Oxidation of Cobaltous Cysteine.

Abstract

Michaelis and Barron 1 and Michaelis and Yamaguchi 2 have shown that cobalt reacts with cysteine to form a product which has a high reduction intensity. They also showed that this reaction product can be oxidized with air, ferri-cyanide, and phenolindophenol to a brown cobaltic cysteine complex. We have found that cobaltous cysteine reacts in a different manner toward each one of these oxidants. Indigo disulfonate produces a quantitative conversion of cobaltous cysteine to the cobaltic cysteine complex. All other oxidants except those which contain a quinone group result in the formation of the cobaltic cysteine complex and cystine in different proportions.

Oxidation with ferricyanide produces a unique type of electrometric titration curve because all of the cobaltous cysteine is removed from solution by addition of a half of the total amount of oxidant. The last half of the curve represents oxidation of cysteine to cystine.

Cysteine reacts with quinone and with the quinone group of dibromophenol-indophenol with the formation of an addition product between the dye and the thiol group similar to the addition of an aromatic thiol group to quinone. One molecule of cysteine reacts with 1 molecule, that is, 2 equivalents of the dye. Ninety per cent of cobaltous cysteine is oxidized to the cobaltic cysteine complex with dibromophenolindophenol, and 10% of the cysteine combines with the dye. The amounts of cobaltic complex and cystine formed by each oxidant are such that the amounts of ferricyanide, oxygen, and indophenol required for a given amount of cysteine are all equal to two-thirds the amount of the cysteine.