Beat H Meier

Roland Riek
email: roland.riek@phys.chem.ethz.ch

Anja Böckmann
email: a.bockmann@ibcp.fr

Emerging structural understanding of amyloid fibrils by solid state NMR

Keywords: amyloid fibrils, structure, NMR, amyloid-β

Amyloid structures at atomic resolution remained elusive for a long time, which was mainly due to the extensive polymorphism they present, as well as to their polymer properties hampering structural studies by classical approaches. Progress in sample preparation, as well as solid-state NMR methods, recently enabled the determination of high-resolution 3D structures of fibrils, as for example amyloid-β involved in Alzheimer's disease. Notably, the simultaneous but independent structure determination of Aβ1-42 by two independent laboratories with virtually identical results has highlighted how structures can be obtained on which further functional investigation can be based.

Amyloid (see glossary) fibrils are a product of self-assembly of proteins. Some of them are related to diseases, most prominently amyloid-β (see glossary) and tau to Alzheimer's disease (see glossary) {Glenner:1984uw, Masters:1985wb} but also play a role as functional amyloids {Maji:2009hv}. Their first design as soluble protein is to serve a determined function, which is not always known. Fibrils present interesting objects from a structural biology point of view. While it is well known that proteins can adopt different threedimensional (3D) structures in the context of their function by differences in the local environment such as pH, salt, membranes or ligand binding, amyloidogenic peptides and proteins undergo a change in secondary, tertiary and quaternary structure through aggregation triggered only by a concentration change of the amyloid protein.

Resisting classical structure-determination approaches, such aggregates were long assumed to all adopt a generic fold consisting of two layers of inter-molecular β-sheets {Dobson:2003el, Petkova:2002dr} or variations like a serpentine {Kajava:2004da} with monomers stacking on top of each other to form the amyloid fibril. However, it has recently become apparent that protein fibrils sample a comparable conformational space as globular and membrane proteins, and include complex combinations of β-strands in the case of amyloids, and also α-helices for fibrils formed in other contexts {Sborgi:2015iv, Dvir:2009fu, TayebFligelman:2017kv}. Structural insight into the fold of fibrils was delayed due to the nature of these objects. Protein fibrils do not normally show long-range order; they are often bent and flexible along their axis (see Figure 1), and resist thus to atomic resolution structural studies by x-ray diffraction. With the advent of solid-state NMR (see glossary) as a structural technique for (biological) solids, structures of protein fibrils at atomic resolution became accessible, and diverse folds of these objects were revealed. While solid-state NMR allows structural studies of large objects lacking long-range order, local order is still a prerequisite, and first studies by this technique were hampered by suboptimal sample preparations, as well as the inherent propensity of many fibrils to form structurally distinct polymorphs (see glossary), which are difficult to separate. The establishment of rigorous production and purification techniques, as well as seeding (see glossary) approaches, allowed recently to acquire high-resolution NMR spectra of protein fibrils adequate for the assignment of the NMR resonances. The measurement of structural restrains then allowed the determination of the 3D structures. We review here the recently determined amyloid structures with a focus on amyloid-β fibrils involved in Alzheimer's disease.

Amyloid structures have local order but limited long-range order

Multimeric protein superstructures can result in highly ordered entities (certain virus capsids and envelopes, ferritins, bacterial needles etc.), but can also form less ordered assemblies, which for example bend and curve irregularly, as is the case for most protein fibrils. Protein fibrils indeed show a quasi one-dimensional growth, and if the fibrils can consist of several protofibrils, only few specific lateral interactions between entire fibrils exist, even if fibrils often bundle, but likely via non-specific contacts. These features prevent in most cases atomic-resolution structural studies by established techniques, as x-ray crystallography, and complicate electron microscopy, as the absence of long-range order (e.g. exact translational symmetry) prevents diffraction and electron-density reconstruction to be used with their full potential. Despite these elements of randomness, other types of long-range, like a constant cross section and a constant supertwist are often observable. It should also be noted that for peptides fibrils consisting of smaller peptides, crystalline objects could be grown, and their structure has been determined {Sawaya:2007em} and that quite high resolution cryoelectron microscopy on Aβ1-42 has been obtained recently {Schmidt:2015df}. Besides the technical challenges introduced in the preceding paragraph, the ability to prepare samples showing a single polymorph presented a major breakthrough for structural studies. Polymorphism is pervasive in many fibrils, and leads typically to the observation of several signals per single residue in the solid-state NMR spectra because the conformations are distinct in the different polymorphs, and chemical shifts respond in a particularly sensitive manner to differences in dihedral angles. Exceptions can be fibrils with more than one symmetrically inequivalent molecules in the fibril {Fitzpatrick:2013do} where multiple peaks can be observed per atom even in the absence of polymorphism, or polymorphs with almost identical local conformation but a different supertwist {Seuring:2017hg} or quaternary structure {Fitzpatrick:2013do} that can not be distinguished by NMR because they have a close-to-identical local structure. Both exceptions are not applicable for the cases discussed here. In the presence of many polymorphs, the NMR spectra become therefore crowded and their analysis becomes difficult due to the strong overlap of signals.

Still, for some fibrils, only a single polymorph has been observed under physiological conditions, as for example in HET-s(218-289) (see glossary) {Wasmer:2008gc, vanMelckebeke:2010kz}. In many other cases polymorphism is often observed {Gath:2014ff, Walti:2016fn, Petkova:2005eh, Lu:2013bb} and pure protein preparation and the tight control of fibrillization conditions or seeding techniques are key to obtain a homogeneous sample. In this context it is interesting to note that peptide preparations using synthesizers typically results in less homogeneous fibrils than from recombinant sources {Finder:2010jw} and that preparations from recombinant sources without any addition of amino acids to the native sequence are considered the most relevant. Also, reproducible conditions are central for NMR studies, as several different samples have to be prepared for structural studies {Lu:2013bb, vanMelckebeke:2010kz}. Reproducibility is best guaranteed by tight control of the physico-chemical conditions including salt content, metal ions, buffer composition, temperature and pH or/and by seeding fresh material with a "mother" fibril sample.

Once a sample condition with a single polymorph is established, it is unclear whether the selected polymorph is relevant in the context of disease. A correlation between patientderived material and the NMR sample is therefore necessary by using conformation-specific antibodies {Walti:2016fn}, spectroscopic signatures {Anonymous:2007ju} or by showing biological activity such as transmissibility potential {Tuttle:2016hl}. Alternatively, seeding the stable isotope labeled monomeric material using brain homogenates from deceased patients may result in disease-relevant structural studies {Lu:2013bb, Qiang:2017ek}. Still their relevance remains difficult to establish since other forms could be formed during seeding and NMR sample preparation (see also below). Within the context of the various polymorphisms observed in vitro, it is interesting to note that also in vivo different strains and structural polymorphisms have been observed with distinct biological properties, e.g. The resulting 3D structures represented by a bundle of conformers then deliver an atomic picture of the protein fibrils (see Figure 4C). In NMR, the resolution of the structure depends on the number of distance restraints (see glossary). In the A𝛽 structures discussed here, 500-1000 restraints were used. Not only the number but also the quality of the restrains is of importance. The more the restraints are distributed over the entire structure, the more reliable the structure will be. If structures can be solved with a lower number of restraints, it often remained unclear which parts were well defined without showing a representative bundle of structures. This has resulted in the past in models which approximate nature was difficult to assess to the outsider.

Q15-G37 V39-H13 F4-A30 L34-G37 L17-G37 E3-G29 E3-G29 F4-G29 L34-G37 M35 M35 M35 G33 K28-E3 Q15-V39 A30-F4 V40 G29-E3 K28-E3 K28-E3 G38

Atomic-resolution 3D structures yield insight into fibril architecture

The two structures of Aβ1-42, as well as the structure of the Osaka mutant (see glossary) Aβ1-40E22Δ are shown in Figure 4. One can see that the structures are rather complex and distinct from the simple β-strand-turn-β-strand fold suggested initially. Both represent dimeric folds, in line with the mass-per-length determined by STEM, with the backbones of the Osaka mutant Aβ1-40E22Δ being highly intertwined, and Aβ1-42 showing less intermolecular contacts. In both structures, the hydrophobic residues are mainly located in the inside of the protein, as also observed in globular proteins. The polar and charged residues point mainly to the outside. If charged residues point to the inside, their charge is compensated as for example in the salt bridges between residues H6, E11, H13 and V40 in the Osaka mutant Aβ1-40E22Δ. In Aβ1-42, no charged residues are found in the interior of the protein fibril. The most fundamental difference between the two structures lies in the attachment of the N-terminal domain to the fibrillar core. Indeed, the N-terminus is attached via a salt bridge between residues D3 and K28 in the Osaka mutant Aβ1-40E22Δ, whereas in Aβ1-42 it shows β-strand structure, but remains at least partially flexible, as no extensive network of restraints can be observed between the N-terminus and the amyloid core in Aβ1-42. In both structures, glycine residues are used for turns, which allows the backbone to easily adopt a kringel-or horseshoe-like structure. In both constructs, V36 represents the center of the structure and is heavily involved in intermolecular contacts. We have speculated with respect to early disease onset that the familial mutations in the loop around residue E22 result in an exposure or not of K28 to the outside, so it can interact with the N-terminal residues {Schledorn:2015ds}. This attachment of the N-terminal may present a hallmark for early-onset, but these hypotheses remain to be explored experimentally. In Aβ1-42, K28 is indeed also oriented toward the exterior of the fibril, but is interacting with the carboxy-terminal of A42. It will be interesting to investigate the Osaka mutant E22Δ of Aβ1-42.

An important success of the solid-state NMR method was that a second, independent NMR structure determination carried out in parallel for essentially the same polymorph of Aβ1-42 (as revealed by highly similar chemical shifts {Ravotti:2016fv}) yielded a highly similar structure (albeit the construct used has a methionine residue at position 0 {Walsh:2009kg}).

The small differences in the structure and chemical shifts observed might be attributed to this additional methionine residue.

Interactions with amyloid dyes

The available structures of amyloid fibrils open an avenue towards the study of amyloidmarker interactions and their structure-based rational design. Recent solid-state NMR studies (both by ligand-induced chemical shift perturbations as well as ligand-amyloid intermolecular polarization transfers) identified the binding mode of the well known amyloid dye

Concluding Remarks

The availability of atomic-resolution structures opens the doors for a molecular-level understanding of amyloid function and the role of amyloids in disease. Markers (e.g. for imaging) can be developed as well as drugs that specifically interact with defined fibrils and regulate their growth and cell-to-cell propagation. Still the experiments are time-consuming and request in-vitro preparation of in the order of 10 milligram of fibril samples. The combination of NMR under fast sample rotation -which reduces sample amount requirements by more than an order of magnitude {Agarwal:2014jr} -and information from cryo electron microscopy may remove some of these limitations.

Trends Box

• Complete atomic-resolution structures of disease-related amyloid fibril have become available, in particular for amyloid β. They are based on solid-state NMR restraints combined with electron-microscopic mass-per-length data.

• There is mounting evidence that functional amyloids play an important role in biology and medicine

• Amyloids show a higher structural complexity than previously assumed and this opens an avenue for rational drug design. Drugs need to be selective to not interfere with functional proteins in amyloid form.

• Alzheimer's disease: neurodegenerative disease that impairs memory and usually starts slowly but worsens over time.

Solid-state NMR: Solid-state NMR addresses materials which are not soluble, but which can be hydrated. All material with very long rotational correlation times are considered in this context as solid.

Polymorphism: Protein monomers can assemble into different fibrils, which are of comparable stability, and thus present energetically almost equal alternative structures.

Minor variations in the environmental conditions determine which polymorph is produced in vitro.

Seeding: seeding is used to prepare homogenous samples which contain only one polymorph, by propagating the most stable fibril form through several generations of seeds.

HET-s(218-289):

The prion domain of the fungal prion HET-s involved in heterokaryon compatibility. It was the first amyloid to have its 3D structure determined.

Isotope labeling: Biomolecular NMR studies are generally carried out on 13 C and 15 N labeled molecules, since these isotopes show a spin-1/2. In order to label the proteins, they are produced in the presence of isotope-labeled precursors, often 13 C-glucose and 15 Nammonium chloride.

STEM mass-per-length: scanning transmission electron microscopy can determine the molecular mass per unit length of the fibril. From this quantity, the number of monomers per 4.7 Å repeat unit can be determines

Chemical shift:

The exact resonance frequency of a nuclear spin is determined by its electronic environment. The difference between the frequency of a given nuclei with the one of a reference compound (for proteins typically DSS) is called the chemical shift and is typically expressed in ppm (parts-per-million) of the resonance frequency.

Figure 1 :

 1 Figure 1: EM pictures (negative stain) of two different protein fibrils, HET-s(218-289) (A) and Aβ1-42 (B), scale bars indicate 200 nm.

  different toxicity and transmissibility, and one may thus argue that the various polymorphs represent different strains of a disease {Bousset:2013im,Qiang:2017ek, Prusiner:2012fp, Soto:2012hc}.

Figure 2 :

 2 Figure 2: [ 13 C, 13 C]-correlation spectra of different polymorphic mixtures of Aβ1-42 fibrils. The black spectrum in (A) is from a sample which shows a broad distribution with at least 6 polymorphs, while the red spectrum (in A and B) is from a sample with a single polymorph and the blue spectrum (shown in (B)) is from a sample with several polymorphs. The number of polymorphs can best be seen in the serine region of the spectra shown in (C) and are marked by an arrow in the red extract. This region shows a single cross peak between the 13 Cα and the 13 Cβ atoms for each serine residue. Since Aβ1-42 has two serine residues only two cross peaks are expected as found for the red spectrum attributed to a fairly pure single polymorph (2 serines), while the blue sample contains the red polymorph and, in addition, a second one, and the black spectrum at least three polymorphs (i.e. 3*2 serine

Figure 3 :

 3 Figure 3: Osaka mutant Aβ1-40E22Δ spectra. (A,B) Extracts from 2D PAR (see glossary) spectra recorded on fully labeled (black) and diluted (green) samples. Intermolecular cross peaks can be identified by the signal attenuation in the green spectra. (C) 1D PAR traces extracted at the Ile32 Cδ1-resonance at 14.8 ppm where intramolecular crosspeaks retain

  13 C (ppm) full intensity. The signal labeled in blue denotes an ambiguous intramolecular peak not used in the calculation but confirmed by the structure. (D) 1D trace from PAIN (see glossary) spectrum from mixed 15 N and 13 C sample (orange), compared to an NCA spectrum recorded on a fully labeled 15 N and 13 C sample in black. The trace is taken at the Q15 N𝜀2 frequency (108.86 ppm). Resonances where intermolecular cross peaks (orange) are much stronger are due to lateral intermolecular contacts (Q15-G37, Q15-G38), peaks with similar intensity may be contributions from the adjacent layer or 13 C natural abundance intramolecular contributions. Spectrally unambiguous peaks are labeled in (A-D), and are shown in (E) (with the color code red for intermolecular peaks, black for intramolecular and blue for ambiguous with respect to intra/inter) on the backbone structure of Aβ1-40E22Δ with two monomers in yellow and violet. Figure adapted from {Schutz:2015gi}.

Figure 4 :

 4 Figure 4: Structures of amyloid-β: (A) Osaka mutant Aβ1-40E22Δ, view perpendicular to fiber direction, white balls denote hydrophobic residues, green balls polar (including glycine), red and blue balls negatively and positively charged residues, respectively, (B) Aβ 1-42 from the "red" polymorph of Fig. 2 (from ref. {Walti:2016fn}); only residues 15-42 are shown (in yellow the N-terminal tail is added on one monomer). (C) Bundle of 10 best conformers of (B) of residues 15-42, (D) Overlay of the structures obtained by Wälti et al. {Walti:2016fn} (blue) and Colvin et al. {Colvin:2016jx} (red) from samples formed under similar conditions showing a very similar chemical-shift fingerprint. The sample used by Colvin contains an additional methionine residue at the N-terminal end of the amino-acid sequence (residue 0).

Figure 5 :

 5 Figure5{Herrmann:2015en}. Using the ligand-induced chemical-shift perturbations in combination with ligand-amyloid polarization transfers the structure of the complex was modeled and the binding mode revealed (Fig.5). The spacing of the negative charges for different polythiophenes depends on the polymer backbone, and can be adapted. For compounds where the negative charges are spaced by 9 Å, the binding is observed for every monomer of HET-s which forms two layers in this amyloid. Polythiophenes with a spacing of 4.5 Å have been designed as well, and can bind to sites where every layer of the amyloid displays the same positive charge. Charge alone, however, does not seem sufficient for a high binding affinity; a groove running along the fibril in HET-s(218-289) near the Lys binding site seems to play a role in interactions mediated via the neighboring backbone atoms.

•••

  How large is the conformational space available for the peptides and proteins forming fibrils. Is there a hierarchy of polymorphism with mayor forms distinguishing themselves in the position of the secondary structural elements {Gath:2014ff} and minor variations, e.g. only with a different twist of the fibrils which are formed from quasi-identical monomers {Seuring:2017hg}. • What is the relevance of fibrils and fibrillar tangles in the context of amyloidrelated disease? Are they the primary cause or a consequence of the disease? Is it consequently desired to dissolve or to stabilize fibrils for therapy? Fibrillar tangles often consist of several proteins. Are they polymorphically uniform for a given protein, e.g. amyloid-β? • What is the medical and therapeutic importance of molecular polymorphism: are different polymorphs responsible for different strains of the disease that distinguish themselves by different clinical symptoms and request different therapy and medication. What is the biological role of oligomeric aggregates either on the pathway to fibrils or off this pathway? It is known that fibril formation proceeds between cells following typical pathways {Brundin:2010hz}. Is prion-like propagation of the fibril structure between cells the relevant mechanism? Is the polymorph conserved? and early fibril studies.{Wasmer:2008gc, vanMelckebeke:2010kz} • Recently, proton detection under fast MAS (>60 kHz) has shown its usefulness.{Huber:2012dr, BarbetMassin:2014er, Bockmann:2015da} Their use in fibrils structure determination has to be demonstrated yet. Its main advantages are lower demands on the sample amount and advantages for dynamics studies.However, the enhanced risk for rotor crashes may raise issues with biosafety for some fibrils due to the production of aerosols. {Bousset:2015cp, Brundin:2010hz} Glossary Amyloid: protein aggregates which fold such that many copies of that protein come together and form fibrils. Historically, amyloids were defined by their reactions with dyes like Congo red or thioflavin-T. Biophysically an amyloid is defined by a core with a cross-β spine with a 4.7 Å periodicity {Westermark:2007cb}. amyloid-β: peptides of 36-43 amino acids, stemming from the cleavage of the amyloidprecursor protein, that are central in Alzheimer's disease as the main component of the amyloid plaques found in the brains of patients.

Acknowledgements:

This work was supported by the ETH Zurich, the Swiss National Science Foundation (Grant 200020_159707 and 200020_146757), the French ANR (ANR-12-BS08-0013-01, ANR-14-CE09-0024B).

References:

Dipolar couplings: Spins are magnetic dipoles that interact with each other through the dipolar coupling, equivalent to the interaction of two bar magnets. The dipolar coupling strength depends on the inverse 3 rd power of the internuclear distance.

PAR:

A radiofrequency pulse sequence which allows the recording of spectra displaying longrange distance restraints between 13 C atoms.

PAIN: Following the same principles as PAR, this pulse sequence allows the measurement of distances between 15 N and 13 C atoms.

Distance restraints: NMR structure calculation proceeds via the measurement of distances which are then used in structure calculation as restraints in order to calculate, via triangulation and dedicated programs, the 3D structure of a protein compatible with as many distances (restraints) measured as possible.

Osaka mutant: A deletion mutant of Aβ lacking E22. The Osaka mutant Aβ1-40E22Δ is one of the Aβ familial mutants which cause early-age onset of the disease.


K Johannes
email: kevyn.johannes@cstb.fr

X Faure

A Bontemps

D Quenard

USE OF PCM IN GLASS BLOCK WALLS FOR SUMMER THERMAL COMFORT

Keywords: simulation, experimentation, phase change materials, energy storage Nomenclature Cp Mass heat capacity, J.kg -1 .K -1 Greek symbols T Temperature, °C ? Flux density, W.m -² Tf Fusion temperature, °C E Hz Horizontal radiation, W.m -² Indexes and exponents L Latent heat, J.kg -1

Summer thermal comfort is often a problem encountered in internal insulation buildings with low inertia. To lessen these problems, Phase Change Materials (PCM) can be used and introduced in walls. Some studies have already been carried out on this subject introducing PCM in concrete wall, in plaster board, in light internal components… conclusive that it is a great solution to improve building inertia. In this paper, experiments on a glass block wall containing PCM are carried out. The experimental setup is described and results are discussed for three MCP. These experimental results are used to assess a one-dimensional model developed in TRNSYS, which is used to study how phase change materials affect model houses thermal performances. Some differences are observed between experimental and numerical results and are explained by several reasons.

INTRODUCTION

Thermal comfort in residential building is essential as regards an occupant. However, most of the buildings which have been built during the last thirty years in France suffer from overheating in summer, generally compensated by the use of air-conditioning which entails energy over consumption. The current context aims to reduce the energy needs without threaten the thermal comfort level (RT2005). Consequently, it is important to find solutions. Different scientific studies have shown that inertia building control could have an undeniable impact on thermal comfort while limiting the energy resort. In 1980, a study realised on isotherm partition wall integrated into the building showed that the replacement of storage classical materials by 'chliarolithe' -a phase change material -led to improve the performance of the systems; in other words, led to an increase of energy savings, a comfort improvement by temperatures stabilization, while reducing the material mass necessary, until a factor of six [START_REF] Bourdeau | Etude de l'intégration à l'habitat d'éléments de paroi isothermes[END_REF]. Later, different researchers worked on the PCM impregnation in plaster boards or other construction materials [START_REF] Salyer | Advanced phase change materials for passive solar storage applications[END_REF][START_REF] Shapiro | PCM thermal storage in drywall using organic phase change material[END_REF][START_REF] Babich | DSC studies of energy storage materials. Part 3. Thermal and flammability studies[END_REF]Banu & al., 1998, Tyagi and Buddhi;[START_REF] Xu | Modeling and simulation on the thermal performance of shape-stabilized phase change material floor used in passive solar buildings[END_REF] in order to store or keep out stock the energy coming from solar radiation and internal contributions. Recently, [START_REF] Cabeza | Use of microencapsulated PCM in concrete walls for energy savings, Energy and Buildings[END_REF] has studied on the introduction of a microencapsulated PCM in concrete walls; temperature fusion is 26 °C for a latent heat of 110 kJ.kg -1 . The experimentation which takes place at Lleida in Spain, and consisted of two trial cells, explains that PCM allows a reduction of inside and surface temperatures. Thanks to this experiment, the author explains that the microencapsulation introduction in concrete does not deteriorate mechanical properties of this one. As regards [START_REF] Lai | How phase change materials affect thermal performance: hollow bricks[END_REF], he has researched the influence of a PCM when it's integrated in earthenware bricks situated outside on flat roof. Results are also positive. On the one hand, PCM allows to reduce the temperature on the brick surface -inside-by 4,5 °C compared to a no treated brick. But, on the other hand, this study does not deal with the temperature evolution inside the room. [START_REF] Ahmad | Nouveaux composants actifs pour la gestion énergétique de l'enveloppe légère des bâtiments. Couplage matériaux à changement de phase, super-isolation, apports solaires[END_REF]Ahmad [ , 2006] ] has worked on inertia of 1 m 3 trial cells. She has characterized intern inertia brought by paraffin, a fatty acid and an hydrated salt, showing again the interest of such a coupling with classical materials of building (outside insulation). Thus, our study proposed is based on this principle of trial cell. PCM addition is realised in a glass brick division, which is assimilated with a dividing wall for two cells; process which can be extended to partition wall for rooms or offices… The experimental device and achieved results will be presented in this paper, showing different PCM influence on thermal inertia.

EXPERIMENTAL SET UP

Cells structure

The experimental set up is consisted of two double cells facing south: the reference and the PCM one. Every cell is composed of two juxtaposed cells and separated by their common wall made of glass bricks and filled or not with PCM (figure 1). The separated wall is consisted of a sixteen glass bricks assembling, the whole rigidified by an aluminium frame (figure 2). Every cell of the double one is also composed of a double glazing facing south and of four light weight walls made of a steel sheet metal 75/100 e , 20 mm of vacuum insulation panel (VIP) and a steel sheet metal 75/100 e . Three phase change materials are studied: a fatty acid, a paraffin and an hydrated salt whose fusion temperatures announced by manufacturers are respectively 21 °C, 25°C and 27,5 °C.

Cells instrumentation and measurements

Ten fluxmeters, beforehand calibrated, are set in the middle of every insulation wall and a fluxmeter is put on both sides of the glass block partition wall in the middle. Every fluxmeter presents a temperature sensor. An additional thermocouple is set in the middle of every cell to measure the inside temperature. Cells are located outside; consequently they undergo climatic variations which symbolize boundary conditions for numerical simulation. Weather parameters are recorded thanks on the experimentation site. An autonomous acquisition station set in a tight box with an alimentation choice: battery or mains, allows to acquire flux sensors exits, to digitise, treat them and finally to safeguard the results. In our case, data are directly collected thanks to an interface network between the measurement station and a micro-computer fits out with a specific software of communication (PC208W Datalogger support software).

Experimental results

During the period of measures, three divisions in glass bricks are trailed with the three PCM selected. Figures 3 and4 present the different temperatures evolution according to time, and the vertical solar radiation part crossing the double glazing. Every period of measures, characterizing a different PCM, shows an inside temperature reducing according to the reference case, reduction between 3°C and 5°C according to the PCM state -completely melt or not. On figure 3, between may 25 th and 26 th , for close outside conditions, the division temperature which was initially 28 °C reached 35 °C, showing that PCM is entirely liquid May 26 th . This day, the temperature variation was 4°C between the reference cell and the cell with PCM. The PCM influence on inside temperature evolution is difficult to assess because boundary conditions are different. Moreover, experimental results show temperature inside the cells is very high. This phenomenon is due to important solar contributions. So, to characterize PCM influence on cell thermal comfort with more precision, we have to reduce solar contributions and we can possibly overventilate during night but we must especially submit PCM to the same boundary conditions. Numerical modelization can also answer questions and that is the reason why the followed study deals with the development of an one-dimensional model in TRNSYS environment and presents at last a comparison with experimental results.

Numerical model description

Studies on simple cells [START_REF] Ahmad | Nouveaux composants actifs pour la gestion énergétique de l'enveloppe légère des bâtiments. Couplage matériaux à changement de phase, super-isolation, apports solaires[END_REF] realised were carried out thanks to a 3D model developed by Josikalo [ 1999]. Although achieved results are close to real phenomena, simulation time is always important (from 2 to 3 hours to symbolise one year). Thus, we have decided to develop an one-dimensional model in TRNSYS environment in order to reduce calculation time. The model assesses thermal behaviour of a wall consisted of 7 layers maximum: 3 externs layers on both sides of a phase change material. The multi-layers wall is modelled by a resistance/capacity approach (figure 5). Every material layer, except PCM one, is modelled by a 3R4C network whereas the capacity number in the PCM layer has to be defined. (100 maximum). Connection between every material layer is taken into account by a predefined contact resistance. Boundary conditions (Fourier conditions) are ambient temperature and radiation one of every wall side. An incident flux can also be chosen on both sides of the wall. As regards PCM change of state modelization, we can think about three possible solutions: Kondo's model [2000] on the approximation of Cp graph by straight lines, a square model or Cp real characteristics measured.

T rm1 Finally, this model was two studies main topic, which allow to assess coherence between achieved results. First of all, it was the subject of a Benchmark [START_REF] Imcpbat | Rapport intermédiaire du projet PREBAT[END_REF] assessing a simple wall with PCM or no. This Benchmark explains that a single wall model gives good results. Then, [START_REF] Faure | RC Network approach for Hybrid envelops in Positive Energy Building[END_REF] show the model gets coherent results when we assess by numerical simulation, the phase change material contribution on summer thermal behaviour of a 40 m 3 room. Consequently, the next stage of this analysis consists in comparing results achieved by model with the experimental ones described in the experimental part of this study.

Φ 1 Couche 1 à 3 3R4C Couche PCM Couche 5 à 7 3R4C Cpcm 1 Cpcm i R contact R contact T side2 T rm2 T side1 Φ 2
T f -2 T f -4,5 T f T f +0,5 C p [J/kg .K] T [°C] Mass heat Capacity Latent heat

Comparison between numerical and experimental results

The dividing wall model was coupled with the 'Type 56 building' available in TRNSYS to simulate the double cell. A first comparison whose results are not described in this paper shows the model is faithful to the double cells behaviour when the dividing wall doesn't present any PCM. Results showed in figure 7 take into account PCM in glass wall. Boundary conditions used during the simulation are real data measured in-situ -outside temperature (T ext ) and horizontal sunning presented on the last picture announced. It's interesting to notice that inside air temperature with PCM (T PCM-sim ) presents the same dynamics as experimental air temperature (T PCM-exp ). However, one phase difference can be noticed and entail problems of inertia and phase change phenomenon representation. Several points can explain this issue: The first one is linked to the simulation which has been realised with Kondo's model; because PCM experimental values were not available. Fusion temperature and heat latent used are data fixed by manufacturer thanks a canning calorimetric measure at the rate of 3 °C/min. But, measures realised thanks to micro differential scaling calorimetry process (figure 8 cycle close to reality at the rate of 0,3 °C/min) revealed fusion temperature given by the manufacturer is not really the one measured. Moreover, this noting was highlighted by an AIE study [2005]. So, you have to be careful when you decide to measure mass thermal capacity evolution and you must take the closest measures context as possible to real entreaties conditions. Moreover, surfusion problem doesn't appear in the model. The second point is linked to the fact that experimentally, the fusion front material is not homogeneous. Figure 9 shows phase change material is liquefied in the middle of the brick, on the edges sometimes, explaining that simulation hypothesis on an homogeneous fusion front is not valid in case of glass bricks. Finally, the solid phase migration towards the bottom of the glass bricks highlights natural convection phenomena which increase heat transport from one side to the other one. With surfusion, observed "smoothing" of the phase change phenomenon is not n umerically represented.

CONCLUSION

The experimental aspect of this study allows to highlight several points. First of all, it's important to notice the phase change materials impact on inside temperature damping, reducing the air-conditioning use. Besides, we have shown that our experimental set up should be improved in order to characterize PCM. So, it's important to reduce solar contributions, to ventilate during the night and especially to compare PCM influence with equal boundary conditions. Moreover, an important point was raised concerning the measure of mass thermal capacity. It must be realised in the same conditions as its real load otherwise important differences could appear, especially on the fusion crest segment. On a numerical point of view, we have developed an one-dimensional model which will take into account, later, the surfusion problem, and the real characteristics measured at a slow rate. In spite of that, the model gives coherent results. Then, it will allow to characterize phase change materials with equal boundary conditions, to assess these materials impact at a building scale, to study their site, the maximum thickness and other criteria…PCM conditioning in glass bricks needs numerical models more precise, which take into account migration phenomena and natural convection.

Figure 1 :

 1 Figure 1: Twin cells separated by a glass bricks wall containing or not PCM Figure 2: Glass bricks wall containing PCM

Figure 3 :Figure 4 :

 34 Figure 3: Thermal behaviour comparison between reference cell and the PCM one (fatty acid)

Figure 5 :

 5 Figure 5: Wall RC network with a phase change material

Figure 6 :

 6 Figure6: Specific heat evolution depending on temperature according toKondo & al.

Figure 7 :

 7 Figure 7: Comparison between inside temperatures with and without PCM for a paraffin

Figure 8 :

 8 Figure 8: PCM specific heat evolution depending on solicitation speed Figure 9: No homogeneous PCM fusion in glass block stones.

ACKNOWLEDGEMENTS

The authors would like to thanks the French Environmental Agency (ADEME) for it financial support as part of PREBAT program.