Light-induced conformational changes of polypeptides. Random copolymers of .GAMMA.-benzyl-L-glutamate with m- and p-phenylazobenzyl-L-aspartates.

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Copolymers of γ-benzyl-L-glutamate with m- and p-phenylazobenzyl-L-aspartates were prepared for investigating the effect of side-chain photoisomerization on their conformations. A study was made of the circular dichroism associated with the n,π* transition of the amide group, as well as the extrinsic bands associated with the azo chromophore. The intensity of the 222-nm amide band seems unaffected by the interaction with the side-chain chromophore. The circular dichroism spectra before irradiation indicate that poly[β-(m-phenylazobenzyl)-L-aspartate] (mPALA) adopts a left-handed α-helical form and the other copolymers a right-handed form in 1,2-dichloroethane. After irradiation, remarkable changes of the 222-nm band were observed for the meta copolymer with 89.3% azo residues and mPALA. The increased absolute ellipticity for the former indicates that photoisomerization enhances the helicity of the polymer. Decrease in ellipticity for the latter indicates that photoisomerization induces significant instability of the left-handed α-helical conformation. In the course of subsequent relaxation of mPALA, an abrupt change in the CD spectra was found above 50% of cis followed by a gradual change with decreasing cis%.
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