Test of the librational entropy mechanism of elasticity of the polypentapeptide of elastin. Effect of introducing a methyl group at residue 5

Abstract

The polypentapeptide analogue, (Val₁-Pro₂-Gly₃-Val₄-Ala₅)_n, has been synthesized to test the proposed librational entropy mechanism of elasticity which is based on the polypentapeptide of elastin, (Val₁-Pro₂-Gly₃-Val₄-Gly₅)_n. The mechanism utilizes the motional capacities of the Val₄-Gly₅-Val₁ segment of the elastin polypentapeptide in a β-spiral conformation in which the segment is suspended between β-turns. Proton and carbon-13 nuclear magnetic resonance spectroscopies have been used to verify purity and to demonstrate that the Pro₂-Gly₃β-turn is retained in the Ala₅ analogue. In addition, the temperature profiles for aggregation are found to be similar for both polypentapeptides. Aggregation of the Ala₅ analogue, however, so immobilizes the polypeptide as to give no carbon-13 nuclear magnetic resonance spectrum, whereas a broadened spectrum is observed for the aggregated state of the polypentapeptide of elastin. Scanning electron micrographs are used to compare the aggregated states of the two polymers. On settling, the polypentapeptide of elastin forms a smooth-appearing sheet whereas the addition of a methyl group at residue 5 results in a granular precipitate. In contrast to the elastomeric polypentapeptide of elastin, the Ala₅ analogue, on cross-linking with γ-irradiation, simply fragments when stress/strain studies are attempted. Thus the introduction of a methyl group at residue 5 allows common conformational features to exist but destroys the elasticity in a manner consistent with the proposed librational entropy mechanism. The effects of this and other analogues of the polypentapeptide of elastin are discussed in terms of accessible configurations, and it is argued that by means of the β-spiral conformation the polypentapeptide of elastin provides a unique configurational entropy which is utilized to produce a new type of entropic elastomer.