Molecular dynamics study of the conformational behavior of a representative elastin building block: Boc-Gly-Val-Gly-Gly-Leu-OMe

Structural and physical basis for the elasticity of elastin

Elastin function is to endow vertebrate tissues with elasticity so that they can adapt to local mechanical constraints. The hydrophobicity and insolubility of the mature elastin polymer have hampered studies of its molecular organisation and structure-elasticity relationships. Nevertheless, a growing number of studies from a broad range of disciplines have provided invaluable insights, and several structural models of elastin have been proposed. However, many questions remain regarding how the primary sequence of elastin (and the soluble precursor tropoelastin) governs the molecular structure, its organisation into a polymeric network, and the mechanical properties of the resulting material. The elasticity of elastin is known to be largely entropic in origin, a property that is understood to arise from both its disordered molecular structure and its hydrophobic character. Despite a high degree of hydrophobicity, elastin does not form compact, water-excluding domains and remains highly disordered. However, elastin contains both stable and labile secondary structure elements. Current models of elastin structure and function are drawn from data collected on tropoelastin and on elastin-like peptides (ELPs) but at the tissue level, elasticity is only achieved after polymerisation of the mature elastin. In tissues, the reticulation of tropoelastin chains in water defines the polymer elastin that bears elasticity. Similarly, ELPs require polymerisation to become elastic. There is considerable interest in elastin especially in the biomaterials and cosmetic fields where ELPs are widely used. This review aims to provide an up-to-date survey of/perspective on current knowledge about the interplay between elastin structure, solvation, and entropic elasticity.

Application of Thermoresponsive Intrinsically Disordered Protein Polymers in Nanostructured and Microstructured Materials

Modulation of inter- and intramolecular interactions between bioinspired designer molecules can be harnessed for developing functional structures that mimic the complex hierarchical organization of multicomponent assemblies observed in nature. Furthermore, such multistimuli-responsive molecules offer orthogonal tunability for generating versatile multifunctional platforms via independent biochemical and biophysical cues. In this review, the remarkable physicochemical and mechanical properties of genetically engineered protein polymers derived from intrinsically disordered proteins, specifically elastin and resilin, are discussed. This review highlights emerging technologies which use them as building blocks in the fabrication of highly programmable structured biomaterials for applications in delivery of biotherapeutic cargo and regenerative medicine.

Structure-Property Relationships of Elastin-like Polypeptides: A Review of Experimental and Computational Studies

Elastin is a structural protein with outstanding mechanical properties (e.g., elasticity and resilience) and biologically relevant functions (e.g., triggering responses like cell adhesion or chemotaxis). It is formed from its precursor tropoelastin, a 60-72 kDa water-soluble and temperature-responsive protein that coacervates at physiological temperature, undergoing a phenomenon termed lower critical solution temperature (LCST). Inspired by this behavior, many scientists and engineers are developing recombinantly produced elastin-inspired biopolymers, usually termed elastin-like polypeptides (ELPs). These ELPs are generally comprised of repetitive motifs with the sequence VPGXG, which corresponds to repeats of a small part of the tropoelastin sequence, X being any amino acid except proline. ELPs display LCST and mechanical properties similar to tropoelastin, which renders them promising candidates for the development of elastic and stimuli-responsive protein-based materials. Unveiling the structure-property relationships of ELPs can aid in the development of these materials by establishing the connections between the ELP amino acid sequence and the macroscopic properties of the materials. Here we present a review of the structure-property relationships of ELPs and ELP-based materials, with a focus on LCST and mechanical properties and how experimental and computational studies have aided in their understanding.

Elastic fibers: formation, function, and fate during aging and disease

Elastic fibers are extracellular components of higher vertebrates and confer elasticity and resilience to numerous tissues and organs such as large blood vessels, lungs, and skin. Their formation and maturation take place in a complex multistage process called elastogenesis. It requires interactions between very different proteins but also other molecules and leads to the deposition and crosslinking of elastin's precursor on a scaffold of fibrillin-rich microfibrils. Mature fibers are exceptionally resistant to most influences and, under healthy conditions, retain their biomechanical function over the life of the organism. However, due to their longevity, they accumulate damages during aging. These are caused by proteolytic degradation, formation of advanced glycation end products, calcification, oxidative damage, aspartic acid racemization, lipid accumulation, carbamylation, and mechanical fatigue. The resulting changes can lead to diminution or complete loss of elastic fiber function and ultimately affect morbidity and mortality. Particularly, the production of elastokines has been clearly shown to influence several life-threatening diseases. Moreover, the structure, distribution, and abundance of elastic fibers are directly or indirectly influenced by a variety of inherited pathological conditions, which mainly affect organs and tissues such as skin, lungs, or the cardiovascular system. A distinction can be made between microfibril-related inherited diseases that are the result of mutations in diverse microfibril genes and indirectly affect elastogenesis, and elastinopathies that are linked to changes in the elastin gene. This review gives an overview on the formation, structure, and function of elastic fibers and their fate over the human lifespan in health and disease.

Coarse-grained model of tropoelastin self-assembly into nascent fibrils

Elastin is the dominant building block of elastic fibers that impart structural integrity and elasticity to a range of important tissues, including the lungs, blood vessels, and skin. The elastic fiber assembly process begins with a coacervation stage where tropoelastin monomers reversibly self-assemble into coacervate aggregates that consist of multiple molecules. In this paper, an atomistically based coarse-grained model of tropoelastin assembly is developed. Using the previously determined atomistic structure of tropoelastin, the precursor molecule to elastic fibers, as the basis for coarse-graining, the atomistic model is mapped to a MARTINI-based coarse-grained framework to account for chemical details of protein-protein interactions, coupled to an elastic network model to stabilize the structure. We find that self-assembly of monomers generates up to ∼70 ​nm of dense aggregates that are distinct at different temperatures, displaying high temperature sensitivity. Resulting assembled structures exhibit a combination of fibrillar and globular substructures within the bulk aggregates. The results suggest that the coalescence of tropoelastin assemblies into higher order structures may be reinforced in the initial stages of coacervation by directed assembly, supporting the experimentally observed presence of heterogeneous cross-linking. Self-assembly of tropoelastin is driven by interactions of specific hydrophobic domains and the reordering of water molecules in the system. Domain pair orientation analysis throughout the self-assembly process at different temperatures suggests coacervation is a driving force to orient domains for heterogeneous downstream cross-linking. The model provides a framework to characterize macromolecular self-assembly for elastin, and the formulation could easily be adapted to similar assembly systems.

The liquid structure of elastin

The protein elastin imparts extensibility, elastic recoil, and resilience to tissues including arterial walls, skin, lung alveoli, and the uterus. Elastin and elastin-like peptides are hydrophobic, disordered, and undergo liquid-liquid phase separation upon self-assembly. Despite extensive study, the structure of elastin remains controversial. We use molecular dynamics simulations on a massive scale to elucidate the structural ensemble of aggregated elastin-like peptides. Consistent with the entropic nature of elastic recoil, the aggregated state is stabilized by the hydrophobic effect. However, self-assembly does not entail formation of a hydrophobic core. The polypeptide backbone forms transient, sparse hydrogen-bonded turns and remains significantly hydrated even as self-assembly triples the extent of non-polar side chain contacts. Individual chains in the assembly approach a maximally-disordered, melt-like state which may be called the liquid state of proteins. These findings resolve long-standing controversies regarding elastin structure and function and afford insight into the phase separation of disordered proteins.

Elastin-like polypeptides as models of intrinsically disordered proteins

Elastin-like polypeptides (ELPs) are a class of stimuli-responsive biopolymers inspired by the intrinsically disordered domains of tropoelastin that are composed of repeats of the VPGXG pentapeptide motif, where X is a "guest residue". They undergo a reversible, thermally triggered lower critical solution temperature (LCST) phase transition, which has been utilized for a variety of applications including protein purification, affinity capture, immunoassays, and drug delivery. ELPs have been extensively studied as protein polymers and as biomaterials, but their relationship to other disordered proteins has heretofore not been established. The biophysical properties of ELPs that lend them their unique material behavior are similar to the properties of many intrinsically disordered proteins (IDP). Their low sequence complexity, phase behavior, and elastic properties make them an interesting "minimal" artificial IDP, and the study of ELPs can hence provide insights into the behavior of other more complex IDPs. Motivated by this emerging realization of the similarities between ELPs and IDPs, this review discusses the biophysical properties of ELPs, their biomedical utility, and their relationship to other disordered polypeptide sequences.

Tuning self-assembly in elastin-derived peptides

Elastin-derived peptides are gaining increasing interest as potential biomaterials. Previous studies have demonstrated that short elastin-derived peptides are able to self-assemble into fibrils as the entire elastin protein. The motif responsible for that is the XGGZG motif at least three-fold repeated. In this work we have synthesized and studied, at molecular and supramolecular levels, four pentadecapeptides obtained by switching the X and Z residue with leucine and/or valine. We found that the four peptides formed different supramolecular structures corresponding to specific molecular conformations. Our results show that not only the residue type but also the exact position occupied by the residue in the motif is crucial in driving the self-aggregation. The aim of this work is to provide the basis for designing elastin-derived peptides with tunable supramolecular architecture.

Role of polyproline II conformation in human tropoelastin structure

In this review, we present a comprehensive overview of the molecular studies on human tropoelastin domains accomplished by Tamburro and co-workers in the last decade. The used approach is the reductionist approach applied to human tropoelastin and is based on the observation that the tropoelastin gene exhibits a cassette-like organization, with a regular alternation of cross-linking and hydrophobic domains putatively responsible for the elasticity of the protein. The peculiar structure of human tropoelastin gene prompted us to study the isolated domains encoded by the exons of tropoelastin, with the perspective to get deep insights into the structural properties of the whole protein. At the molecular level, the results clearly evidence large flexibility of the polypeptide chains in the hydrophobic domains, which oscillate between rather extended and folded conformations. An important role was assigned to poly-proline II conformation considered as the hinge structure in the dynamic conformational equilibrium suggested for the hydrophobic domains. For the lysine-rich cross-linking domains, the structural studies exactly localized α-helix along the polypeptide sequence. Furthermore, at supramolecular level, these studies showed that several domains are able to self-assemble in two different aggregation patterns, the fibrous elastin-like structure for some proline-rich hydrophobic domains and the amyloid-like for some glycine-rich hydrophobic domains. Accordingly, the studies suggest that the reductionist approach was a valid tool for studying a complex protein, such as elastin, elucidating not only the structure but also the specific role played by its constituent domains.

On the inverse temperature transition and development of an entropic elastomeric force of the elastin mimetic peptide [LGGVG](3, 7)

We report on a molecular dynamics simulation based study of the thermal and mechanical properties of the elastin mimetic peptide [LGGVG](n) (n = 3, 7). Our findings indicate that this peptide undergoes an inverse temperature transition as the temperature is raised from ~20 °C to 42 °C. The thermal behavior is similar to what has been observed in other well studied short mimetic peptides of elastin. Both [LGGVG](n) (n = 3, 7) peptides exhibit an increase in the number of side chain contacts and peptide-peptide hydrogen bonds when the temperature is raised from ~20 °C to 42 °C. These observations are accompanied by a decrease in the number of proximal water molecules and number of peptide-water hydrogen bonds. This work also reports on a comparison of the thermal and mechanical properties of [LGGVG](3) and [VPGVG](3) and quantifies the interaction with surrounding waters of hydration under mechanically strained conditions. It is demonstrated, via a quasi-harmonic approach, that both model peptides exhibit a reduction in the population of low-frequency modes and an increase in population of high-frequency modes upon elongation. The shift in population of frequency modes causes the peptide entropy to decrease upon elongation and is responsible for the development of an entropic force that gives rise to elasticity. These observations are in disagreement with a previously published notion that model elastin peptides, such as [VPGVG](18), increase in entropy upon elongation.

Modular design in natural and biomimetic soft materials

Under eons of evolutionary and environmental pressure, biological systems have developed strong and lightweight peptide-based polymeric materials by using the 20 naturally occurring amino acids as principal monomeric units. These materials outperform their man-made counterparts in the following ways: 1) multifunctionality/tunability, 2) adaptability/stimuli-responsiveness, 3) synthesis and processing under ambient and aqueous conditions, and 4) recyclability and biodegradability. The universal design strategy that affords these advanced properties involves "bottom-up" synthesis and modular, hierarchical organization both within and across multiple length-scales. The field of "biomimicry"-elucidating and co-opting nature's basic material design principles and molecular building blocks-is rapidly evolving. This Review describes what has been discovered about the structure and molecular mechanisms of natural polymeric materials, as well as the progress towards synthetic "mimics" of these remarkable systems.

Elastomeric polypeptide-based biomaterials

Elastomeric proteins are characterized by their large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Motivated by their unique mechanical properties, there has been tremendous research in understanding and manipulating elastomeric polypeptides, with most work conducted on the elastins but more recent work on an expanded set of polypeptide elastomers. Facilitated by biosynthetic strategies, it has been possible to manipulate the physical properties, conformation, and mechanical properties of these materials. Detailed understanding of the roles and organization of the natural structural proteins has permitted the design of elastomeric materials with engineered properties, and has thus expanded the scope of applications from elucidation of the mechanisms of elasticity to the development of advanced drug delivery systems and tissue engineering substrates.

A never-ending love story with elastin: a scientific autobiography

The author describes, in a quite unconventional way, the most important results achieved in the last 50 years in the field of elastin structure–elasticity relationships, beginning with the first invaluable findings of Partridge on desmosines and isodesmosines until the most recent theories on elastomeric proteins. The author also relates a scientific autobiography characterized by his greatest passion, elastin.

Kinetics and thermodynamics of type VIII beta-turn formation: a CD, NMR, and microsecond explicit molecular dynamics study of the GDNP tetrapeptide

We report an experimental and theoretical study on type VIII beta-turn using a designed peptide of sequence GDNP. CD and NMR studies reveal that this peptide exists in equilibrium between type VIII beta-turn and extended conformations. Extensive MD simulations give a description of the free energy landscape of the peptide in which we retrieve the same two main conformations suggested by the experiments. The free energy difference between the two conformational states is very small and the transition between them occurs within a few kT at 300 K on a nanosecond timescale. The equilibrium is mainly driven by entropic contribution, which favors extended conformations over beta-turns. This confirms other theoretical studies showing that beta-turns are marginally stable in water solution because of the larger entropy of the extended state unless some stabilizing interactions exist. Our observations may be extended to any type of beta-turn and have important consequences for protein folding. A comparison of our MD and CD results also suggests a possible type VIII beta-turn CD signature indicated by one main band at 200 nm, close to that of random coil, and a fairly large shoulder at 220 nm. Last, our results clearly show that the XXXP motif can only fold into a type VIII beta-turn, which is consistent with its fairly strong propensity for this type of turn. This important finding may help for peptide design and is in line with recent studies on bioactive elastin peptides.

Circular dichroism studies on repeating polypeptide sequences of abductin

The secondary structure of abductin was investigated by CD and NMR studies of several synthetic peptides. Results obtained with these peptides showed the dominant conformations to be the polyproline II (PPII) structure in aqueous solution and different types of beta-turns in the less polar solvent trifluoroethanol. Accordingly, a preliminary structure-elasticity relationship for abductin, not unlike that currently accepted for elastin, is proposed.

The dissection of human tropoelastin: from the molecular structure to the self-assembly to the elasticity mechanism

After a historical introduction the authors describe their most recent results on the structure, assembly and elasticity of elastin. Recent results obtained by analyzing the conformation of polypeptide sequences encoded by the single exons of human tropoelastin demonstrated the presence of labile conformations such as poly-proline II helix (PPII) and beta-turns whose stability is strongly dependent on the microenvironment. Stable, periodic structures, such as alpha-helices, are only present in the poly-alanine cross-linking domains. These findings give a strong experimental basis to the understanding of the molecular mechanism of elasticity of elastin. In particular, they strongly support the description of the native relaxed state of the protein in terms of trans-conformational equilibria between extended and folded structures as previously proposed [Int. J. Biochem. Cell. Biol. 31 (1999) 261]. The same polypeptide sequences have been analyzed for their ability to coacervate and to self-assembly. Although the great majority of them were shown to be able to adopt more or less organized structures, only a few were indeed able to coacervate. Studies carried out by transmission electron microscopy showed the polypeptides to adopt a variety of supramolecular structures going from a filamentous organization (typical of elastin) to amyloid-like fibers. On the whole, the results obtained gave significant insight to the roles played by specific polypeptide sequences in self-assembly and possibly in elasticity.

Structure Distribution in an Elastin-Mimetic Peptide (VPGVG)3 Investigated by Solid-State NMR

Elastin is an extracellular-matrix protein that imparts elasticity to tissues. We have used solid-state NMR to determine a number of distances and torsion angles in an elastin-mimetic peptide, (VPGVG)3, to understand the structural basis of elasticity. C-H and C-N distances between the V6 carbonyl and the V9 amide segment were measured using 13C-15N and 13C-1H rotational-echo double-resonance experiments. The results indicate the coexistence of two types of intramolecular distances: a third of the molecules have short C-H and C-N distances of 3.3 +/- 0.2 and 4.3 +/- 0.2 A, respectively, while the rest have longer distances of about 7 A. Complementing the distance constraints, we measured the (phi, psi ) torsion angles of the central pentameric unit using dipolar correlation NMR. The -angles of P7 and G8 are predominantly ~150, thus restricting the majority of the peptide to be extended. Combining all torsion angles measured for the five residues, the G8 C chemical shift, and the V6-V9 distances, we obtained a bimodal structure distribution for the PG residues in VPGVG. The minor form is a compact structure with a V6-V9 C=O-HN hydrogen bond and can be either a type II -turn or a previously unidentified turn with Pro (phi = -70, psi= 20 +/- 20) and Gly ( phi= -100 +/- 20, psi = -20 +/- 20). The major form is an extended and distorted beta-strand without a V6-V9 hydrogen bond and differs from the ideal parallel and antiparallel beta-strands. The other three residues in the VPGVG unit mainly adopt antiparallel beta-sheet torsion angles. Since (VPGVG)3 has the same 13C and 15N isotropic and anisotropic chemical shifts as the elastin-mimetic protein (VPGXG)n (X = V and K, n = 195), the observed conformational distribution around Pro and Gly sheds light on the molecular mechanism of elastin elasticity.

Elastin: molecular description and function

Elastin, the protein responsible for the elastic properties of vertebrate tissues, has been thought to be solely restricted to that role. As a consequence, elastin was conventionally described as an amorphous polymer. Recent results in the biomedical, biochemical and biophysical fields have lead to the conclusion that the presence of elastin in the extracellular space has very complex implications involving many other molecules. The present review describes the current state of knowledge concerning elastin as an elastic macromolecule. First, the genetic, biological, biochemical and biophysical processes leading to a functional polymer are described. Second, the elastic function of elastin is discussed. The controversy on elastin structure and elasticity is discussed and a novel dynamic mechanism of elasticity proposed. Finally, pathologies where the elastin molecule is involved are considered. This updated description of functional elastin provides the required background for the understanding of its pathologies and defines clearly the properties a substance should possess to be qualified as a good elastic biomaterial.

Structure and dynamics of elastin building blocks. Boc-LG-OEt, Boc-VGG-OH

Short di- and tripeptides such as Boc-LG-OEt, Boc-VG-OEt and Boc-VGG-OH, corresponding to abundant repetitive sequences in elastin, have been extensively studied both in solid state, by X-ray diffraction, and in solution by circular dicroism and nuclear magnetic resonance. Furthermore, theoretical procedures such as simulated annealing and molecular dynamics were also performed on these peptides. In general, the results indicate that no one single structure (be folded or extended) could be representative for these sequences in the protein, but rather that a multiplicity of interconverting conformers, ranging from folded to extended structures, should be considered. In any case, these structures, e.g. beta-turns, polyglycine II and beta-conformations, are those previously suggested to participate to conformational equilibria of elastin.

Swelling and viscoelastic properties of osmotically stressed elastin

The swelling and viscoelastic properties of purified elastin were studied in aqueous solutions of superswelling agents or osmotic deswelling agents to develop models to study the behavior of elastin at frequencies not easily accessible by direct measurement. Increasing the concentration of any of the deswelling solutes (glucose, sucrose, sodium chloride, ammonium sulphate, dextran, and polyethylene glycol) increased the tensile storage and loss moduli. The viscoelastic behavior was independent of solute when compared on the basis of swelling behavior. The data collected at various solute concentrations at 37 degrees C could be reduced to one master curve, and the master curves for elastin in each of the deswelling solutes were themselves superposable. The ability to reduce the data indicates that dehydration can be used to model elastin's viscoelastic behavior at high frequencies or over short times. The viscoelastic behavior of elastin in the superswelling agents [potassium thiocyanate (KSCN), dimethyl sulfoxide (DMSO), and ethylene glycol (EG)] depended on the solute and was independent of swelling behavior. In KSCN the behavior of elastin seemed to be a continuation of the pattern established by the deswelling agents in that an increase in swelling was accompanied by a decrease in both moduli, and the viscoelastic spectra were reducible to one master curve. In high concentrations of DMSO and EG the spectra were not reducible. KSCN appears a suitable superswelling solute to model elastin's viscoelastic behavior at low frequencies or over long times.

Conformational modeling of elastin tetrapeptide Boc-Gly-Leu-Gly-Gly-NMe by molecular dynamics simulations with improvements to the thermalization procedure

Molecular Dynamics Simulations (MD) at Constant-Temperature or Constant-total Energy for the conformational Global-Minimum (GM) of elastin tetrapeptide Boc-Gly-Leu-Gly-Gly-NMe have been performed. The thermalization problem concerning the initial state of Constant-Temperature MD has been solved developing two effective strategies. In the first one, the run starts from the room-temperature state reached by Molecular Dynamics Simulated Annealing (SA). In the second one, one starts from the annealed-state at low-temperature and performs a long constant-low-temperature run until the initial conformer is perfectly equilibrated. Then, the low-temperature equilibrated-state is used as initial state for MD at room-temperature. heuristic criteria on order to define the onset of steady-state have been established monitoring the hystories of collective parameters (e.g., the total energy, temperature, end-to-end distance, etc.) and their amplitude fluctuations. Moreover, the equilibrium between the system and the heat bath is verified analyzing the total linear momentum conservation by the time evolution of center mass velocity. The slow drift of total energy during Constant-total Energy MD has been corrected using a loose coupling between the system and the heat bath. Moreover, we have verified that the roto-translational motions do not affect significantly the properties of molecular vibrations. The librations of peptide unit inside the type II beta-turn [Gly1]C = 0 ... HN[Gly4], previously detected, were confirmed. Large -Gly-Gly- chain motions were identified and modeled as fluctuations occurring between the tetratepeptide GM and the saddle-point corresponding to the transition state of the conversion toward the extended-chain conformation. All these peptide motions could contribute to the elasticity mechanism of elastin.

Insect cuticular proteins

Insect cuticles are composite structural materials with mechanical properties optimal for their biological functions. The bulk properties of cuticles are to a large extent determined by the interactions between the various components, mainly the chitin filament system and the proteins. The various cuticular types show pronounced differences in mechanical properties, and it is suggested that these differences can be related to the properties of the individual proteins and to the degree of secondary stabilization (sclerotization). The amino acid sequences, which have been obtained for insect cuticular proteins either by direct sequencing of purified proteins or by deduction from corresponding DNA-sequences, are listed according to insect order and species. Extensive sequence similarity is observed among several cuticular proteins obtained from different insect orders. Other cuticular proteins are characterized by repeated occurrence of a few small motifs consisting mainly of hydrophobic residues. The latter group of proteins has so far only been reported from stiff cuticles. The possible relevance of the various motifs and repeats for protein interaction and the mechanical properties of cuticles is discussed.

The effects of heating on the mechanical properties of arterial elastin

Autoclaving is a standard way of purifying arterial elastin for mechanical testing, but recent evidence suggests that heating native elastin might affect its mechanical behavior. We therefore examined the quasi-static tensile properties of pig arterial tissue to see if the mechanical properties of native elastin are altered by autoclaving. From an analysis of the shapes of the stress-extension ratio curves of tissues before and after 8 h of autoclaving, we determined that the mechanical characteristics of elastin dominated the behavior of unautoclaved arterial tissue at wall stresses around 25+/-5 kPa. Autoclaving did not change the tangential modulus of the tissue at this wall stress (+/-4% 95% CI), indicating that elastin can be heated during purification without affecting its mechanical behavior. Autoclaved tissue was tested daily to determine the effects of prolonged heating of autoclaved elastin. Between tests the elastin was incubated at either 80 degrees C (experimental group) or 37 degrees C (control group). After 6 days the average modulus of the control group was unchanged from the initial value, while the average modulus of the experimental group was 7%+/-2% (95% CI) lower. At shorter times the modulus of the experimental group was not significantly reduced. The slight decrease in modulus suggests a slow chemical degradation may occur with prolonged heating, but its time course and magnitude are such that it would not affect standard mechanical tests.

Synthesis and structural studies of a pentapeptide sequence of elastin. Poly (Val-Gly-Gly-Leu-Gly)

Poly (Val-Gly-Gly-Leu-Gly), a polypeptide mimicking the physico-chemical properties of the glycine-rich regions of elastin, has been synthesized and studied both in solution and in the aggregated state. By comparison, also the conformation of different "monomeric" units has been investigated. The polymer showed increased disorder with respect to the "monomers", the molecular conformation being accounted for by a more or less random collection of isolated beta-turns. Nevertheless, in the solid state the polymer is able to adopt supramolecular structures reminiscent of those found for elastin.