Coacervation of sequential polypeptide models of tropoelastin. Synthesis of H-(Val-Ala-Pro-Gly)n-Val-OMe and H-(Val-Pro-Gly-Gly)n-Val-OMe.

Syntheses of two sequential polytetrapeptide models, H-(Val-Ala-Pro-Gly)n-Val-OMe and H-(Val-Pro-Gly-Gly)n-Val-OMe via the p-nitrophenyl ester method are described. The p-nitrophenyl ester method gave high yields (85%--100%) of large molecular weight polymers. H-(Val-Pro-Gly-Gly)n-Val-OMe exhibited the interesting property of coacervation on raising the temperature of aqueous solutions while H-(Val-Ala-Pro-Gly)n-Val-OMe precipitates irreversibly under similar conditions. Whereas non-dialyzed lower molecular weight polymers of H-(Val-Pro-Gly-Gly)n-Val-OMe with n = 8 to 40 did not coacervate, but did show a transition to increased intramolecular order on raising the temperature of aqueous solutions above 50 degrees C, the dialyzed higher molecular weight polymer, n greater than 40, does coacervate beginning at about 50 degrees C. This demonstrates the molecular weight dependence of coacervation and also suggests the importance to coacervation of side chain interactions in the Val-Pro sequence. The increase in intramolecular order, seen as the formation of a 14-atom hydrogen-bonded ring, occurs simultaneously with the Val-Pro hydrophobic side chain association.