Kinetic study of conformational transition accompanied by isomerization of spiropyrans bound to poly(l-glutamic acid) side chains.

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Kinetic studies have been made of the conformational transition of the backbone of spiropyran-modified poly(L-glutamic acid) (PSLG) from α-helix to random coil, accompanied by the isomerization of spiropyrans in the side chains from spiropyran form (SP) to merocyanine form (MC) in hexafluoro-2-propanol (HFP) in dark adaptation after visible light irradiation, by means of the measurements of absorption and circular dichroism spectra. The SP→ MC isomerization reaction of side-chain spiropyrans in PSLG deviated from first-order kinetics in the earlier stage, where the α-helix→ random coil transition of the backbone correlatively occurred. Then, the isomerization proceeded in first-order kinetics under the conformational state of the backbone merely in random coil. The kinetic analyses of the SP→MC isomerization were carried out on the basis of a binary competitive reaction kinetics, in which two species of SPs bound to the side chains of local PSLG in the backbone conformations of α-helix and random coil, respectively, are involved. As a result, it was derived that the rate of the SP→MC isomerization in the α-helix backbone was appreciably smaller than that in the random coil backbone, and depended on the spiropyran content in PSLG, indicating that the isomerization rate for PSLG with a low spiropyran content of 0.15 was considerably smaller than that for PSLGs with higher spiropyran contents above 0.25. Moreover, the solvent effect on the isomerization rates has been investigated, using the mixed solvent of HFP/MeOH. Consequently, the analytical results led to the inference that the isomerization rates of the spiropyran side chains are governed by molecular conformations of PSLG.