PHOTO-INDUCED FORMATION OF α-HELICAL STRUCTURE OF POLY(L-GLUTAMIC ACID) CONTAINING PARAROSEANILINE IN THE SIDE CHAINS

Abstract

A photoresponsive polypeptide has been prepared by condensation reaction of poly (L-glutamic acid) (PGA) with pararoseaniline (Rose) in dimethylformamide solution (Rose-modified PGA). The Rose side chains in the polymer exhibited the photo-ionization under uv light irradiation, yielding triphenylmethyl cations, which vanished completely in the dark. The helix content of Rose-modified PGA, 4.3% in the dark, was increased to 7.2% by uv light irradiation at pH9.7, owing to the compensation for carboxylate anion (glutamic acid side chain) repulsions resulting from the production of the triphenylmethyl cation on the polymer chains by uv irradiation. The photo-induced changes of the conformation of Rose-modified PGA at pH9.7 were reversible and in correlation with the change in absorbance at 560nm of Rose side chain groups. On the other hand, the Rose side chains in the polymer exhibited no photo-ionization under uv light at pH5.7, owing to the neutralization effect in the low pH aqueous solution. As a result, uv irradiation did not induce any variations in conformation of Rose-modified PGA at pH5.7.