Photoinduced electron transfer and free radical formation in dry keratin protein

Abstract

Dry keratin protein was subjected to flash photolysis at 265 nm in the absence of oxygen. Transient species with absorption maxima at 410 and 520 nm were observed by diffuse reflectance following flash photolysis. The species with an absorption maximum at 410 nm increases rapidly at a rate faster than the time response of the laser flash photolysis apparatus. By contrast, the species with an absorption maximum at 520 nm increases relatively slowly with a first-order rate constant of 1.5 × 10 5 s −1. Little or no decay was apparent in both species over the first 100 μs following excitation. Comparison of the absorption spectra observed in this work with previously published spectra for the tyrosine and tryptophan free radicals in aqueous solution suggests that the species observed in keratin following flash photolysis with an absorption maximum at 410 nm is the tyrosine free radical, whereas the species with an absorption maximum at 520 nm is the tryptophan free radical. Emission in the near IR from 1 μm to 2 μm was also observed following flash photolysis. The emission increases with a first-order rate constant of 2.4 × 10 6 s −1 and then decays with an initial first-order decay constant of 1 × 10 5 s −1.