A mechanistic study of the photooxidation of L-cystine and its residues in fibrous proteins

The disulphide chromophore of the amino acid L-cystine absorbs radiation at 250 nm. The optical reflectance spectrum of crystalline L-cystine following irradiation with UVC at 254 nm shown in the figure exhibits two absorption bands near 600 and 420 nm. The figure also shows that a similar spectrum is obtained following UVC irradiation of dry Merino wool keratin, which contains high concentrations (35 mol% half cystine) of cystine residues near the fibre surface. In previous work on gamma-irradiated single crystals of L-cystine dihydrochloride, Akasaka observed two absorption peaks in the optical spectrum at 550 and 400 nm. He found that the primary radiolytic event is removal of an electron from cystine to give the radical cation (the 550 nm absorber), followed by capture of the electron by another disulphide group to give the radical anion (the 400 nm absorber). The similarity of the positions of the absorption bands suggest that the same process occurs when L-cystine is exposed to UVC. It is interesting that cystine appears to undergo a similar ionisation process when exposed to radiation over a wide range of wavelengths which includes UVC, X- and gamma-rays. The radioprotective ability of disulphides and thiols against ionising radiation in biological systems is well known, and it appears that protection against UVC occurs via a similar mechanism