Interaction of poly- L-proline with lithium bromide

Abstract

The role of solvation in stabilizing the poly proline II helix as well as the related question of the mechanism of action of the neutral salts (specifically LiBr) in destabilizing the poly proline II conformation have been investigated. Results indicate that water does not have a unique structural role in the stabilization mechanism. Direct evidence is presented demonstrating that the conformational pattern of poly- L -proline in aqueous and anhydrous LiBr solutions is due to a specific type of binding of the salt to the peptide linkages. A LiBr-poly proline complex has been isolated from both aqueous and anhydrous LiBr solvents, and dilute solutions of the complex in solvents of low water content are found to give optical rotatory and viscosity properties qualitatively similar to those observed in the concentrated salt systems. Results are consistent with a binding of LiBr at the peptide linkages, leading to a lowered barrier to rotation of the rings about the peptide bonds. LiBr-polymer complexes of poly- L -hydroxy proline, poly sarcosine, poly- DL -alanine, poly glycine and gelatin have also been isolated and the weight fractions of bound LiBr determined. The physico-chemical properties of polypeptide chains in concentrated LiBr solutions are discussed.