PHOTOCHROMIC POLYPEPTIDES AS SYNTHETIC MODELS OF BIOLOGICAL PHOTORECEPTORS - A SPECTROSCOPIC STUDY

L-Glutamic acid polypeptides containing photochromic nitrospiropyran b ound to the side chains at various percentages (''local'' concentratio n) have been synthesized and investigated as possible artificial model s of biological photoreceptors. Absorption and fluorescence spectrosco py have been utilized to investigate the photophysical and photochemic al properties of nitrospiropyrans, both inserted in the polypeptide ch ain and in solution as ''free'' dye. Conformational variations produce d by dark storage and light exposure of the photochromic polypeptides have been studied by means of circular dichroism. Dark-kept ''free'' d yes in hexafluoro-2-propanol solution in the merocyanine form (''open' ' form) give rise to molecular aggregates, which have been characteriz ed as merocyanine dimers. The equilibrium constant between the monomer and the dimer, K, and their molar extinction coefficients, epsilon, a t several wavelengths have been determined. Fluorescence measurements on ''free'' and polypeptide-bound nitrospiropyrans suggest that the di merization process between merocyanines is favored when the photochrom ic units are inserted in the polypeptide chain and that under these co nditions an efficient energy transfer from the monomer (donor) to the dimer (acceptor) occurs. By varying ''local'' as well as total nitrosp iropyran concentration, it has been shown that the dimeric species res ult from intermolecular interactions between photochromic groups inser ted in the same polypeptide chain. The alpha-helix --> random coil tra nsition of the polypeptide structure after dark storage has eventually been shown to be the result of the dimerization process and not of th e dark isomerization per se from the ''closed'' spiropyran form to the ''open'' merocyanine form of the dye.