| Abstract: | Photochromic polypeptides, with 16 to 56% azobenzene groups in the side chains, have been prepared by reaction of poly(L -glutamic acid) with p-aminozaobenzene, both in the presence of dicyclohexyl carbodiimide/N-hydroxybenzotriazole and of pivaloyl chloride. Analogous modification reactions carried out on poly(L -aspartic acid) were unsuccessful owing to the formation of N-succinimide rings. In trimethylphosphate, all the azopolypeptides exhibit the α-helix CD pattern. Irradiation produces the trans-to-cis isomerization of the azo side chains, but does not induce any variations of the backbone conformation. In water, the CD spectra indicate the presence of appreciable amounts of α helix in 16 and 21% mol azo-containing poly-(L glutamates), while a β structure is present in a 36% mol azopolypeptide. Light produces conformational changes of the polypeptide conformation which are completely reversed in the dark. The extent and kind of photobehavior depend on the azo content and the pH value at which irradiation is carried out. The light-induced effects are discussed on the basis of the pH-induced order-disorder conformational transitions. In fact, the pK values and the transition curves of the dark-adapted samples were found to be different from those of the irradiated ones. |
