DYE‐SENSITIZED SELECTIVE PHOTO‐OXIDATION OF CYSTEINE

The phtalein derivative cresol red and the fuchsin dye crystal violet have been found to sensitize the selective photo-oxidation of cysteine, both free and incorporated into a protein. The specificity for cysteine of these dyes was checked in aqueous solution over the pH range 2.5-9, as well as in aqueous acetic acid. In acidic solutions, cysteine is quantitatively converted to cysteic acid; on the contrary, in neutral or alkaline media, the photo-oxidative process is competed by a dark reaction, which causes the oxidation of cysteine to cystine. The irradiation of lysozyme, after reduction of a single disulphide bridge, in 16%acetic acid solution and in the presence of either sensitizer yielded a disulphonic derivative, which displayed about 40% enzymic activity. Moreover, fully reduced lysozyme, after photo-oxidation under the same conditions, appeared to contain eight cysteic acid residues per protein molecule and was devoid of enzymic activity. Therefore, this method appears to be suitable for the selective and quantitative modification of the cysteinyl residues in biologically active polypeptides and for investigating their importance for the biological function.