat, the thermal decomposition and racemization of amino acids is one of the major concerns regarding their chemical integrity.

Constraints for the Delivery of Extraterrestrial Chiral Amino Acids

The earliest shock experiments performed by Tingle et al. used samples from the Murchison meteorite [START_REF] Tingle | The fate of organic matter during planetary accretion: preliminary studies of the organic chemistry of experimentally shocked Murchison meteorite[END_REF]. The authors gave impact shocks, with a pressure of 36 GPa generated by a solid propellant gun, to polished pieces of a Murchison sample and reported that 70% of the organic species in the meteorites were decomposed by the impact shock. Sekine et al. reported that dehydration and carbonization of organic materials started already at pressures of 7-8 GPa [START_REF] Sekine | An experimental study on impact-induced alterations of planetary organic simulants[END_REF]. They concluded that heat generated by impact shock could be a major factor that controls the alteration.

A shock experiment that focused on amino acids was first conducted by Peterson et al. [START_REF] Peterson | Modification of amino acids at shock pressures of 3.5 to 32 GPa[END_REF]. In this study, several amino acids were mixed with the mineral matrices of powdered Murchison (CM2) and Allende (CV3) meteorites and subjected to impact shocks up to 32 GPa. The authors reported on the decomposition of most of the amino acids at approximately 20 GPa with the formation of secondary amino acids. Sugahara and Mimura showed that 4 amino acids (Gly, α-Ala, α-ABA, α-AIB), which had been mixed with serpentine powder to imitate the carbonaceous chondrite matrix, decomposed rapidly by 92-95% at 18.4 GPa and almost fully decomposed at 35.3 GPa [START_REF] Sugahara | Shock-induced pyrolysis of amino acids at ultra high pressures ranged from 3[END_REF]. The results indicate that the shock decomposition of amino acids is a dynamic and rapid process and independent from the chemical structure of the amino acids. The critical shock pressure of the decomposition of amino acids is consistent among several studies and is about 18-20 GPa.

The racemization of amino acids by impact shock is another important concern regarding the delivery of chiral amino acids to the early Earth. Using enantiopure amino acids, Peterson et al. found that racemization proceeds slower than decomposition of amino acids via the shock experiments [START_REF] Peterson | Modification of amino acids at shock pressures of 3.5 to 32 GPa[END_REF]. The authors showed that racemization did not occur until 7 GPa and the chirality was retained at >60% even after >99% decomposition of the original amino acids at 25 GPa.

Since decomposition occurs faster than racemization, the major constraints for the delivery of chiral amino acids depend on the conditions of the decomposition. As discussed above, the shock pressure of 18-20 GPa would be one of the important constraints. If we consider the vertical oceanic impacts of carbonaceous asteroids or comets, this shock pressure range corresponds to an impact velocity of <5 km/s for carbonaceous asteroids and <6 km/s for comets, based on Hugoniot data [e.g., [START_REF] Marsh | LASL Shock Hugoniot Data[END_REF][START_REF] Stewart | Shock properties of H2O ice[END_REF]. These impact velocities were relatively low compared with the average impact velocities of asteroids (15 km/s) [START_REF] Chyba | Impact delivery and erosion of planetary oceans in the early inner Solar System[END_REF] and shortperiod comets (24 km/s) [START_REF] Hughes | The velocity distributions of periodic comets and stream meteoroids[END_REF]. However, there are several factors related to natural asteroidal and cometary impacts that can reduce the shock pressure dramatically. These factors were discussed in detail by Sugahara and Mimura [START_REF] Sugahara | Peptide synthesis triggered by comet impacts: A possible method for peptide delivery to the early Earth and icy satellites[END_REF], underlining that oblique impact, deceleration, and fragmentation in the atmosphere could contribute effectively to reduce the shock pressure to below 20 GPa. Furukawa et al. provided a detailed discussion about decomposition and racemization of amino acids in relation to the various heat production processes at impact shock [START_REF] Furukawa | Racemization of valine by impact-induced heating[END_REF].

On the other hand, impacts can also act as a driving force for molecular evolution. Sugahara and Mimura demonstrated that comet impacts could oligomerize amino acids to form peptides up to tri-peptides by shock experiments on frozen mixtures of amino acids, water ice, and silicate (forsterite) at 77 K [START_REF] Sugahara | Peptide synthesis triggered by comet impacts: A possible method for peptide delivery to the early Earth and icy satellites[END_REF][START_REF] Sugahara | Glycine oligomerization up to triglycine by shock experiments simulating comet impacts[END_REF]. Shock experiments on cometary ice analogues by Martins et al. revealed the synthesis of amino acids [START_REF] Martins | Shock synthesis of amino acids from impacting cometary and icy planet surface analogues[END_REF]. A theoretical study predicted the synthesis of a Gly-containing molecule by impact shock [START_REF] Goldman | Synthesis of glycinecontaining complexes in impacts of comets on early Earth[END_REF]. Thus, extraterrestrial impacts do not always result in the complete decomposition of amino acids.

Amplification of ee in Terrestrial or Extraterrestrial Environments

In the previous chapters, we discussed the symmetry breaking of amino acids by CPL and their possible delivery by extraterrestrial impacts. However, the inducible ee by CPL that could have been delivered to early Earth after surviving the impact shock would have been very small. Soai et al. proposed the enhancement of such small initial ee by asymmetric autocatalysis [START_REF] Soai | Asymmetric autocatalysis and amplification of enantiomeric excess of a chiral molecule[END_REF]. They demonstrated that the small ee (2%) of chiral 5-pyrimidyl alkanol was increased significantly (88%) by the autocatalytic reaction involving diisopropylzinc and pyrimidine-5-carboxaldehyde. Kawasaki et al. further investigated this autocatalytic reaction [START_REF] Kawasaki | Enantioselective synthesis of near enantiopure compound by asymmetric autocatalysis triggered by asymmetric photolysis with circularly polarized light[END_REF]. They started the experiments by irradiating racemic pyrimidyl alkanol with CPL (λ = 290-390 or 313 nm), followed by chiral amplification of the initial small ee by asymmetric autocatalysis to obtain nearly enantiopure (>99.5% ee) compounds. Moreover, the same research team demonstrated that chiral deuterated Gly (Gly-α-d), α-methyl-Ala (α-methyl-d3-Ala), and even carbon isotopically chiral alcohols, in which the relative energetic difference between 13 C and 12 C is smaller compared to the relative energetic ratio between D and H, can be used as chiral initiators for asymmetric autocatalysis [START_REF] Kawasaki | Asymmetric autocatalysis induced by meteoritic amino acids with hydrogen isotope chirality[END_REF][START_REF] Kawasaki | Asymmetric autocatalysis triggered by carbon isotope ( 13 C/ 12 C) Chirality[END_REF].

Another proposed scenario includes an extended Viedma model that combines CPL irradiation and Viedma ripening [e.g., 179], and the chirality transfer from a chiral methyl-Val to proteinogenic amino acids (Phe) during their synthesis with a copper catalyst [START_REF] Levine | Enantioselective synthesis and enantiomeric amplification of amino acids under prebiotic conditions[END_REF].

Perspective for the Upcoming Space Missions

Space missions have significantly advanced our knowledge about the Solar System in the last decades. Extraterrestrial samples, such as meteorites and interplanetary dust particles (IDPs), collected on the Earth cannot fully exclude the risk of terrestrial contamination. They are also considered to have been affected to some extent by impact shock during entry. Sample return missions have therefore the great advantage to obtain pristine matter with limited terrestrial contamination and the record of the original locality.

Sample Return Missions: Hayabusa2, OSIRIS-REx, and MMX

Currently, two major sample return missions Hayabusa2 and OSIRIS-Rex are ongoing. Hayabusa2 is the second sample return mission from an asteroid (Figure 7a), following Hayabusa, the first successful sample return mission of the Japanese Aerospace Exploration Agency (JAXA). Although the sampling mechanism of Hayabusa failed and terrestrial contamination has been revealed, few authentic particles from the targeted asteroid Itokawa have been returned. Initial chemical and mineralogical analysis of these asteroidal grains provided unprecedented information on the formation and evolutional history of the asteroid [e.g., [START_REF] Tsuchiyama | Three-dimensional structure of Hayabusa samples: Origin and evolution of Itokawa regolith[END_REF][START_REF] Nagao | Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples[END_REF][START_REF] Yurimoto | Oxygen isotopic compositions of asteroidal materials returned from Itokawa by the Hayabusa mission[END_REF][START_REF] Nakamura | Itokawa dust particles: A direct link between S-type asteroids and ordinary chondrites[END_REF][START_REF] Noguchi | Incipient space weathering observed on the surface of Itokawa dust particles[END_REF]. The Hayabusa2 spacecraft was successfully launched on December 3, 2014 and is now heading for the near-Earth carbonaceous-type (C-type) asteroid (162173) Ryugu. Hayabusa2 is scheduled to arrive at Ryugu in the middle of 2018, collect at least 100 mg of surface samples at multiple places, and return the asteroidal samples to Earth at the end of 2020. One of the scientific goals of Hayabusa2 focuses on the origin and evolution of volatile components in the early Solar System and in the asteroidal parent body [START_REF] Tachibana | the Hayabusa2 Project Team, Hayabusa2: Scientific importance of samples returned from C-type near-Earth asteroid (162173) 1999 JU3[END_REF][START_REF] Watanabe | Hayabusa2 Mission Overview[END_REF]. For this purpose, on-site geological observations of the asteroid by remote sensing from the spacecraft in the km to mm scale will be linked to the in situ surface measurements by the lander and with the analyses of the retuned samples (atomic scale).

OSIRIS-REx is a NASA sample return mission from an asteroid (Fig. 7b). The OSIRIS-REx spacecraft was successfully launched on September 8, 2016. The target asteroid is the B-type near-Earth asteroid Bennu, belonging to a rare spectral subgroup of carbonaceous asteroids [START_REF] Lauretta | OSIRIS-REx: Sample Return from Asteroid (101955) Bennu[END_REF]. OSIRIS-REx is planning to arrive at Bennu in mid-2018 and to return at least 60 g of asteroid regolith sample in the middle of 2023. OSIRIS-REx shares scientific goals with Hayabusa2. The prime objective of the mission is to understand the role of organic matter and water in primitive asteroids and their impact on the origin and early evolution of life on Earth. Thus, the synergetic comparison of the collected data of both sample return missions is expected to considerably advance our understanding of the evolution of organic molecules in space [START_REF] Tachibana | the Hayabusa2 Project Team, Hayabusa2: Scientific importance of samples returned from C-type near-Earth asteroid (162173) 1999 JU3[END_REF].

In both sample return missions, the lessons learned from the past sample return missions will be utilized. A critical concern in sample return missions is the elimination of terrestrial contamination. For the Stardust mission and its returned cometary grains, a comprehensive assessment of the terrestrial contamination throughout the mission was preformed [START_REF] Sanford | Assessment and control of organic and other contaminants associated with the Stardust sample from comet 81P/Wild 2[END_REF]. However, the terrestrial contamination, in which ε-amino-n-caproic acid (EACA) that originated from Nylon-6 of the shipping bags [START_REF] Glavin | Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography-time of flight-mass spectrometry[END_REF] was found in the sampling aerogel, has questioned the unambiguous detection of Gly in the returned cometary grains [START_REF] Elsila | Cometary glycine detected in samples returned by Stardust[END_REF] (see also Chapter 4). In the case of the Hayabusa mission, an assessment of the contamination of the curation facilities was performed [START_REF] Yada | Hayabusareturned sample curation in the Planetary Material Sample Curation Facility of JAXA[END_REF]. The analyses of organic compounds, including amino acids in 5 individual carbonaceous (category 3) particles (~50 to 100 μm in diameter) resulted in no detection of organic molecules [START_REF] Naraoka | Preliminary organic compound analysis of microparticles returned from Asteroid 25143 Itokawa by the Hayabusa mission[END_REF][START_REF] Uesugi | Sequential analysis of carbonaceous materials in Hayabusa-returned samples for the determination of their origin[END_REF]. The carbonaceous matter recovered from Hayabusa's sample container turned out to be of terrestrial origin, potentially including contamination from the spacecraft and the sampling processes [e.g., [START_REF] Yabuta | X-ray absorption near edge structure spectroscopic study of Hayabusa category 3 carbonaceous particles[END_REF][START_REF] Naraoka | ToF-SIMS analysis of carbonaceous particles in the sample catcher of the Hayabusa spacecraft[END_REF]. The knowledge obtained from the first Hayabusa mission has been used to design the follow-up Hayabusa2 sampling system [START_REF] Okazaki | The Hayabusa2 SMP Team, Hayabusa2 sample catcher and container: Metal-seal system for vacuum encapsulation of returned samples with volatiles and organic compounds recoveredfrom C-type asteroid Ryugu[END_REF][START_REF] Sawada | Hayabusa2 SMP Team, Hayabusa2 sampler: Collection of asteroidal surface material[END_REF]. In addition, comprehensive analyses of terrestrial contamination pathways, including the chirality of molecules, have been systematically conducted for both Hayabusa2 and OSIRIS-Rex missions to accumulate extensive knowledge about potential contamination sources [START_REF] Sugahara | Hayabusa2 project team, Amino acid analysis of witness coupons collected from IASA curation room for the quality control of Hayabusa2 return sample procedure[END_REF][START_REF] Dworkin | OSIRIS-REx contamination control strategy and implementation[END_REF].

JAXA is planning a new mission named MMX (Martian Moon eXplorer) that will explore the Martian moons Phobos and Deimos (Fig. 7c). The aim of this mission is to reveal the formation processes of the Martian moons and to understand the delivery processes of water and organics to the inner planets [START_REF] Hyodo | On the impact origin of Phobos and Deimos. I. Thermodynamic and physical aspects[END_REF][START_REF] Kikuchi | 3D mapping by active stereo sensor on sampling mission of asteroid surface[END_REF]. In this mission, a lander is to be sent to the surface of Phobos that will sample the surface using a robot arm and return these samples back to Earth. The spacecraft is expected to be launched in the early 2020's.

In Situ Analysis: ExoMars 2020

Another space mission aims for the in situ analysis of organic matter. ESA's ExoMars 2020 mission will be launched in 2020 and plans to deposit a lander -the ExoMars rover -on the Martian surface, that is equipped with a drilling system and several instruments for atmospheric and geophysical/chemical analysis [START_REF] Vago | Pasteur Instrument Teams, Landing Site Selection Working Group, and Other Contributors, Habitability on early Mars and the search for biosignatures with the ExoMars rover[END_REF] (Figure 7d). The main scientific targets of the ExoMars 2020 mission are the search for physical and chemical biosignatures of extinct life as well as for abiotic/prebiotic organics that indicate a habitability past of Mars. The Mars Organic Molecule Analyzer (MOMA) on board the ExoMars rover was designed for this purpose [START_REF] Goetz | the MOMA Science Team, MOMA: the challenge to search for organics and biosignatures on Mars[END_REF][START_REF] Goesmann | the MOMA Science Team, The Mars Organic Molecule Analyzer (MOMA) instrument: Characterization of organic material in Martian sediments[END_REF]. MOMA is the largest instrument on the rover and able to analyze a wide range of organic compounds with high sensitivity. Subsurface samples will be provided by drilling down to 2 m below the Martian surface. This is what distinguishes the ExoMars 2020 mission from previous Mars missions. Organic compounds in Martian subsurface samples will be volatilized and analyzed by a mass spectrometer that allows for the operation in two different modes: (1) The Gas Chromatography-Mass Spectrometry (GCMS) mode and (2) the Laser Desorption Mass Spectrometry (LDMS) mode. The GCMS will allow the sample to be heated for evaporation and/or thermochemical decomposition (pyrolysis), so that the organic compounds can be further derivatized by chemical reagents and separated on different GC columns. The LDMS mode will promote the prompt desorption of organic molecules into the gas phase by intense UV laser pulses followed by their direct analysis using MS. For the GCMS mode, two derivatization reagents are prepared, N, N-methyl-test-butyl-dimethylsilyltrifluoroacetamide (MTBSTFA) and N,N-Dimethylformamide dimethyl acetal (DMF-DMA), which allow for the derivatization of organic molecules that contain a labile polar functional group, e.g., -OH, -COOH, -NH, -SH, or -PH, including amino acids. MOMA is equipped with a chiral capillary column (CP Chirasil Dex CB) that enables the resolution of enantiomers.

One critical concern of space missions aiming for in situ analysis on planetary bodies is terrestrial contamination imported by the rover and/or spacecraft. NASA's Mars Science Laboratory (MSL) mission that operated the Curiosity rover on Mars has implemented strategies to control and reduce the risk of contamination in the process of exploration [START_REF] Grotzinger | Mars Science Laboratory Mission and science investigation[END_REF][START_REF] Conrad | The Mars Science Laboratory Organic Check Material[END_REF]. MSL reported that organic compounds unintentionally originated from a reagent brought to Mars to allow for the detection of organic volatiles in Martian soil [START_REF] Leshin | MSL Science Team, Volatile, isotope, and organic analysis of Martian fines with the Mars Curiosity Rover[END_REF]. Precautions and assessing the characteristics of organic contamination prior to each mission are therefore necessary to avoid false positive detection of biomolecules and to fulfil planetary protection regulations.

Perspective

We have outlined how CPL can induce ee in amino acids by asymmetric photolysis and asymmetric synthesis. The series of experimental results using circularly polarized synchrotron radiation in combination with the CD and anisotropy data of amino acids have confirmed the ability of CPL to induce an initial enantiomeric imbalance into interstellar/circumstellar organic molecules. Thus, it is a possible scenario that amino acids with a reasonably small ee induced by CPL became incorporated into comets and asteroids, being delivered to the early Earth, where they have triggered subsequent chiral amplification in suitable terrestrial environments. However, the analyses and interpretation of meteoritic ees remains controversial due to the potential terrestrial contamination and analytical challenges. The analyses of pristine asteroid samples obtained by the Hayabusa2 and OSIRIS-REx sample return missions and the in situ analyses on Mars by the ExoMars 2020 mission, as well as the MMX mission to Martian moons, are considered to be highly important to provide solid evidence about the optical purity of organic molecules, especially amino acids in extraterrestrial materials. The obtained data are expected to give crucial insights into the linkage between extraterrestrial organic molecules and the origins of biomolecular homochirality. [START_REF] Meinert | Photonenergy-controlled symmetry breaking with circularly polarized light[END_REF]). The induced ee showed the opposite signs between 200 and 184 nm, which are closely correlated with the reserved signs in the anisotropy spectra. Fig. 4. Experimental setup for asymmetric synthesis of amino acids in interstellar ice analogues (modified from Meinert et al. [START_REF] Meinert | Photochirogenesis: Photochemical models on the absolute asymmetric formation of amino acids in interstellar space[END_REF]). The representative interstellar (ISM) gas molecules are deposited on an MgF2 window at 80 K in the center of the high vacuum chamber. VUV-CPL from a synchrotron light source irradiates the ice sample on the MgF2 window. During the experiments, in situ IR-spectra are taken to monitor the composition and the thickness of the ice. After the irradiation, the sample is warmed up to room temperature and the remaining residue analyzed by enantioselective gas chromatography.

Figures and

Fig. 1 .

 1 Fig.1. Representation of right-handed circularly polarized light (R-CPL). The direction of the magnetic field vector is perpendicular to the electric field vector shown in green arrow.

Fig. 2 .

 2 Fig. 2. Circular dichroism (CD) (a) and anisotropy (g) spectra (b) of D-amino acids in their amorphous solid-state in the VUV and UV region (modified from Meinert et al. [85]).

Fig. 3 .

 3 Fig. 3. Anisotropy spectra of L-and D-alanine in VUV and UV region (a), and the GC×GC chromatograms of rac-[ 13 C] alanine films that were irradiated with L-and R-CPL at wavelengths of 200 and 184 nm (b) (modified from Meinert et al.[START_REF] Meinert | Photonenergy-controlled symmetry breaking with circularly polarized light[END_REF]). The induced ee showed the opposite signs between 200 and 184 nm, which are closely correlated with the reserved signs in the anisotropy spectra.

Fig. 5 .

 5 Fig.4. Experimental setup for asymmetric synthesis of amino acids in interstellar ice analogues (modified from Meinert et al.[START_REF] Meinert | Photochirogenesis: Photochemical models on the absolute asymmetric formation of amino acids in interstellar space[END_REF]). The representative interstellar (ISM) gas molecules are deposited on an MgF2 window at 80 K in the center of the high vacuum chamber. VUV-CPL from a synchrotron light source irradiates the ice sample on the MgF2 window. During the experiments, in situ IR-spectra are taken to monitor the composition and the thickness of the ice. After the irradiation, the sample is warmed up to room temperature and the remaining residue analyzed by enantioselective gas chromatography. Fig.5. Structures of major extraterrestrial amino acids up to C6 in carbonaceous chondrites.

Fig. 6 .

 6 Fig. 6. Flow diagram of the two-dimensional chiral high-performance liquid chromatography (2D-HPLC) system (a), and the enantiomeric separation of amino acids in the Yamato 791191 meteorite by the 2D-HPLC system (b) (modified from Hamase et al. [160]). The grey bars in the first dimension chromatogram indicate the on-line collected fractions via a multi-loop device that are transferred to the enantioselective second dimension column.

Fig. 7 .

 7 Fig. 7. Mission image of Hayabusa2 (a), OSIRIS-REx (b), MMX (c), and ExoMars 2020 (d) (image courtesy of JAXA for (a) and (c), NASA for (b), and ESA for (d), respectively).

  Fig.1

  Fig.5

Fig. 7

 7 Fig.7

1 Manuscript proposal -Reviewfor the Special Issue of Biochimica et Biophysica Acta Journal -Proteins and Proteomics entitled "D-amino acids: biology in the mirror", featuring highlights of the IDAR2017 meeting. Acknowledgements This work was funded by the JSPS Overseas Research Fellowships (H.S) and a Grantin-Aid for Scientific Research on Innovative Areas (Research Project on Evolution of Molecules in Space, No. 25108006 : H.S. and Y.T.). We thank the Agence National de la Recherche for support under grant number ANR-16-C829-0015. This work was furthermore funded by the French Government and the Agence National de la Recherche, Investissement d'Avenir UCA JEDI (ANR-15-IDEX-01).