One-Dimensional Aromatic Crystals in Solution VII. Conformational Analysis of Poly(β-9-anthrylmethyl L-aspartate) in Solution

Abstract

Main-chain and side-chain helical conformations of a novel aromatic polypeptide, poly(β-9-anthrylmethyl L-aspartate), poly[Asp(OCH2-9-Ant)], were studied experimentally and theoretically. A series of random copolymers of β-9-anthrylmethyl L-aspartate, Asp(OCH2-9-Ant), and γ-benzyl L-glutamate, Glu (OBzl), having varying molar ratios were prepared. Their circular dichroism (CD) spectra showed an abrupt change with increasing the mole fraction of Asp(OCH2-9-Ant) from 0.6 to 0.8, indicating the inversion of the helix sense from a right-handed to a left-handed one. Thus the helix sense of poly[Asp(OCH2-9-Ant)] was concluded to be left-handed. Empirical energy calculations were performed on the side-chain conformation of the polypeptide, setting the main chain in a left-handed α-helical conformation. Five rotational angles were varied from 0° to 360° with an interval of 30° and seven low-energy side-chain conformations were found. Those conformers were further optimized by the Simplex energy-minimization procedure. Theoretical CD curves were computed for the seven minimum-energy conformations on the basis of the exciton theory. Of the seven candidates two conformers gave reasonable CD pattern, and of the two the lower energy conformer was chosen as the most probable helix conformation of poly-[Asp(OCH2-9-Ant)] in solution.