An insoluble intermediate in the biosynthesis of elastin and its relationship to tropoelastin.
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The cyclododecapeptide, (Ala1-Pro2-Gly3-Val4-Gly5-Val6)2, was synthesized and its secondary structure was evaluated from extensive studies in dimethyl sulphoxide, trifluoroethanol and water using NMR methods. A selective decoupling technique in 13C-NMR has been utilized in order to assign the C=O carbon resonances. Temperature dependence of the peptide NH protons and the solvent perturbation of the peptide NH and C=O resonances show the occurrence in all solvents of a beta-turn (a 10-membered H-bond between the Val4 NH and Ala1 C=O) and a gamma-turn, an 11-membered H-bond between the Gly3 NH and the Gly5 C=O; and a possible 14-membered H-bond between the Ala1 NH and the Val4 C=O in dimethyl sulphoxide and trifluoroethanol. These secondary structural features are compared with the linear polyhexapeptide and found the the beta-turn and the gamma-turn are the common conformational features of these peptide systems.
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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.
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