Synthetic fragments and analogues of elastin. I. The synthesis

Conformational Study and Hydrogen Bonds Detection on Elastin-Related Polypeptides Using X-ray Photoelectron Spectroscopy

The chemical bonds of the pentapeptide sequence of elastin ValGlyGlyValGly (VGGVG), both in its monomer and polymer forms, were correlated with their XPS spectra through a well-established curve-fitting procedure. To aid in this correlation, the C1s, O1s, and N1s chemical shifts of the Boc-VGGVG-OEt, were validated by theoretical calculations, performed in the framework of the Koopman approximation of HF/6-31G molecular orbitals, leading to the "preferred" conformation of the protected monomer. Then the same curve-fitting procedure was adopted for interpreting the XPS spectra of the polypentapeptide as a powder, and the XPS results obtained both for monomer and polymer compounds were compared with those obtained by FT-IR. The polymer was then analyzed after deposition onto a silicon substrate, Si(100), either from methanol or water suspensions and the presence of hydrogen bonds was detected at the polymer/substrate interface and between the polymer chains. The "surface rearrangement" that could be inferred from XPS results strongly confirms that derived from AFM images previously obtained under the same experimental conditions. In particular, the observed amyloid conformation is stabilized by hydrogen bonds to water molecules included in the structure while the formation of the beaded string structure observed in deposits from methanolic suspension is probably mediated by hydrogen bonds to the hydrated silicon surface.

AFM Study of the Elastin-like Biopolymer Poly(ValGlyGlyValGly)

In this paper, we report an AFM study on the supramolecular structures adopted by the synthetic polypentapeptide poly(ValGlyGlyValGly), whose monomeric sequence is an abundant, simple building block of elastin. The polypeptide was analyzed by deposition from both methanolic and aqueous suspensions, showing different behaviors. In methanol, the polypeptide is able to evolve, in a time-dependent way, from layers to ribbons to beaded filaments. When the equilibrium is reached, the formation of well-defined dendritic structures is also observed. This restructuring of the polypentapeptide seems to be reminiscent of a sort of Rayleigh instability. When deposited from aqueous suspensions, the polypeptide self-assembles either in fibrillar networks or in amyloid-like patterns, both of them being found in elastin or elastin-related polypeptides. As a general finding, poly(ValGlyGlyValGly) seems to constitute an excellent mimetic of the supramolecular properties of native elastin.

Elastin-based biopolymers: chemical synthesis and structural characterization of linear and cross-linked poly(OrnGlyGlyOrnGly)

Poly(OrnGlyGlyOrnGly) was synthesized by classical procedures in solution. The monomeric sequence -OrnGlyGlyOrnGly- was chosen as a modification of -ValGlyGlyValGly-, typical of elastin, to impart primary amine functionality, susceptible to cross-linking with appropriate bifunctional reagents. Herein we focus on the cross-linking of poly(OrnGlyGlyOrnGly) with glutaraldehyde. The polymers, both linear and cross-linked, were characterized and investigated for their molecular and supramolecular properties. Circular dichroism studies performed on linear poly(OrnGlyGlyOrnGly) revealed a variety of conformations similar to elastin. At a supramolecular level, different kinds of aggregates were found such as the elastin-like twisted-rope pattern of filaments and fibrils, together with other specific morphologies, similar to those recently identified in some elastin-mimetic polypeptides.

Chemical synthesis of cross-linked poly(KGGVG), an elastin-like biopolymer

Previous studies afforded on peptides and polypeptides containing repetitive sequences of elastin have largely demonstrated that their molecular and supramolecular properties are fully representative of those of tropoelastin, the soluble, linear precursor of elastin itself. In the attempt to synthesize cross-linked elastin-mimetic polypeptides, the repeating sequence VGGVG (V: valine; G: glycine), typical of elastin, was modified to incorporate lysine residues, yielding the polymer poly(KGGVG) (K: lysine). This imparts primary amine functionality susceptible to cross-linking reaction with appropriate bifunctional cross-linking reagents. We report herein the chemical synthesis and cross-linking of poly(KGGVG) with glutaraldehyde (GTA) and with disuccinimidyl glutarate (DSG). In both cases, the characterization of the polymers, both linear and cross-linked, has been carried out by CD spectroscopy and transmission electron microscopy measurements. The obtained results, although not conclusive, demonstrate that poly(KGGVG), both linear and cross-linked, may be considered very similar to tropoelastin and mature elastin, as concerns its molecular and supramolecular properties.

Synthesis and structural characterization of poly(LGGVG), an elastin-like polypeptide

Poly(LGGVG) a potential elastin-like biomaterial has been synthesized and studied both in solution (by circular dicroism and nuclear magnetic resonance) and in the aggregated state (by transmission electron microscopy). For sake of comparison, also the conformation of the protected (Boc-LGGVG-OEt) and free (H(2)(+)-LGGVG-OH) 'monomers' has been investigated. While in the latter ones the presence has been evidenced of more or less stable type II beta-turns, the polymer showed a conformational ensemble, possibly comprising type II beta-turns, type I beta-turns and open (unordered) structures. At supramolecular level, twisted-rope aggregates were observed by transmission electron microscopy for the polymer. Thus, the title compound has shown to possess, at both molecular and supramolecular level, physico-chemical properties very similar to those of elastin, so to give some confidence that it could really constitute the precursor of an artificial substitute of elastin itself.

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.

Solid-state studies on synthetic fragments and analogues of elastin

A series of synthetic fragments and analogues of elastin have been investigated, in the solid state, by means of differential scanning calorimetry and thermally stimulated current. Most of the polypeptides were shown to possess both amorphous regions and segments of long-range order. Water, which interacts preferentially with the amorphous zones, behaves as plasticizer, i.e. facilitates the localized motions of polypeptide chains. The results obtained have been correlated with elastin elasticity, in particular as far as the fundamental destructuring role of water is concerned.

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.

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

The conformational behavior of the synthetic peptide, Boc-Gly-Val-Gly-Gly-Leu-OMe, containing the X-Gly-Gly and Gly-Gly-X (X = Val or Leu) repeating sequences and constituting a fragment of elastin was investigated by molecular mechanics and molecular dynamics (MD) simulation. The results suggest that, irrespective of the approximations used, the molecule shows a manifold of low energy conformations characterized by gamma-turns and type II beta-turns. Furthermore, MD simulations point out a conformational floppiness due to very low barriers between different conformations. Experimental CD measurements in a virtually apolar medium (dioxane--epsilon = 2.209), which better mimics the vacuum conditions of the simulation, support the theoretical results. The general emerging picture, indicating the molecule as characterized by a combination of flexibility with conformational preferences, is in agreement with previous experimental findings and enriches of new aspects the description of the microscopic behavior of this molecule suggesting more detailed interpretation of previous data.

Phase transitions and chain dynamics, in the solid state, of a pentapeptide sequence of elastins

Differential scanning calorimetry (d.s.c.) and thermally stimulated current (t.s.c.) have been applied to the study of thermal transitions and dielectric relaxations of a pentapeptide sequence: Gly-Leu-Gly-Gly-Val of elastin. The manifestation of the glass transition has been observed by both techniques. The analysis of the fine structure of t.s.c. spectra reveals the existence of local order in the amorphous phase upon physical ageing. In the 'true' amorphous phase, cooperative motions of sequences of various length are observed. The corresponding activation parameters are characteristic of the 'structure' of the amorphous phase and might be used as reference for further studies.

Polypeptide models of elastin: CD and NMR studies on synthetic poly(X-Gly-Gly)

Poly(X-Gly-Gly), simple structural models for the hydrophobic, proline-devoid, regions of elastin, have been synthesized and studied by circular dichroism and NMR spectroscopies. The results gave evidence of type II beta-turns as the only ordered structure present in the polymers. The stability of the turns has been shown to decrease on hydration and to increase in the series Leu less than Ala less than Val less than Ile.

Spectroscopic studies on elastin-like synthetic polypeptides

Spectroscopic studies on synthetic polypeptides containing the unit-X-G-G (X=V or L) are reported. The sequences, constituting either fragments or model of elastin, were shown to adopt type II beta-turns together with an ensemble of unordered conformations. Furthermore, it was found that the stability of the beta-turns was depending on the nature of the X residue, on the hydration of the chain and, in the case of the sequence G-V-G-G-L, was decreasing by increasing the length of the chain.