The photodegradation of wool keratin. II. Proposed mechanisms involving cystine

Abstract

A possible mechanism for the formation of cysteine, cysteic acid and partially oxidized cystine species in keratin following exposure to WC radiation at 254 nm is proposed. The primary photolysis products are the radical ions RSSR(·+) and RSSR(·-), and it is suggested that partially oxidized cystine species are formed from reactions of the radical cation only. The mechanism is supported by Fourier transform IR (FT-IR) and optical reflectance spectroscopy of WC-treated wool exposed to air, oxygen and nitrogen atmospheres, and by previous electron spin resonance (ESR) studies on disulphides exposed to both UV and ionizing radiation. The mechanism supports the Symons assignment for the sulphur radical 'species X' as [RS(SR)SR]̇, rather than RSṠ. Exposure of wool to less energetic UV radiation produces different products via a quenching mechanism involving the formation of the radical anion RSSR(·-) only, which precludes the formation of oxidized cystine intermediates. The formation of S-sulphonate residues at longer wavelengths suggests that a competing singlet oxygen process may also occur.