Side-chain conformations in dry and liquid-crystalline forms of racemic poly-γ-benzyl glutamate

Abstract

A description is given of the X-ray diffraction analysis both of dry fibres of racemic poly-γ-benzyl glutamate and of liquid-crystalline solutions of racemic poly-γ-benzyl glutamate in dimethylformamide and acetophenone. It appears that regular side-chain conformations occur in these systems, presumably due to the strong interactions between the benzyl groups on the ends of the side chains. Least-squares analysis of the reciprocal space co-ordinates of reflections in the X-ray diffraction patterns from these systems has given precise unit-cell parameters. The axial repeat (except for some very weak reflections) is 64 Å, suggesting the presence of α-helical molecules with 43 residues in 12 turns of the helix. Equatorial Patterson and Fourier synthesis to a resolution of about 2 to 3 Å has been carried out to confirm models of the side-chain structures deduced from the distribution of intensity in the observed diffraction patterns. The models involve regular stacks of benzyl groups from about five side chains which are formed by interaction between adjacent molecules of opposite screw sense and in which every second benzyl group originates from the same molecule. Benzyl groups in side chains from one molecule interdigitate with side chains from the adjacent molecules of opposite screw sense. The separation between centres of successive benzyl groups agrees with results on the benzyl group interactions in the ω-form of poly-β-benzyl- l-aspartate and in the “complex” phase of poly-γ-benzyl- l-glutamate.