Crystallinity and conformational changes in poly(β-benzyl L-aspartate)

The crystallinity of poly(β-benzyl L -aspartate) is highly variable, in a series of specimens prepared under various conditions: films cast from chloroform solutions at various evaporation rates, films cast comparatively slowly from chloroform–trifluoroacetic acid solutions, films prepared from dichloroacetic acid solution by treatment with ethyl alcohol, precipitates formed from trifluoroacetic acid solution by addition of ether. Film cast slowly from chloroform is in the highly crystalline ω form. In contrast, the conformation of the benzene rings in the ω helix obtained from the α helix by heating is distorted to some extent in comparison with the structure of the highly crystalline ω form. Crystallization and conformational changes from the α to the ω form, and from the ω to the β form upon heating, are correlated with the dispositions of the side chains, the packing of the benzene rings, and the motion of the side chains. The main chain of the α helix is distorted into the ω form when its side chains are in a favorable conformation. The α helix is stable in the disordered conformation, and it is distorted to some extent at high temperature.