Light-induced Conformational Changes of Polypeptides. Photoisomerization of Azoaromatic Polypeptides

Copolymers of β-benzyl l-aspartate with β-m- or p-(phenylazo)benzyl l-aspartate were prepared in order to investigate the effect of side-chain photoisomerization on their conformations. These polymers in 1,2-dichloroethane show a CD band at 222 nm characteristic of left-handed α-helices. After irradiation, remarkable changes of the 222 nm band were observed for both meta and para copolymers with more than 50 % azo residues. The decrease in ellipticity observed for the meta copolymers indicates that photoisomerization induces a significant instability in the left-handed α-helical conformation. On the other hand, the para copolymers change their sign from positive to negative by irradiation, confirming the reversal in the helix sense. In the course of subsequent relaxation of the irradiated para copolymer with 59% azo residues, an abrupt change in the CD spectra was found above 50% cis followed by a gradual change on decreasing the content of cis isomers. Concerning the polymers with azoaromatic side chains including copolymers with γ-benzyl l-glutamate, the helical sense seems to affect appreciably the sign of the side-chain CD bands.