Is the alkaline cleavage of disulfide bonds in peptides an alpha-beta elimination reaction or a hydrolysis?

The cleavage of oxidized glutathione by alkali has been studied as representative of the cleavage of protein disulfides. This process is quite different when studied in 10-4N or 2-10-1N NaOH. At low alkali concentration no spectral changes are noted; at higher hydroxyl concentration the appearance of persulfide groups (followed at 335 nm), the formation of thiocyanate (arising from cold cyanolysis of persulfide groups) and the absorbance at 240 nm follow the same kinetics. The amount of half-cystine, recovered as cysteic acid after a 3-h reaction, is significantly lower than calculated. These results confirm that oxidized glutathione undergoes beta-elimination at high pH values, and that persulfide groups absorb not only at 335 nm (as already known) but also at 240 nm where, under our conditions, the contribution of other absorbing species is not very high.