Random copolymers of α‐benzyl‐<scp>L</scp>‐aspartate with <i>ortho</i>‐ and <i>para</i>‐nitrobenzyl‐<scp>L</scp>‐aspartates. Intrinsic and extrinsic dichroic bands and the left‐to‐right‐handed α‐helical transition

Abstract

Abstract Copolymers of benzylaspartate with the corresponding para ‐nitro and ortho ‐nitro derivatives are known to undergo, at increasing nitro content, a transition from a left‐handed α‐helical conformation to a right‐handed α‐helical conformation. The circular dichroism associated with the n , π* transition of the amide group, as well as the extrinsic bands associated with the nitrobenzyl chromophore, have been studied for these copolymers as a function of the nitro content. The circular dichroism associated with the 330‐nm extrinsic band is shown to present an abrupt change, which parallels the change in the 222‐nm band characteristic of the left‐ to right‐handed helix transition. The intensity of the 222‐nm CD band seems unaffected by the interaction with the side‐band chromophore. The circular dichroism associated with the 330‐nm band is much stronger in the right‐handed conformation and is unaffected by increasing interactions between side chains. Interpretation in the light of the possible mechanism for optical activity of helical molecules seems to indicate that the 330‐nm extrinsic effect is essentially due to the Condon, Altar, and Eyring (CAE) mechanism and μ‐ m perturbation (in Schellmann terminology) on the side‐chain chromophore n , π* transition.