Guanines and related nucleobases such as guano sine, deoxyguanosine, isoguanosine and so on (simply referred to as G hereafter) are fascinating chemical tools, owing to their ability to create programmable supra molecular networks through their selfassociation to form Gquartets or Gribbons, i.e. macrocyclic arrange ments of four guanines or virtually infinite linear chains of guanines, respectively 1 (Box 1). These selfassembled structures are stabilized by the formation of Hbonds at both the Watson-Crick 2 and Hoogsteen [START_REF] Hoogsteen | The structure of crystals containing a hydrogen-bonded complex of 1-methylthymine and 9-methyladenine[END_REF] faces, making guanine a highly versatile tool for the assembly of supra molecules, with steadily increasing applications in both nanotechnology and chemical biology.

The history of guanine

The discovery of the unique properties of guanines can be traced back to the turn of the twentieth cen tury (Fig. 1). Stubborn scientists were determined to elucidate the chemical nature of nucleic acids [START_REF] Tipson | The chemistry of the nucleic acids[END_REF] , first isolated in 1871 by Miescher [START_REF] Miescher | Hoppe-Seyler's[END_REF] . A laborious isolation of guanylic acids from bovine pancreas was achieved by Hammarsten in 1894 (reF. [START_REF] Hammarsten | Zur Kenntniss der Nucleoproteide[END_REF] ), but uncertainties remained as to the monomeric or polymeric nature of this novel, nonprotein natural substance. Independent efforts by Levene and Jacobs 7 (who coined the term tetranucleo tide hypothesis) and Bang [START_REF] Bang | Untersuchungen über die Guanylsäure[END_REF] towards the end of the first decade of the 1900s provided ever purer guanylic acid and, in turn, solid evidence supporting the mono molecular nature of guanylic acid. They noted, as did others, the tendency of concentrated solutions to 'gelatinize' in a vivid manner, describing guanylic hydrogels as "an amorphous, somewhat gelatinous precipitate (that) turns into a semiliquid jelly" [START_REF] Buell | Crystalline guanine nucleotide[END_REF] . The precise structural nature of the viscous guanylic hydrogel linchpin was eventually unveiled by Davies and colleagues in 1962 (reF. [START_REF] Gellert | Helix formation by guanylic acid[END_REF] ), in a report describing the very first Xray crystal structure of a Gquartet. It seems unlikely that even the most clever of scientists would have envisioned at that time the tre mendous potential of this 'semiliquid jelly' to, a century later, become a key player in supramolecular chemistry, genetics, chemical biology and catalysis.

Native versus synthetic G-quartets

The current fascination with Gquartets undoubt edly originates from their involvement in the forma tion of higherorder nucleic acid structures, known as Gquadruplexes. The discovery of Gquadruplexes led to a paradigm shift in genetics because they "con tribute spatial and temporal dimensionalities to linear sequence information" (Maizels) [START_REF] Maizels | G4-associated human diseases[END_REF] . This paves the way for a new view of DNA in which it is not merely a barcode waiting to be scanned but actively controls its own expression according to a new -and not yet fully understood -dimensionality provided by local formation of quadruplexes. DNA then has a structurefunction relationship and joins RNA (for which this has long been accepted). In this Review, we do not aim to discuss at any length the roles of the Gquartets when

Applications of guanine quartets in nanotechnology and chemical biology

Loic Stefan 1 * and David Monchaud 2 * Abstract | Guanine and related nucleobases such as guanosine, deoxyguanosine and isoguanosine are notable molecular tools for designing functional supramolecular assemblies. This popularity originates in their ability to self-assemble via a unique topological pluralism -as isolated nucleobases, discrete macrocyclic quartets and virtually infinite linear ribbons -that endows them with a considerable functional versatility. Many programmes have been launched to fine-tune the chemical properties of guanine derivatives, to make them usable under different experimental conditions, such as in organic or aqueous environments, and responsive to external stimuli, such as ionic strength, pH, light or temperature. These strategies aim to translate the chemical information encoded in a basic guanine unit into programmable, higher-order supramolecular architectures. Spectacular results have been recently obtained in various chemical fields, from supramolecular chemistry to chemical biology , from soft matter to catalysis. In this Review , we detail these advances and demonstrate how these multidisciplinary investigations cast a bright light on the diversity that guanines, synthetic guanines and related nucleobases uniquely offer in terms of both structure and function.

embedded in Gquadruplexes or the prevalence of quad ruplex structures in the human genome, transcriptome and ncRNAome (known as g4ome) [START_REF] Cammas | RNA G-quadruplexes: emerging mechanisms in disease[END_REF] . These topics have been frequently covered in a number of other authori tative reviews [START_REF] Balasubramanian | Targeting G-quadruplexes in gene promoters: a novel anticancer strategy?[END_REF][START_REF] Bochman | DNA secondary structures: stability and function of G-quadruplex structures[END_REF][START_REF] Rhodes | G-quadruplexes and their regulatory roles in biology[END_REF][START_REF] Hänsel-Hertsch | DNA G-quadruplexes in the human genome: detection, functions and therapeutic potential[END_REF][START_REF] Kwok | Detecting RNA G-quadruplexes (rG4s) in the transcriptome[END_REF] and readers are directed there. Here, we focus on both natural and nonnatural Gquartetsor synthetic Gquartets (SQ) -used as molecular platforms for nanobiotechnological purposes.

The field of Gbased molecular tools was launched in 1995 independently by Davis et al. [START_REF] Davis | Self-assembled ionophores from isoguanosine[END_REF] and Gottarelli et al. [START_REF] Gottarelli | Self-assembly in organic solvents of a deoxyguanosine derivative induced by alkali metal picrates[END_REF] , who demonstrated that lipophilic guanine (Lipog) derivatives (5′tertbutyldimethylsilyl2′, 3′isopropylideneisoguanosine (1) and 3′,5′didecanoyl 2′deoxyguanosine (2), respectively; Fig. 2a) fold into stable, cationtemplated SQ in organic media. Indepth NMR investigations performed with both systems indicated that potassium ions trigger the formation of higherorder assemblies, [(1) 4 K + (1) 4 ] and [(2) 4 K + (2) 4 ], respectively, in which the potassium ion is sandwiched between two layers of SQ (socalled octamers; Fig. 2b). Both studies demonstrated the specific ion binding of SQbased systems, with a better chelation of potassium (K + , 1.33 Å) versus the smaller sodium (Na + , 0.98 Å) (and no chelation for the even smaller lithium (Li + , 0.68 Å)), confirming the privileged shape and charge recognition of K + by Gbased suprastructures [START_REF] Pinnavaia | Alkali metal ion specificity in the solution ordering of a nucleotide, 5′-guanosine monophosphate[END_REF] . Interestingly, Davis et al. soon demonstrated the importance of the chemical nature of the Gderivatives: while guanosines are selec tive for K + , isoguanosines selectively interact with larger cations such as caesium (Cs + , 1.67 Å) and radium (Ra 2+ , 1.48-1.70 Å), adapting the nature of its supramolecu lar assembly from octamer to decamer, [(1) 5 Cs + (1) 5 ] [START_REF] Cai | Binding cesium ions with nucleosides: templated self-assembly of isoguanosine pentamers[END_REF] and [(1) 5 Ra 2+ (1) 5 ] [START_REF] Van Leeuwen | Selective 226 Ra 2+ ionophores provided by self-assembly of guanosine and isoguanosine derivatives[END_REF] , respectively (and, thus, from quar tets to quintets). This opened the way to assemble a Gbased toolbox in which to select Gderivatives in an applicationdriven manner (see below).

Efforts were, thus, subsequently invested to finetune the chemical structure of LipoGs to endow

Box 1 | h-bonded guanines

Guanine (and related guanosine, deoxyguanosine) shines by its hydrogen-bond (H-bond) donating/accepting capability, with three acceptor (a) and three donor (D) sites, which enables both watson-Crick and Hoogsteen faces to participate in the creation of a broad array of H-bond networks 152 . the iconic guanine/cytosine (G≡C) base pair follows the rule described by watson and Crick 2 , involving three H-bonds on the sole guanine's watson-Crick face. when both faces are implicated, the supramolecular assemblies depend on the experimental conditions: in the presence of cation (e.g. K + ), the guanosine (G) quartet formation is privileged, given that alkali metals can neutralize the electrostatic repulsion of the inwardly pointing guanine's carbonyl groups within the quartet inner cavity [START_REF] Davis | G-quartets 40 years later: from 5′-GMP to molecular biology and supramolecular chemistry[END_REF][START_REF] Pinnavaia | Alkali metal ion specificity in the solution ordering of a nucleotide, 5′-guanosine monophosphate[END_REF] . the stability of the macrocyclic edifice thus originates in an interplay between the formation of eight H-bonds, the cation-mediated electronic stabilization, along with the delocalization of the nucleobase electrons (resonance), either π-electrons or σ-electrons or both (the resonance-assisted hydrogen bonding (raHB) 153 or the charge separation theory 154 , respectively). in the absence of cations, G-ribbon formation is privileged: two types of ribbon are possible (type a and type B), which differ from the orientation of the guanine's molecular dipole (arising from the highly polarized aromatic surface of the nucleobase, yellow arrows) [START_REF] Davis | G-quartets 40 years later: from 5′-GMP to molecular biology and supramolecular chemistry[END_REF]155 . the nature of the solvent will favour ribbon with or without global dipole moment.

G-quartet (K + G-quartet) G-ribbon

G4ome

the g4ome comprises all g-quadruplex (g4)-forming sequences identified in the genome (g4-DNA) and both the coding and non-coding transcriptome (g4-rNA).

LipoG lipophilic guanine derivatives aimed at being used in organic solvents, masking hydrophilic moieties (e.g. the hydroxyl groups of the guanosine's ribose) through the use of aliphatic protecting groups.

them with special properties. For instance, Rivera and colleagues designed monomeric guanines that selfassemble into SQ with an extended Hbond net work (increasing from 8 to 12 Hbonds) [START_REF] Gubala | Expanding the Hoogsteen edge of 2′-deoxyguanosine: consequences for G-quadruplex formation[END_REF] to improve their stability. They synthesized a LipoG derivative with an expanded Hoogsteen face, adding an acetyl phenyl appendage on its C8position (8(3acetyl phenyl)(3′,5′bisisobutyryl)2deoxyguanosine (3); Fig. 2a). This allows for the formation of a synthetic Gquartet in which the amine moieties located in the outer side of the SQ, and, thus, readily accessible to Hbonddonating groups [START_REF] Otero | Guanine quartet networks stabilized by cooperative hydrogen bonds[END_REF] , are involved in additional intramolecular interactions. In addition to higher sta bility, the resulting SQ display extended πsurfaces that result in the formation of K + promoted hexadecameric columnar assemblies ([(3

) 4 K + (3) 4 K + (3) 4 K + (3) 4 ]
). These assemblies can be topologically switched to octameric assemblies by replacing K + with strontium (Sr 2+ , [(3) 4 Sr 2+ (3) 4 ]) [START_REF] Martín-Hidalgo | Metallo-responsive switching between hexadecameric and octameric supramolecular G-quadruplexes[END_REF] , in a reversible manner, on the basis of electrostatic consideration only (Fig. 2c). Spada and col leagues also exploited the strategic C8modification of LipoGs to introduce a photoswitchable styryl moiety (3′,5′Odiacetyl8styrylguanosine (4); Fig. 2a), which enables control over the K + promoted formation of the [(4) 4 K + (4) 4 ] octamers and their lightinduced dis assembly [START_REF] Lena | Triggering of guanosine self-assembly by light[END_REF] . Irradiation at λ ex = 365 nm indeed triggers the EtoZ conformational switch of the 8styrylguanosine, creating a steric clash that precludes πstacking inter actions between SQ planes and, thus, causes the supra molecular complex to disaggregate. Upon irradiation at λ ex = 254 nm, the conformational switch is reversed (ZtoE conversion) and the octamer restored, mak ing the system fully reversible (no cycling wastes) and controllable (Fig. 2d). C8modified LipoGs were also useful for gaining insights into the many exter nal factors that influence the topological dynamism of resulting SQ. Using 8bromo2′,3′Oisopropylidene 5′tertbutyldimethylsilylguanosine (5) (Fig. 2a), Meijer and colleagues confirmed the expected roles of solvent polarity and of the nature of the cation, but also highlighted a subtle yet significant influence of the Coulombic energy of cation-anion pairs (e.g. KCl, KBr or KI) used for templating SQ formation [START_REF] González-Rodríguez | G-quadruplex self-assembly regulated by Coulombic interactions[END_REF] . Ion pairseparation energy, although largely negligi ble in water, is important in organic buffers, given that it may have an impact on the nature of the resulting supramolecular assemblies (octamers, dodecamers or hexadecamers). Brown and colleagues further sub stantiated these observations [START_REF] Reddy | Co-existence of distinct supramolecular assemblies in solution and in the solid state[END_REF] , demonstrating that the topological pluralism of LipoG might also be sub tly influenced by the method of investigation (in this case, the differences between solution and solidstate NMR), highlighting again the need to exercise caution when studying highly dynamic supramolecular systems. Anecdotally, 5 was a synthetic intermediate in a strat egy implemented by Davis and colleague to take control over the topological diversity of Gbased assemblies [START_REF] Kaucher | N2, C8-disubstituted guanosine derivatives can form G-quartets[END_REF] . Attempting to irreversibly freeze octamers via covalent crosslinking, alkene appendages were inserted on the C8 and N2 positions of 5. The resulting Ba 2+ promoted octamer was then subjected to olefin meta thesis, but this very elegant strategy proved unsuccessful. [START_REF] Davis | Self-assembled ionophores from isoguanosine[END_REF] , Gottarelli (2) [START_REF] Gottarelli | Self-assembly in organic solvents of a deoxyguanosine derivative induced by alkali metal picrates[END_REF] , Rivera (3) [START_REF] Gubala | Expanding the Hoogsteen edge of 2′-deoxyguanosine: consequences for G-quadruplex formation[END_REF] , Spada (4) [START_REF] Lena | Triggering of guanosine self-assembly by light[END_REF] and Meijer (5) [START_REF] González-Rodríguez | G-quadruplex self-assembly regulated by Coulombic interactions[END_REF] used as models to gain insight into the parameters that govern G-quartet formation. b, c | LipoGs (2, 3) were used to demonstrate the capacity of K + -promoted G-quartets to adopt higher-order suprastructures, such as octamers and hexamers. c, d | A variety of strategies were implemented to gain control over G-quartet topological pluralism (tetramer, octamer, hexadecamer), in a reversible manner, using either metallo-responsive or light-responsive LipoGs. SQ, synthetic G-quartets.

It was later revisited and applied efficiently for the synthesis of artificial ion transporters (see below).

A further possibility that enables greater control over the nature of the Gquartetbased suprastructures is to assemble multiple guanines within the same chem ical scaffold. Lipophilic Gdimers and Gtetramers were designed to construct higherorder assemblies in a precise manner. The selfassembly of a covalently tethered C8aryl Gdimer derived from 5 led to dis crete quadruplextype octamers (whose structure was confirmed by singlecrystal Xray diffraction). The resulting structures were found to be highly stable, even in protic (methanol, up to 100%) or polar apro tic (dimethyl sulfoxide, DMSO, up to 50%) solvents [START_REF] He | Construction of a cross-layer linked G-octamer via conformational control: a stable G-quadruplex in H-bond competitive solvent[END_REF] . To build supramolecular ion channels, Matile and col leagues synthesized Gdimers to assemble artificial ion channels but faced tremendous solubility issues, which likely originated from the use of nonglycosylated purine moieties [START_REF] Chen | Toward supramolecular ion channels formed by oligonucleotide analogs: Hydrophobic guanine dimers[END_REF] . Davis and colleagues successfully addressed this issue, assembling four 5′amino2′,3′isopropylidene guanosines (6) around an alternating calix [4]arene scaffold [START_REF] Sidorov | Toward artificial ion channels: self-assembled nanotubes from calix [4] arene-guanosine conjugates[END_REF] . The resulting G4calix (Fig. 3a) orientates two guanines above and two guanines below the plane defined by the calixarene core (in an 'up-down-up-down' manner), so that the spontaneous, Na + promoted assem bly of multiple G4calix moieties results in the forma tion of micrometrelong, SQbased nanotubes. Sherman and colleague followed a similar approach, using the alkyne-azide cycloaddition to attach four 5′azido2′, 3′isopropylideneguanosines (7) to a Crambowl shaped cavitand displaying four propargyloxy groups [START_REF] Cram | Hemicarcerands that encapsulate hydrocarbons with molecular weights greater than two hundred[END_REF] to assemble lipophilic Gtetramers, referred to as TASQ (templateassembled synthetic Gquartets) [START_REF] Nikan | Template-assembled synthetic G-quartets (TASQs)[END_REF] . The allup organization of the cavitand template makes TASQ the first example of discrete, intramolecular SQ. The presence of cations is not mandatory for the SQ forma tion; however, TASQ efficiently chelate Na + or Sr 2+ , in a complex in which the quartet acts as a lid on top of a

H H H H O O N N N N N N N N N N N N N N N H N O H 2 N O O O N N N H N O H 2 N O O O N N N H N O H 2 N O O O N N N H N O H 2 N O O O H N O O O N N N N O N N N N N O HN N N N N O N N N N N O NH H H H H H H H H H O O O OR O H N O O HN O O O O N N N N O N N N N N O HN N N N O N N N N N O NH H H H H H H H H H H N H O O O O O O O O O NH O O O O O O O O O N N N NH O NH 2 O O N 3
+ O O O O O O O N H O N N N N H O NH 2 O O O N H N N N N H O H 2 N O O O H N N N N H N O NH 2 O O O H N N N N H N O H 2 N O O O H 2 N N N N NH O NH 2 O O O O O O O O O O O O O O O O C 11 H 23 C 11 H 23 C 11 H 23 C 11 H 23 H H H H O O N N N HN N N N N N N N N N N N N O N N N N N O HN N N N N O N N N N N O NH H H H H H H H H H O O O O O O O O O Na +
Fig. 3 | tetrameric lipophilic guanosines assemblies. a | Guanosine (G)4-calix synthesis from (6) [START_REF] Sidorov | Toward artificial ion channels: self-assembled nanotubes from calix [4] arene-guanosine conjugates[END_REF] and spontaneous, Na + -promoted self-association leading to the formation of nanotubes based on the intermolecular synthetic G-quartets (SQ) thanks to the alternate (up-down-up-down) orientation of the 1,3-alt-calix [4]arene template. b | Template-assembled synthetic G-quartets (TASQ) synthesis from ( 7) [START_REF] Nikan | Template-assembled synthetic G-quartets (TASQs)[END_REF] and representation of its 'open' (centre) and 'closed' (right) conformation, in which an intramolecular SQ is folded, owing to the all-up orientation of the cavitand template.

Nature reviews | Chemistry R e v i e w s basketlike ion receptor (Fig. 3b), while larger cations such as Cs + trigger TASQ dimerization [(TASQ)Cs + (TASQ)] [START_REF] Nikan | Cation-complexation behavior of template-assembled synthetic G-quartets[END_REF] .

Lipophilic versus water-soluble G-quartets

LipoGs have been instrumental in understanding the factors that govern Gquartet formation and stability, thanks to the broad variety of techniques implementa ble in organic solvents that are often more challenging to apply in water (see below). However, watersoluble guanines open up opportunities in terms of applications, notably for investigations in biological systems. From a structural point of view, the simplest hydrophilic mon omer is guanosine monophosphate (GMP; Fig. 4). Early investigations on Gquartet formation were performed using concentrated GMP solutions (25 mg mL -1 ) [START_REF] Gellert | Helix formation by guanylic acid[END_REF] . The structure of GMPbased, hydrophilic Gquartets was further investigated later on, through modern analytical techniques. For instance, using a combination of scan ning electron microscopy (SEM) and NMR, Walmsley and colleagues demonstrated that Na + indeed prompts GMP to form Gquartets [(GMP) 4 Na + ] that then spon taneously selfassemble into micrometrelength rods comprising thousands of stacked Gquartets. Meanwhile, K + leads to the formation of both rodlike lattices (formed of stacked [(GMP) 4 K + ], similar to those formed with Na + ) and twisting fibres comprising continuous helices of hydrogenbonded guanines, with 15 Gs per turn 36 (Fig. 4a).

The puckering of both the C2′endo/C3′endo sugars alternates along the strands and the righthanded helical twist of the suprastructure were confirmed by Wu et al., using a combination of 1D and 2D NMR spectro scopy techniques [START_REF] Wu | On the helical structure of guanosine 5′-monophosphate formed at pH 5: is it left-or right-handed?[END_REF][START_REF] Wu | Helical structure of disodium 5′-guanosine monophosphate self-assembly in neutral solution[END_REF] . GMPdimers and GMPtetramers have also been investigated. For example, the natu rally occurring GMPdimer c-di-gMP 39 , which is a cen tral bacterial messenger, was not observed to fold into Gquartets or Gquadruplexes under the conditions found inside cells (300 mM K + and 10 mM cdiGMP). This observation discounts any possible biological role of cdiGMP higherorder assemblies in the bacterial cell cycle. However, the presence of substoichiometric quantities of aromatic intercalators (such as proflavine [START_REF] Nakayama | c-di-GMP can form remarkably stable G-quadruplexes at physiological conditions in the presence of some planar intercalators[END_REF] or thiazole orange [START_REF] Nakayama | Thiazole orange-induced c-di-GMP quadruplex formation facilitates a simple fluorescent detection of this ubiquitous biofilm regulating molecule[END_REF] ; Fig. 4b) leads to the formation of highly stable cdiGMP quadruplexes with a cagelike structure in which the aromatic inducer is intercalated between two layers of Gquartets. Sherman and colleagues scrutinized the structure of tetrameric GMP, following the lipophilic TASQ approach and exploiting the phosphate group of the 2′deoxyguanosine3′monophosphate to link gua nines to the cavitand template 41 (Fig. 4c). As above, the presence of cations is not mandatory for the Gquartet formation of watersoluble TASQ; this new molecu lar tool was not exploited for cation chelation but as a DNAmimicking platform to investigate the interactions between Gquartets and g4-ligands (vide infra).

Overall, the wealth of information collected over the past 25 years with both lipophilic and hydrophilic Gquartets has provided invaluable insights into the thermodynamic, dynamic and structural parameters that govern the assembly of Gquartet structures. The analytical techniques available to study these struc tures have become increasingly effective and accurate, making the area a playground for the development and application of cuttingedge technologies. From the pioneering crystallographic works of Gellert et al. [START_REF] Gellert | Helix formation by guanylic acid[END_REF] that provided the very first details about the structure of a Gquartet, many different practical and theo retical techniques (often combined) have been applied to gain insights into the kinetics and thermodynamics of Gbased assemblies. This includes other Xraybased techniques (for example, smallangle Xray scattering (SAXS)) [START_REF] Gonnelli | Small-angle X-ray scattering study of self-assembling lipophilic guanines in organic solvents: G-quadruplex formation and cation effects in cyclohexane[END_REF] , along with various microscopy techniques: atomic force microscopy (AFM) was applied to charac terize the selforganization of GMP on mica [START_REF] Kunstelj | Self-organization of guanosine 5′-monophosphate on mica[END_REF] , SEM was applied to investigate the folding parameters and global topology of micrometrelong Na 2 (5′GMP) assemblies [START_REF] Hightower | Supramolecular structure and polymorphism of alkali metal salts of guanosine 5′-monophosphate: SEM and NMR study[END_REF] and scanning tunnelling microscopy (STM) was applied in combination with density functional theory (DFT) calculations to assess whether and how Gquartets can be assembled on surfaces (either Au(iii) [START_REF] Otero | Guanine quartet networks stabilized by cooperative hydrogen bonds[END_REF][START_REF] Zhang | Solventless formation of G-quartet complexes based on alkali and alkaline earth salts on Au(111)[END_REF] or highly ori ented pyrolytic graphite (HOPG) [START_REF] González-Rodríguez | Persistent, well-defined, monodisperse, π-conjugated organic nanoparticles via G-quadruplex self-assembly[END_REF] surface) in solventless conditions, with simple or sophisticated guanine deriva tives, in the absence or presence of Na + , K + or Ca 2+ cations. Spectroscopic techniques provided invaluable clues about Gbased selfassemblies in solution; these include Fourier transform infrared (FTIR) [START_REF] Gao | Temperature and pressure limits of guanosine monophosphate self-assemblies[END_REF] , infrared multiple photon dissociation (IRMPD) [START_REF] Azargun | Guanine tetrads: an IRMPD spectroscopy, energy resolved SORI-CID, and computational study of M(9-ethylguanine) 4 + (M = Li, Na, K, Rb, Cs) in the gas phase[END_REF][START_REF] Azargun | The intrinsic stabilities and structures of alkali metal cationized guanine quadruplexes[END_REF][START_REF] Fraschetti | Spectroscopic evidence for a gas-phase librating G-quartet-Na + complex[END_REF] and RAMAN [START_REF] Mudroňová | Effect of ribose versus 2′-deoxyribose residue in guanosine 5′-monophosphates on formation of G-quartets stabilized by potassium and sodium cations[END_REF] spectroscopies, along with electronic and vibrational circular dichroism for instance (ECD [START_REF] Panda | Circular dichroism study of supramolecular assemblies of guanosine 5′-monophosphate[END_REF] and VCD [START_REF] Goncharova | Stacked and continuous helical self-assemblies of guanosine monophosphates detected by vibrational circular dichroism[END_REF] , respectively) combined with degenerate coupled oscillator (DCO) calculations [START_REF] Goncharova | Stacked and continuous helical self-assemblies of guanosine monophosphates detected by vibrational circular dichroism[END_REF] and NMR [START_REF] Kwan | Probing hydrogen bonding and ion-carbonyl interactions by solid-state 17 O NMR spectroscopy: G-ribbon and G-quartet[END_REF][START_REF] Wong | Probing the Na + binding site in a calix[4] arene-guanosine conjugate dimer by solid-state 23 Na NMR and quantum chemical calculation[END_REF] , notably diffusionordered spectroscopy (DOSY) NMR, which were successfully implemented to characterize the cationtemplated GMP aggregation in nearphysiological conditions. Finally, theoretical mod els were provided via molecular dynamics (MD) [START_REF] Mukhopadhyay | Design rules for the generation of stable quartet phases of nucleobases over two-dimensional materials[END_REF][START_REF] Šponer | Folding of guanine quadruplex molecules-funnel-like mechanism or kinetic partitioning? An overview from MD simulation studies[END_REF] and molecular mechanics (MM) [START_REF] Gresh | Channeling through two stacked guanine quartets of one and two alkali cations in the Li + , Na + , K + , and Rb + series. Assessment of the accuracy of the SIBFA anisotropic polarizable molecular mechanics potential[END_REF] , notably for both describ ing cationtemplated Gquartet formation and simulating in silico the movement (channelling) of cation (e.g. K + ) between two Gquartet layers. Dispersioncorrected DFT was also found to be a quantum chemical computational method of choice for deciphering the parameters (desol vation energy, cation size) at the origin of the exquisite K + affinity of Gquartets [START_REF] Zaccaria | The role of alkali metal cations in the stabilization of guanine quadruplexes: why K + is the best[END_REF][START_REF] Paragi | Cooperativity in the self-assembly of the guanine nucleobase into quartet and ribbon structures on surfaces[END_REF] . This wealth of information has been progressively harnessed to design ever more versa tile Gbased molecular tools that have found applications in the fields of soft matter, nanotechnology and chemical biology, which will be discussed in the following sections.

Nanotechnological applications

As described above, the first application developed with guanines relied on their capability to firmly che late cations. Lipophilic SQ were thus exploited as ion ophores, capable of extracting cations from water into organic solvents (most often chlorinated solvents, such as chloroform or dichloromethane). Model studies were performed with K + and Na + , but the demonstration that Gquartets efficiently chelate radioactive metals such as caesium ( 137 Cs + ) and radium ( 226 Ra 2+ ) generated a tre mendous strategic interest. Notably, it was shown that the isoguanosine derivative (1) could selfassemble to form nanoscale decamers that were capable of captur ing Cs + ([(1) 5 Cs + (1) 5 ]) and Ra 2+ ([(1) 5 Ra 2+ (1) 5 ]), even in the presence of a millionfold excess of other cati ons (Na + , K + , Mg 2+ ) [START_REF] Cai | Binding cesium ions with nucleosides: templated self-assembly of isoguanosine pentamers[END_REF][START_REF] Van Leeuwen | Selective 226 Ra 2+ ionophores provided by self-assembly of guanosine and isoguanosine derivatives[END_REF] , and, thus, suggesting a possible new application in the remediation of nuclear wastes. with aromatic inducers (here, thiazole orange) that intercalate between two layers of G-quartets [START_REF] Nakayama | Thiazole orange-induced c-di-GMP quadruplex formation facilitates a simple fluorescent detection of this ubiquitous biofilm regulating molecule[END_REF] . c | Tetrameric GMP can be assembled intramolecularly to provide water-soluble template-assembled synthetic G-quartets (TASQ) that fold into intramolecular SQ without cations and serve as a platform for investigating the G-quartet interactions of G4-ligands (here, BRACO-19) [START_REF] Bare | Synthesis of a single G-quartet platform in water[END_REF] .

Nature reviews | Chemistry

R e v i e w s

Davis and colleagues later developed ionophores with improved chelating properties, providing binding sites for both cations and anions [START_REF] Kotch | Water-mediated association provides an ion pair receptor[END_REF] . The dimeric assembly of 1,3altcalix [4]arenetemplated Gtetramer (8) indeed traps Na + within the intermolecular SQ plane, while Cl -is chelated by two neighbouring amide groups (Fig. 5a).

This study, performed in watersaturated chloroform, highlighted the key role of water in the overall stabil ity of the supramolecular edifice, lowering the energy required for extracting the ions from bulk water to the ionophore's binding site, participating in salt binding between the quartet layers and filling the coordination spheres of bound ions. Lipophilic SQ were also exploited as ion channels [START_REF] Davis | Recent advances in the transmembrane transport of anions[END_REF] their lipophilicity means that they are readily incorporated into membranes. Davis and colleagues constructed transmembrane pores based on a quadruplex archi tecture. These pores were formed by the assembly of the Gmonomer 5′(3,5bis(allyloxy)benzoyl)2′, 3′isopropylidene guanosine (9), followed by crosslink ing of neighbouring guanines within the quadruplex architecture using rutheniumcatalysed, ringclosing metathesis 62 (Fig. 5b). This approach leads to a cova lently crosslinked quadruplex that avoids any potential problems arising from the dynamic nature of quad ruplex formation/disassembly. The capacity of the resulting quadruplex, comprising sixteen units of (9), to transport Na + across a liposome membrane was qualitatively demonstrated by both basepulse assay and [START_REF] Gubala | Expanding the Hoogsteen edge of 2′-deoxyguanosine: consequences for G-quadruplex formation[END_REF] Na NMR spectroscopic experiments. The approach of covalently capturing the formed quad ruplexes was subsequently extended using a photocyclo addition strategy. Combining 2′,3′isopropylidene guanosine moieties with metamethoxycinnamic acid provided Gmonomeric units primed for crosslink ing. Once assembled in SQ in the presence of K + , an UVpromoted [2+2] cycloaddition of the cinnamate chains led to the formation of cyclobutane covalent crosslinks with high chemical yields (>90%) [START_REF] Sutyak | Templating and catalyzing [2+2] photocycloaddition in solution using a dynamic G-quadruplex[END_REF] . However, the iontransportation properties of these new supra structures were not assessed. Matile and colleagues exploited another type of lipophilic SQ to assemble ion channels, using πstacked supramolecular rosettes made of Gmimicking folate quartets [START_REF] Sakai | Dendritic folate rosettes as ion channels in lipid bilayers[END_REF] . The capacity of the obtained channels to transport K + across lipid bilayers highlights the importance of the dynamic nature of quartetbased channels, which uniquely allows for sterically demanding cations to be transported through topologically breathing channels, an effect termed dynamic ion tunnelling. Davis and colleagues also constructed ion channels whose structure relies on the assembly of preformed Gdimers covalently linked by bislithocholamide linkers (10) [START_REF] Ma | Large and stable transmembrane pores from guanosine-bile acid conjugates[END_REF] (Fig. 5c). The capacity of these pores to enable K + transport across phospholipid bilayers was quantified by conductance experiments. It was, thus, shown that the resulting SQ channels formed in this way were heterogeneous in size, with small, 2.6Ådiameter pores (with a corresponding conductance in the pS range) that correspond to discrete SQ and large, 12Ådiameter pores (with conductance in the nS range) that correspond to polymeric SQ (as shown in Fig. 5c). Dash and colleagues subsequently combined these dif ferent approaches, linking lipophilic Gdimers using various bridge structures (polyethylene glycol (PEG), dyes, aromatic linkers) [START_REF] Kumar | Triazole-tailored guanosine dinucleosides as biomimetic ion channels to modulate transmembrane potential[END_REF][START_REF] Kumar | Bis-triazolyl diguanosine derivatives as synthetic transmembrane ion channels[END_REF] to assemble channels that could embedded within phospholipid bilayers and capa ble of transporting cations such as K + for long durations (up to 10 h). Moving beyond ion transportation, Gquartet struc tures have also been exploited for the construction of functional nanostructures. For instance, Dash and col leagues exposed GMP to microwaveassisted thermal treatment in order to assemble carbon dots [START_REF] Ghosh | Chiral carbon dots derived from guanosine 5′-monophosphate form supramolecular hydrogels[END_REF] . While GMP itself does not possess interesting spectroscopic properties, these socalled Gdots were found to have interesting fluorescence upconversion properties and can be readily converted in fluorescent Gdot hydrogels in the presence of K + , which were shown to have high ther mal stability (gel-sol transition temperature >70 °C) and elasticity. Meijer and colleagues used LipoGs to assem ble fluorescent organic nanoparticles (ONP) (Fig. 6a). Cationtemplated selfassembly of oligo(pphenylene vinylene) (OPV) guanosines (11) leads to discshaped octameric nanostructures (1.5 nm high, 8.5 nm wide), with improved fluorescence emission properties in comparison with isolated guanosines (with a twofold to threefold enhancement of quantum yield in organic solvents) and thermal stability (melting temperature >70 °C) [START_REF] González-Rodríguez | Persistent, well-defined, monodisperse, π-conjugated organic nanoparticles via G-quadruplex self-assembly[END_REF] . Mirkin and colleague also exploited Gquartet formation to assemble metallic nanoparticles (Fig. 6b). A disperse solution of 10nm gold nanoparticles coated with 3′thiopropylGMP (12) were combined with a series of short, Grich oligonucleotides to produce, upon Na + addition, micrometrelong architectures that displayed increased plasmon coupling as a result of reduced interparticle distance and higher thermal sta bility (aggregate-sol transition temperature >60 °C) [START_REF] Li | G-quartet-induced nanoparticle assembly[END_REF] . Wu and colleagues assembled even larger nanoobjects by subjecting concentrated GMP solution (33 wt%, i.e. 800 mM) to weakly basic conditions (pH = 8), which results in the formation of Na + containing nano cylinders (26 Å diameter, up to 30 nm long) [START_REF] Wong | Disodium guanosine 5′-monophosphate self-associates into nanoscale cylinders at pH 8: a combined diffusion NMR spectroscopy and dynamic light scattering study[END_REF] . Qu and colleagues later demonstrated that replacing Na + for Sr 2+ enabled the production of micrometrelong nanowires from diluted GMP solution (20 mM), in a reversible, controllable manner (nanowires were broken upon the addition of EDTA) [START_REF] Hu | Metal-mediated fabrication of new functional G-quartet-based supramolecular nanostructure and potential application as controlled drug release system[END_REF] . Decorating these fibres with three dyes, i.e. thioflavin T, thiazole orange and pyronin Y, that constitute a fluorescence resonance energy transfer (FRET) system, the same authors reported on the use of GMPbased nanowires as lightharvesting material (Fig. 6c). Once assembled in the nanostructure, light (λ ex = 450 nm) is absorbed by thioflavin T, transmitted by FRET first to thiazole orange and then pyronin Y (λ em = 573 nm), generating a photocurrent that demon strates the functional relevance of tripledyedoped nanofibres as lightharvesting antennae [START_REF] Pu | G-quartetbased nanostructure for mimicking light-harvesting antenna[END_REF] . The electronic properties of Gbased suprastructures were further stud ied by Wasielewski and colleagues, who investigated the properties of guanines with electronacceptor moieties such as perylene3,4,9,10bis(dicarboximide) (PDI) derivatives. The resulting GPDI selfassembles into octamers upon interaction with K + (Fig. 6a). Femtosecond transient absorption spectroscopy was used to demon strate that [(GPDI)K + (GPDI)] undergoes efficient charge separation upon photoexcitation at λ ex = 525 nm. The lifetime of the chargeseparated intermediate in the octameric assembly was determined to be 1.2 ns, as compared with 98 ps for the monomeric GPDI [START_REF] Wu | Extending photoinduced charge separation lifetimes by using supramolecular design: guanine-perylenediimide G-quadruplex[END_REF] . Using a dimeric system with two guanosines appended to the core perylene diimide, the same authors constructed SQ organic frameworks and showed their ultrafast production of longlived mobile charges (radical ion pairs, or polar ons) upon photoexcitation (λ ex = 500 nm; Fig. 6d). The excellent electrical conductivity of these frameworks was exploited to assemble a SQbased Liion battery, which proved to be robust (<7% loss of efficiency after 300 charge/discharge cycles) [START_REF] Wu | G-quadruplex organic frameworks[END_REF] .

Gquartets also served as a platform for interacting with smallmolecule partners to endow the resulting Gquartet/ligand complex with specific properties. Two examples were discussed above: the interaction of thiazole orange with a cdiGMP octamer (Fig. 4b), in which the resulting quadruplex/ligand complex is fluorescent (and, thus, detectable), and the interaction of TASQ with G4ligands (Fig. 4c), which allows for quan tifying in vitro the parameters that govern the ligand/ target interaction. Combining these two approaches, Sintim and colleagues developed a bioinspired catalytic system for the oxidation of 2,2′azinobis(3ethylbenzo thiazoline6sulfonic acid) (ABTS; Fig. 7). In the pres ence of an intercalator (proflavine), cdiGMP forms a quadruplex that externally binds hemin (protoporphy rin IX iron(III) chloride) [START_REF] Nakayama | A C-di-GMP-proflavine-hemin supramolecular complex has peroxidase activity-implication for a simple colorimetric detection[END_REF] , a wellknown cofactor of

Fluorescence upconversion

this fluorescence spectroscopy (also known as sum frequency generation), is a nonlinear, high-resolution technique characterized by an emission wavelength that is shorter than the excitation wavelength.

Organic frameworks

three-dimensional organic structures in which building blocks are associated either through covalent bonds (covalent organic framework, or CoF) or metal-mediated coordination (metal organic framework, or MoF).

Nature reviews | Chemistry

R e v i e w s hemoproteins, such as horseradish peroxidase (HRP), and a key biochemical tool in the development of the enzymelinked immunosorbent assay (ELISA). The cata lytic activity of Gquartetcontaining biomolecules, i.e. quadruplexDNA (G4DNAzyme), was discovered during investigations that aimed to identify aptamers for catalysing porphyrin metalation. Aptamers with a high affinity for Nmethyl mesoporphyrin IX (NMM) were investigated, given that NMM is a transitionstate ana logue for porphyrin metalation and a potent inhibitor of ferrochelatase, an enzyme that catalyses the insertion of iron into protoporphyrin IX (the last step of haem biosynthesis). Through systematic evolution of ligands by exponential enrichment (SELEX) experiments, Sen and colleague identified a short, 24nt DNA referred to as PS5.M that strongly binds to NMM and catalyses the metalation of mesoporphyrin IX [START_REF] Li | A catalytic DNA for porphyrin metallation[END_REF][START_REF] Li | Toward an efficient DNAzyme[END_REF] . Importantly, it was shown that the catalytic activity of PS5.M is inhib ited by hemin through the formation of a highaffinity PS5.M/hemin complex. This observation drove the authors to assess whether it could elicit hemoproteinlike properties, notably peroxidases. Upon addition of a sto ichiometric oxidant (hydrogen peroxide, H 2 O 2 ), the complex was found to accelerate 250fold the oxida tion of the chromogenic substrate ABTS [START_REF] Travascio | DNA-enhanced peroxidase activity of a DNA aptamer-hemin complex[END_REF] . In light of the Grichness of PS5.M (d[ 5′ GTG 3 TCAT 2 GTG 3 TG 3 TGTG 2 3′ ]) and the K + dependency of its catalytic activ ity, the authors wisely mused about its quadruplex struc ture [START_REF] Travascio | The peroxidase activity of a hemin-DNA oligonucleotide complex: free radical damage to specific guanine bases of the DNA[END_REF] , which offers a defined binding site to hemin that becomes catalytically competent once nestled within this site only. It is intriguing that, more than three decades after its discovery, the precise mechanism of quadruplex DNAzyme remains unclear [START_REF] Stefan | Insights into how nucleotide supplements enhance the peroxidase-mimicking DNAzyme activity of the G-quadruplex/hemin system[END_REF][START_REF] Yamamoto | Characterization of heme-DNA complexes composed of some chemically modified hemes and parallel G-quadruplex DNAs[END_REF][START_REF] Chen | How proximal nucleobases regulate the catalytic activity of G-quadruplex/hemin DNAzymes[END_REF] , despite hundreds of new applications reported each year (for relevant reviews, see reFs 83-85 ). Inspired by Sintim's work, Spinelli and col leagues further exploited the intramolecular assembly of TASQ to provide versatile catalytic systems (TASQzyme) that could be applied with a variety of precatalysts, cofactors and substrates [START_REF] Flack | Prefolded synthetic G-quartets display enhanced bioinspired properties[END_REF] . Monchaud and colleagues used the nonanionic TASQ (e.g. DOTASQ; Fig. 7) (in place of anionic DNA or cdiGMP) to replace the poorly soluble hemin cofactor with the more practically [START_REF] González-Rodríguez | Persistent, well-defined, monodisperse, π-conjugated organic nanoparticles via G-quadruplex self-assembly[END_REF] and charge separation under photoexcitation for guanine perylene-3,4,9,10-bis(dicarboximide) (GPDI) [START_REF] Wu | Extending photoinduced charge separation lifetimes by using supramolecular design: guanine-perylenediimide G-quadruplex[END_REF] . b | Gold nanoparticle assembly triggers by (12) [START_REF] Li | G-quartet-induced nanoparticle assembly[END_REF] , which provides micrometre-long architectures with increased plasmon coupling. c | Guanosine monophosphate (GMP)-based nanowires that act as light-harvesting antenna upon association with fluorescence resonance energy transfer (FRET) dyes triplets [START_REF] Pu | G-quartetbased nanostructure for mimicking light-harvesting antenna[END_REF] , i.e. thioflavin T (ThT), thiazole orange (TO) and pyronin Y (PY). d | G-quadruplex organic frameworks that produce long-lived mobile charges upon photoexcitation, making them prototypes of a G-quartet-based Li-ion battery [START_REF] Wu | G-quadruplex organic frameworks[END_REF] . OPV, oligo(p-phenylene vinylene); PDI, perylene-3,4,9,10-bis (dicarboximide); SQ, synthetic G-quartets.

www.nature.com/natrevchem R e v i e w s convenient 5,10,15,20tetrakis(4sulfonatophenyl) porphyrinato iron (iii) chloride (FeTPPS) [START_REF] Stefan | Harnessing nature's insights: synthetic small molecules with peroxidase-mimicking DNAzyme properties[END_REF] . The con struction of catalytic Gquartets is, however, not limited to assembly from Gdimers and Gtetramers: recently, Davis and colleague showed that GMP itself could assemble to form an efficient thermosensitive catalyst for DNAzymelike reactions, offering higher catalytic rates at low temperature, owing to an improved sta bility of GMPbased Gquadruplex structure at 10 °C versus 30 °C (reF. [START_REF] Harraz | A self-assembled peroxidase from 5′-GMP and heme[END_REF] ). Dash and colleagues reported on the design of catalytic Gquartetbased hydrogels formed by boronateesterfunctionalized guanosines that act as logic gates. K + and Pb 2+ are the on and off inputs, respectively, and the product of the oxidation of 3,3′,5,5′tetramethylbenzidine is the output, thanks to its characteristic absorbance signal at 652 nm that pro vides a convenient readout of the efficiency of the opera tion [START_REF] Bhattacharyya | Supramolecular hydrogel inspired from DNA structures mimics peroxidase activity[END_REF] . Pei and colleagues exploited a similar approach to design bioelectronics nanodevices (i.e. an electrochemi cal glucose sensor based on the concerted action of glu cose oxidase and heminmediated catalytic Gquartets embedded in a hydrogel matrix) [START_REF] Zhong | Self-assembly of enzyme-like nanofibrous G-molecular hydrogel for printed flexible electrochemical sensors[END_REF] .

G-quartet-based soft matter

Soft matter refers to materials whose properties occupy the tenuous frontier between the solid and liquid states. The study of soft matter was popularized by de Gennes, whose work was recognized by the award of the Nobel Prize in Physics 1991 (reF. [START_REF] De Gennes | Soft matter (Nobel lecture)[END_REF] ). These materials have low elastic moduli, enabling them to withstand thermal fluc tuations and external deformations, thanks to internal structural rearrangements that allow for energy dissi pation in response to external stress [START_REF] Nagel | Experimental soft-matter science[END_REF] . They can be pro duced from a variety of organic and inorganic molecular entities (e.g. clay, metal oxides, synthetic and natural polymers and proteins) and encompass a wide diversity of systems, including colloid suspensions, emulsions, foams, surfactants, liquid crystals or gels.

The pioneering works on the ability of GMP to form hydrogels 7,[START_REF] Bang | Untersuchungen über die Guanylsäure[END_REF][START_REF] Gellert | Helix formation by guanylic acid[END_REF] cast a bright light on Gquartets as softmatter building blocks [START_REF] Peters | Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs[END_REF] . For example, Gottarelli et al. studied the formation of columnar aggregates by the lipophilic 3′,5′didecanoyl2′deoxyguanosine (2) [START_REF] Gottarelli | The self-assembly of lipophilic guanosine derivatives in solution and on solid surfaces[END_REF][START_REF] Mezzina | The self-assembly of a lipophilic guanosine nucleoside into polymeric columnar aggregates: the nucleoside structure contains sufficient information to drive the process towards a strikingly regular polymer[END_REF] and sought to characterize them both struc turally and stereochemically. They noticed that [START_REF] Nakayama | A C-di-GMP-proflavine-hemin supramolecular complex has peroxidase activity-implication for a simple colorimetric detection[END_REF] or the intramolecularly folded DOTASQ [START_REF] Stefan | Harnessing nature's insights: synthetic small molecules with peroxidase-mimicking DNAzyme properties[END_REF] upon interaction with hemin, which makes the resulting G-quartet/hemin complex catalytically competent to oxidize the chromogenic 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of H 2 O 2 (as the stoichiometric oxidant), in a DNAzyme-type manner [START_REF] Mergny | DNA quadruple helices in nanotechnology[END_REF] . SQ, synthetic G-quartets.

Logic gates

in electronics, a logic gate is a device that performs a logical operation on one or two inputs and produces a single output. Logic gates are primarily implemented as switches of different possible natures (AND, or, NoT, iNHiBiT etc.) in digital circuits.

Nature reviews | Chemistry

R e v i e w s lyotropic liquid crystal phases were formed in heptane in the presence of potassium picrates [START_REF] Pieraccini | Columnar lyomesophases formed in hydrocarbon solvents by chiral lipophilic guanosine-alkali metal complexes[END_REF] . At high dilution (<6% w/w), an isotropic phase was observed, while medium (6-16% w/w) and high (40-50% w/w) concen trations led to a lefthanded cholesteric phase and a hex agonal phase, respectively (Fig. 8a). Xray measurements confirmed the columnar nature of the Gquartetbased selfassemblies with characteristic peaks at 3.4 Å corre sponding to the stacking of Gquartets and an additional calculated radius of 12 Å for the hexagonal phase, fitting with the Gquartet radius. However, in ionfree condi tions, 2 and other LipoG derivatives also exhibit lyo tropic properties in both toluene and chloroform, in this case due to the formation of Gribbons [START_REF] Giorgi | Gel-like lyomesophases formed in organic solvents by self-assembled guanine ribbons[END_REF] . Recently, Kato and colleagues reported on thermotropic liquidcrystal formation with C8oligothiophene guanine derivatives. In this case, both the nature of the phases and the phase transitions were modulated by potassium ion concen tration. As well as the hexagonal columnar phases based on Gquartet assemblies (as already reported for 2),

Lyotropic liquid crystal

property of an amphiphilic material (mesogen) that exhibits phase transitions upon dissolution in a suited solvent, in a concentration-dependent manner.

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R e v i e w s

Gribbon formation leads to bicontinuous cubic phase rarely observed for πconjugated liquid crystals [START_REF] Gan | Guanine-oligothiophene conjugates: liquid-crystalline properties, photoconductivities and ion-responsive emission of their nanoscale assemblies[END_REF] . Gquartets have also been used to formulate organogels and hydrogels, assembled in organic solvents or aqueous media, respectively. As an example, Yi and colleagues demonstrated that 5′(adamantylcarbamoyl)propion ate2′,3′isopropylideneguanosine (13) can form an organogel even in the absence of cations when working above the critical gelation concentration (>50 mM). The sol-gel transition results from a structural change from Gquartets (assembled at low concentration, <10 mM) to type A Gribbons (Fig. 8b). Gelation occurred when ribbons selfassembled into micrometrewidth hexa gonal columnar structures; a morphological change can be triggered by ultrasonication, which favours twistedflowerlike structures [START_REF] Meng | From G-quartets to G-ribbon gel by concentration and sonication control[END_REF] . Liu and colleagues observed a similar behaviour with a guanine function alized with a 3,4,5tris(ndodecan1yloxy)benzohy drazide ( 14): the quartettoribbon structural transition was solventdependent, occurring above 17% w/w in toluene and above 29% w/w in chloroform. This tran sition was harnessed to build a dynamic organogel. In the presence of cryptand [2.2.2], which is known to chelate K + ions, sequential additions of trifluoroacetic acid and triethylamine trigger switching between the Gquartet formation (sol state) in the presence of base and the Gribbon formation (gel state) in the presence of acid 100 (Fig. 8c). The strategy of using cryptands to modulate Gribbon/Gquartet interconversion was previously described in organic solution by Spada and colleagues with LipoG (2) 101 and a 5′Ooligothiophene guanine derivative 102 . Higherorder, selforganized, hybrid organic/inorganic materials have also been built on Gquartetbased organogels formulated from a guanine-siloxane derivative (15) in the presence of K + or barium (Ba 2+ ) ions 103 . The polymerization of -SiR 2 -OEt into -SiR 2 -O-SiR 2 -was triggered by water, leading to higherordered hybrid materials with micro metric crystallinerod morphologies (Fig. 8d). Similarly, selforganized organic polymers that exhibit microfi bre or microspring morphologies can be accessed from a 5′Omethacrylate guanosine derivative, with the morphology switching dependent on cation driven Gribbon or Gquadruplex adopted assembly, respectively 104 .

The majority of Gquartetbased soft matter des cribed to date focuses on the formation of hydrogels. From the initial pioneering observations of the GMP gelation process 7,[START_REF] Bang | Untersuchungen über die Guanylsäure[END_REF][START_REF] Gellert | Helix formation by guanylic acid[END_REF] , a vast amount of research has been conducted in order to decipher the parameters that govern the formation of hydrogels and improve the scope of their applications. GMPbased gels have mostly been formulated with monovalent or divalent cations (see above); however, the use of trivalent lantha nide ions, including lanthanum (La 3+ ), cerium (Ce 3+ ), europium (Eu 3+ ) and terbium (Tb 3+ ), led to SQbased nanofibrillar networks in acidic conditions 105 . As a result of its high aqueous solubility, GMP alone does not form a gel at neutral pH. However, upon addition of phosphatefree guanosines, gelation occurs and the modulation of the GMP/guanosine ratio can be used to finetune the resulting material properties 106,107 . These gels exhibit unusual thermoassociative behav iour, with the sol-gel transition taking place above a specific temperature 108 . In the presence of different monovalent cations (from Li + to Cs + ), guanosine alone can also form a hydrogel, but it is stable for only a few minutes or hours. This instability can be addressed by the addition of 0.7-1.5 molar equivalents of lipophilic 2′,3′,5′triOacetylguanine (16), which enhances the gel lifetime from hours to years and also increases the sol-gel transition temperature (from 50 °C to >60 °C) 109 (Fig. 9a). Another possibility is to formulate the gel with metal ions such as silver (Ag + ), which increases the sta bility (from hours to months) and the storage moduli (up to 8 kPa) and endows the resulting gel with thixotropy 110 . Addition of isoguanosine is another alternative to turn guanosine gels into thixotropic cogels exhibiting very short recovery times (<20 s) 111 .

Davis and colleagues thoroughly exploited guano sine-borate assemblies 112 , which showed high propensity to form stable hydrogels (with storage moduli >10 kPa at 0.2 wt%; Fig. 9b). These gels have been used to incorpo rate a wide variety of chemicals, notably fluorescent dyes. Even at extremely low concentration (2.5 × 10 -3 molar equivalents), these probes impact the hydrogel properties -decreasing gel stiffness in the case of, e.g. rhodamine B and Rose Bengal, or increasing it in the presence of, e.g. thioflavin T or thiazole orange 113 . Guanosine-borate systems were also used in the preparation of photo reversible hydrogels, in which the photoswitch E4,4′ dioxyethylpiperidinium iodideazobenzene controls the Gquartetbased assembly (λ ex = 436 nm) and disas sembly (λ ex = 365 nm) 114 . Lehn and colleague developed smart, Gquartetbased hydrogels, using watersoluble Gdimer (17). As above, the sol-gel equilibrium of these K + promoted dynamic polymers can be fully controlled by sequential addition of cryptand [2.2.2] and HCl to control the availability of K + to promote Gquartet for mation 115 . Another dynamer was developed to exploit the reactivity of monomeric guanosine5′hydrazide (18) with a library of aldehydes, in order to assemble acylhydrazonebased hydrogels capable of dynamically selfselecting the most stable gel under the experimental conditions 116-118 (Fig. 9c). Barboiu and colleagues reported a doubledynamer system based on the reversible reac tion between guanosines and a bisiminoboronic acid linker to form bisiminoboronateGdimers, whose equilibrium between the cationfree ribbon state and K + promoted Gquartet state led to the production of two types of functional membrane films 119 .

Interesting biotechnological and biological appli cations have been reported with Gquartetbased hydrogels. Dash et al. used Ag + to formulate hydrogels that enable the incorporation of cytochrome c into GMPbased gels 120 . Zhao and colleagues described the preparation of an antimicrobial hydrogel (active against Gramnegative bacteria) by incorporating 2′deoxy2′ fluoroguanosine ( 19) 121 (Fig. 9a). The controlled release of bioactive compounds from Gquartetbased hydrogels has been described as a method for drug delivery. For example, the release of vitamin C and the Gcontaining drugs acyclovir and ganciclovir has been achieved using hydrogels based on guanosine5′hydrazide 122 ,

Storage moduli

indications of the ability of the gel to withstand and store deformation energy in an elastic manner, being dependent on its crosslinking state.

Thixotropy

property of a material (gel, fluid), which is thick or viscous under static conditions, to become thin and liquid when shaken or sheared. guanosine/bromoguanosine (20) (1/1) mixture 123 or 5′deoxy5′iodoguanosine (21) 124 . In the latter case, an intramolecular cyclization that forms 5′deoxyN3,5′cy cloguanosine (22) leads to gel breakdown, triggering the release of acyclovir (>80% release after 3 days) that had previously been incorporated into the stacked Gquartet assemblies through its guanine moiety (Fig. 9d). Sadler and colleagues further exploited this approach, incor porating a guanosine-borate Pt(IV) complex ( 23) into a guanosine-borate hydrogel to exploit the photo activatable anticancer properties of platinum 125 . Under visiblelight irradiation (λ ex = 465 nm), the photo reduction of Pt(IV) to cytotoxic Pt(II) combined with the release of azidyl radicals (Fig. 9e) triggers higher cytotoxicity in cisplatinresistant A2780cis human ovarian cancer cells (IC 50 = 3 µM) than cisplatin and chlorpromazine (IC 50 > 100 µM and 6 µM, respectively), with a better selectivity towards normal MRC5 human fibroblast cells (IC 50 > 50 µM). Rowan and colleagues described the application of hydrogels derived from 8methoxy2′,3′,5′triOacetyl guanosine (24) as a scaffold for tissue engineering. C166 endothelial cells were incubated in cell culture medium DMEM/24 mixtures (between 0 wt% and 2 wt%), the addition of gelatin (1 wt%) providing a tridimen sional cogel within which cells can be incorporated, as observed by confocal microscopy 126 . The choice of benzene1,4boronic acid instead of simple boronic acid to form a guanosine-borate hydrogel provides an efficient support for human dermal fibroblast growth with cell viability >73% when formulated with both K + (triggering the Gquartet formation) and mag nesium (crosslinking the anionic borate moieties ions) 127 . Altogether, these studies confirm the versatility of Gquartetbased soft matter and highlight their high potential as biomaterials.

Nature reviews | Chemistry

Applications in chemical biology

As it may have become apparent from the discussion above, increased understanding and popularity of Gquartets as supramolecular building blocks has also increased our understanding of these structures in their natural setting -within DNA and RNA quadruplexes. The prevalence of G4s in the human genome, transcrip tome and ncRNAome (G4ome) is now firmly demon strated thanks to the recent and spectacular development of sequencing technologies 128 (Fig. 1): G4seq identified >716,000 potential DNA quadruplexforming sequences (QFS) in the human genome 129 and G4 ChIPseq decreased this number to 10,000 G4 sites, because of the topological restriction imposed by the chromatin environment 130 ; rG4seq identified >13,000 QFS in the human transcriptome (polyAenriched RNA) 131 , while G4RPseq extended the analysis to the whole transcrip tome and ncRNAome, demonstrating that G4s are per vasive structures in functional human cells 132 . Therefore, the prevalence of G4s and their established functional relevance [START_REF] Hänsel-Hertsch | DNA G-quadruplexes in the human genome: detection, functions and therapeutic potential[END_REF][START_REF] Kwok | Detecting RNA G-quadruplexes (rG4s) in the transcriptome[END_REF] have made them new stars in genetics and promising targets for therapeutic interventions. Specially dedicated G4ligands [133][134][135] , such as quarfloxin 136 , BRACO19 (reF. 137 ), PhenDC 138 and pyridostatin 139 , were developed in the hope of gaining control over the many cellular processes in which G4s are involved. Most, if not all, programmes that have attempted to identify promising new G4ligands have chemical scaffolds primarily developed as duplex DNA intercalators (acri dine, quinoline, etc.). This approach is somewhat sur prising, given that binding to duplex DNA is the major unwanted interaction that would need to be avoided for a G4ligand to be considered promising. That said, suc cessful strategies have been devised that avoid duplex interaction, such as using ligands with large to very large aromatic cores to maximize overlap with the external Gquartet of a G4 structure and avoid duplex inter calation. Another approach that has emerged -as a result of the many structural studies on quadruplexes -is the use of synthetic Gquartets as bioinspired G4ligands. This approach builds on the observation that the stability of quadruplex structures is frequently related to the number of constitutive Gquartets, and there is a likelikeslike recognition between natural and synthetic Gquartets (Fig. 10).

In order to circumvent the challenging kinetics associated with the assembly of four isolated guanines, the first biomimetic ligands developed were tetrameric TASQ, such as the neutral DOTASQ 140,141 (Fig. 7). Further work led to the development of the first smart quad ruplex ligands, which assemble only in the presence of their native Gquadruplex targets, for example, the cat ionic PNA DOTASQ 142,143 (Fig. 10a). PyroTASQ has been described as a 'twiceassmart' quadruplex ligand 144,145 (Fig. 10a), referring to the fact that it is both a smart quadruplex ligand and a smart, turnon fluorescent probe. The exquisite selectivity of these ligands for their targets is a result of a unique G4triggered intra molecular rearrangement of the ligand. TASQ ligands have been instrumental in the detection and identifica tion of DNA/RNA quadruplexes in human cells using optical imaging (for example, NTASQ was used as a probe to directly visualize DNA/RNA quadruplexes in human cells; Fig. 10a,b) 146,147 and sequencing studies (BioTASQ was used for the pull down of RNA quad ruplexes containing sequences from living human cells, enabling their identification through to the G4RPseq protocol (G4RNAspecific precipitation followed by sequencing); Fig. 10a,c) 132,148 . Together, these studies have both confirmed the existence of quadruplexes in functional human cells and provided tools to assess the ligandinduced changes in the quadruplex landscape upon therapeutic treatments.

Intermolecular SQ have also been used as bio mimetic ligands. Rivera and colleagues reported on the use of quartets selfassembled from the 8aryl2′ deoxyguanosine derivative (25) and showed that this cationic guanine spontaneously folds into K + promoted or Na + promoted hexadecamers, which are subse quently sandwiched between two quadruplex units [G4((25) 4 K + (25) 4 K + (25) 4 K + (25) 4 )G4] 149 (Fig. 10d). These unusual, nanometrelong structures, whose stability is a result of extended networks of πstacking and Coulombic interactions, describe a new possible G4ligand/quadru plex binding mode and make 25 SQ the first example of a biomimetic, selfassembled ligand. Dash and colleagues also reported on the use of Gmonomers and Gdimers (dansyl monoguanosine and dansyl diguanosine) as Gquadruplexinteracting agents 150 . It is thought that these agents are groove binders (rather than themselves forming a quartet structure), but they were found to inter act efficiently with the quadruplex found in the promoter region of the Myc oncogene (a goldstandard target for anticancer strategies) and capable of downregulating its expression in cancer cells.

Conclusions

Guanine was, is and will remain a fascinating study sub ject and likely has many functionalities that are yet to be discovered. Described above are just some of the applica tions that have been developed in the past two decades, and highlight an apparent disconnect between the sim plicity of the guanine chemical scaffold and the diversity of fields in which guanines and their derivatives provide practical solutions. The careful selection of derivatives (guanine itself, guanosine, deoxyguanosine, GMP, etc.), combined with suitable chemical modifications, has enabled the assembly of a toolbox that appears to be virtually unlimited. Chemical modifications mean that guanine is useful in a variety of experimental conditions (organic or aqueous environments) and responsive to external stimuli (cation, pH, light and temperature), which translates into programmable supramolecular properties. However, it is also immediately apparent that we are only beginning to assess the functional diversity of guanine in its entirety and to harness its supramolecular properties in a fully controllable manner.

To move up a gear, we believe that this field will have to shortly meet the following milestones.

Better predictions. Efforts must be invested to design Gbased systems that will strictly obey predefined rules in terms of topological pluralism and stability. We need to understand and harness these rules with the help of in silico models to predict and encode the properties of Gbased devices in the very structure of their basic guanine units.

Improved chemical strategies. The commercial accessi bility of guanine derivatives sustains the use of Gbased molecular tools in various research fields. However, the many examples discussed above highlight that only exquisitely tailored guanines provide devices with controllable properties. The relevance of a Gbased mol ecular toolbox is, to date, far too dependent on hightech chemistry. This might prevent academiatoindustry transitions in the short term.

Alternative chemical approaches. The supramolecular nature of this field of research must be further exploited, notably through alternative chemical strategies, such as dynamic combinatorial chemistry. This approach will undoubtedly offer alternative solutions to technical conundrums and improve the modularity and diversity of Gbased devices.

New fields of applications. Beyond the aforemen tioned examples, more applications might benefit from Gbased materials, from personalized medicine (for example, the use of Grich aptamers as therapeutics) to climatechange issues (for example, Gpolymers har nessing solar energy). This will require the input of sci entists from diverse research domains, making this field a resolutely multidisciplinary research area.

Validated biotech prototypes. Important challenges will have to be met in terms of scalability and cost to translate this research into the industrial arena. The advent of suc cessful applications in the next years will help take the last few steps towards longawaited demonstration of the industrial significance of Gbased molecular devices.

Hammarsten initially described guanine as a "curious nucleic acid" [START_REF] Hammarsten | Zur Kenntniss der Nucleoproteide[END_REF] ; we would wager that he was very far from thinking that, 125 years later, scientific communities from different origins (supramolecular chemistry, poly mer, soft matter, bioelectronics, catalysis and chemical biology) would become curious about this nucleic acid. One thing is certain: no one is more adventurous than the scientist who tries to follow guanine on its way across various chemical lands, along a route whose trajectory is as yet unknown. It is a fascinating, transdisciplinary chemical adventure in a yet undiscovered country from which no wanderer returns.

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Fig. 1 |

 1 Fig. 1 | timeline of the major steps in the discovery of DNA and G-quartets.From the first observation by Pasteur and Virchow on cell division (discarding the notion of 'spontaneous generation'), the history of DNA is marked by many important breakthroughs (grey boxes), chief among them the isolation of DNA ('nuclein') by Miescher 156 , the demonstration of its role as a repository of genetic information by Avery 157 , the elucidation of its duplex structure by Watson and Crick 2 and the central dogma of biology heralded by Crick 158 . Our understanding of G-quartets has developed alongside, while also being fully part of the history of DNA , with its own important steps (blue boxes): the isolation of guanylic acid byHammarsten 6 , the description of guanylic hydrogel byLevene 7 and Bang[START_REF] Bang | Untersuchungen über die Guanylsäure[END_REF] , the elucidation of the guanosine (G)-quartet structure by Davies 10 , the first G-based devices by Davis[START_REF] Davis | Self-assembled ionophores from isoguanosine[END_REF] and Gottarelli[START_REF] Gottarelli | Self-assembly in organic solvents of a deoxyguanosine derivative induced by alkali metal picrates[END_REF] , and the demonstration of the existence of G-quartet-based suprastructures referred to as G-quadruplexes in human cells by Balasubramanian 159,160 .

Fig. 2 |

 2 Fig.2| LipoGs and their higher-order supramolecular assemblies. a | Monomeric lipophilic guanosines (LipoGs) investigated by Davis (1)[START_REF] Davis | Self-assembled ionophores from isoguanosine[END_REF] , Gottarelli (2)[START_REF] Gottarelli | Self-assembly in organic solvents of a deoxyguanosine derivative induced by alkali metal picrates[END_REF] , Rivera (3)[START_REF] Gubala | Expanding the Hoogsteen edge of 2′-deoxyguanosine: consequences for G-quadruplex formation[END_REF] , Spada (4)[START_REF] Lena | Triggering of guanosine self-assembly by light[END_REF] and Meijer (5)[START_REF] González-Rodríguez | G-quadruplex self-assembly regulated by Coulombic interactions[END_REF] used as models to gain insight into the parameters that govern G-quartet formation. b, c | LipoGs (2, 3) were used to demonstrate the capacity of K + -promoted G-quartets to adopt higher-order suprastructures, such as octamers and hexamers. c, d | A variety of strategies were implemented to gain control over G-quartet topological pluralism (tetramer, octamer, hexadecamer), in a reversible manner, using either metallo-responsive or light-responsive LipoGs. SQ, synthetic G-quartets.

  c-di-GMP3,3′-cyclic diguanylic acid discovered by Benziman and colleagues in 1987 (reF.151 ), originally as a regulator of cellulose production in bacteria and now known as a regulator of virulence factor production and biofilm formation.

Fig. 4 |

 4 Fig. 4 | structural studies of water-soluble G-assemblies. a | Monomeric guanosine monophosphate (GMP) can self-assemble in discrete G-quartets in the presence of Na + and K + , but the latter can also trigger the formation of micrometre-long, helical rod-like structures comprising continuous, open-ended G-quartet 36 . b | The dimeric GMP c-di-GMP can form G-quadruplexes comprising four c-di-GMP molecules upon interaction

Fig. 5 |

 5 Fig.5| Applications of G-quartet-based devices as ionophores and ion channels. a | Artificial ion pair (NaCl) receptor that results from the dimerization of 1,3-alt-calix[4]arene-templated guanosine (G)-tetramer (8)[START_REF] Kotch | Water-mediated association provides an ion pair receptor[END_REF] . b | A covalently crosslinked quadruplex, comprising 16 units of the G-monomer (9) 62 linked via olefin metathesis, acts as a Na + transporter through inclusion in a liposome membrane. c | The lipophilic, bis-lithocholamide-based G-dimer (10)[START_REF] Ma | Large and stable transmembrane pores from guanosine-bile acid conjugates[END_REF] self-associates to form synthetic G-quartets (SQ)-based nanopores that allow for K + transportation through phospholipid bilayers.

12 Fig. 6 |

 126 Fig. 6 | the nanotechnological applications of synthetic G-quartets-based nanostructures. a | Decorated lipophilic guanosines self-assemble into octamers with enhanced properties, i.e. enhanced fluorescence for(11) [START_REF] González-Rodríguez | Persistent, well-defined, monodisperse, π-conjugated organic nanoparticles via G-quadruplex self-assembly[END_REF] and charge separation under photoexcitation for guanine perylene-3,4,9,10-bis(dicarboximide) (GPDI)[START_REF] Wu | Extending photoinduced charge separation lifetimes by using supramolecular design: guanine-perylenediimide G-quadruplex[END_REF] . b | Gold nanoparticle assembly triggers by(12) [START_REF] Li | G-quartet-induced nanoparticle assembly[END_REF] , which provides micrometre-long architectures with increased plasmon coupling. c | Guanosine monophosphate (GMP)-based nanowires that act as light-harvesting antenna upon association with fluorescence resonance energy transfer (FRET) dyes triplets[START_REF] Pu | G-quartetbased nanostructure for mimicking light-harvesting antenna[END_REF] , i.e. thioflavin T (ThT), thiazole orange (TO) and pyronin Y (PY). d | G-quadruplex organic frameworks that produce long-lived mobile charges upon photoexcitation, making them prototypes of a G-quartet-based Li-ion battery[START_REF] Wu | G-quadruplex organic frameworks[END_REF] . OPV, oligo(p-phenylene vinylene); PDI, perylene-3,4,9,10-bis (dicarboximide); SQ, synthetic G-quartets.

Fig. 7 |

 7 Fig.7| DNAzyme-type catalysis performed by G-quartet molecular tools. Catalytic guanosine (G)-quartets comprising either proflavine-promoted c-di-GMP octamer[START_REF] Nakayama | A C-di-GMP-proflavine-hemin supramolecular complex has peroxidase activity-implication for a simple colorimetric detection[END_REF] or the intramolecularly folded DOTASQ[START_REF] Stefan | Harnessing nature's insights: synthetic small molecules with peroxidase-mimicking DNAzyme properties[END_REF] upon interaction with hemin, which makes the resulting G-quartet/hemin complex catalytically competent to oxidize the chromogenic 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of H 2 O 2 (as the stoichiometric oxidant), in a DNAzyme-type manner[START_REF] Mergny | DNA quadruple helices in nanotechnology[END_REF] . SQ, synthetic G-quartets.

Fig. 8 |

 8 Fig. 8 | soft matter based on G-quartet molecular devices. a | Amphiphilic guanosine (G; 2) that self-assembles into a lyotropic liquid crystal triggered by a K + -promoted synthetic G-quartets (SQ) formation 96 . b | Cation-free SQ formed by adamantyl-conjugated guanines (13) 99 , whose SQ/ribbon topological transition is controlled by the concentration. c | Dynamic hydrogel (or dynamers) formed by hydrazide-based guanines (14) 100 and controlled by cryptand [2.2.2] and sequential acid/base addition. d | Water-assisted polymerization of siloxane-conjugated guanines (15) 103 to form covalent crystalline nanorods.

Fig. 9 |

 9 Fig. 9 | supramolecular hydrogels based on G-quartet assemblies. a | Guanine derivatives (16) 109 , (17) 115 , (19) 121 , (20) 123 and (24) 126 used as building blocks for hydrogels. b | Guanosine-borate-based hydrogels used for incorporating a wide variety of chemicals 112 . c | Dynamic self-selection of aldehydes from (18) that provides the most stable gel118 . d | Following gel formation, the cyclization of (21) to(22) triggers gel weakening and collapse, allowing the controlled release of acyclovir incorporated into the stacked guanosine (G)-quartet assemblies124 . e | Biological application of a platinum-G-quartet-based hydrogel exhibiting photoactivatable anticancer properties 125 .

Fig. 10 |

 10 Fig. 10 | the chemical biology applications of sQ. a | Chemical structure of several template-assembled synthetic guanosine (G)-quartets (TASQ) used as biomimetic G-quadruplex ligands, i.e. PNA DOTASQ 142 and PyroTASQ 144 , N-TASQ 146,147 and BioTASQ 132,148 . b | Schematic representation of the detection of quadruplexes in human cells using the twice-as-smart N-TASQ ligand. c | Schematic representation of the use of BioTASQ as molecular bait to fish quadruplexes out of human cells prior to being identified by sequencing (G4RP protocol). ** and *** represent the statistical analysis (T test) of the results, p-values (with ** p < 0.01 and *** p < 0.001)132 . d | Self-assembled G4-ligand resulting from the multimerization of (25) 149 that is subsequently sandwiched between two quadruplex units.

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Acknowledgements

L.S. and D.M. thank the Centre National de la Recherche Scientifique (CNRS) for funding. D.M. thanks the Agence Nationale de la Recherche (ANR-17-CE17-0010-01), the European Research Council (H2020-MSCA-IF-2016-750368), the Université de Bourgogne, Conseil Régional de Bourgogne (PARI) and the European Union (Pharmaco-imagerie et agents théranostiques, PO FEDER-FSE Bourgogne 2014-2020 programme) for financial support. The authors also thank their collaborators for the daily passionate scientific discussions and all scientists worldwide involved in the fascinating field of research, to make it lively, thrilling and always moving.

Author contributions

All authors contributed equally to the preparation of this manuscript.

Competing interests

The authors declare no competing interests.

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