Structural Changes in Poly-β-benzyl-L-aspartate

Abstract

DIFFERENTIAL thermal analysis has been applied over a wide field in the study of thermally induced structural changes in polymers¹. This communication reports the application of this technique to an investigation of such changes in poly-β-benzyl-L-aspartate. It is generally recognized that polypeptides can form crystalline, helical structures in the solid state, in particular the α-helix of Pauling, Corey and Branson². The stability of such a helix depends on a number of factors, in particular the nature of the side-chain³, and the degree of polymerization of the molecule⁴. Of particular interest is poly-β-benzyl-L-aspartate, which adopts a relatively unstable left-handed α-helical conformation in the solid state, at room temperature. It has been suggested that the preferential formation of the helix with this anomalous sense results either from steric interference between the side-chain and the main-chain atoms^5,6, or from the competitive hydrogen bonding between the ester carbonyl groups of the side-chain and the amide groups of the main-chain⁷, which precludes the formation of the more common right-handed helix. In this investigation, changes in the heat-energy of the polymer when heated at a constant rate were followed by differential thermal analysis and were correlated with structural changes, characterized by X-ray diffraction and infra-red spectroscopic methods.