A nuclear magnetic resonance study of the equilibria and kinetics of the reaction of penicillamine with cystine and related disulfides

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Abstract
The equilibria and kinetics of the reactions of PSH with cystine were characterized over the pH range 5-8, while the reactions with the disulfides of cysteamine, homocysteine, 2-mercaptoethanol, mercaptoacetic acid, 3-mercaptopropionic acid, and mercaptosuccinic acid were studied at neutral pH. From the pH dependence of the rate of the reaction of PSH with cystine, the reactive species are identified as penicillamine with its amino group protonated and its thiol group deprotonated and cystine and penicillamine-cysteine mixed disulfide with their amino groups protonated. For all the disulfides studied, the extent to which the first reaction occurs is within a factor of 2-3 of that predicted by a random distribution, while the extent to which the second reaction occurs is considerably less than for a random distribution. This is attributed to steric effects due to the two methyl groups next to the sulfur of penicillamine.