Electrochemical Investigations of L-Cysteine Interactions with Bismuth Ions

The interaction of L-cysteine with bismuth compounds bismuth(III) salicylate, bismuth(III) citrate, and bismuth(III) nitrate, was studied at pH 1.0 (0.100 M HNO 3 and 0.100 M HCl) and pH 7.4 MOPS buffer by cyclic voltammetry at glassy carbon and boron-doped diamond electrodes. pH 1.0, at which bismuth (III) exists as the simple Bi 3+ ion, was chosen to approximate the acid strength of stomach contents. pH 7.4, at which bismuth(III) exists as BiO, was used for its similarity to general physiological conditions. The amino acid L-cysteine was chosen because its sulfhydryl group undergoes intense interaction with many metal cations, serving as a model for cysteine-containing proteins in the digestive system. It was determined that Bi(III) and L-cysteine (Cys) form soluble complexes at both pH 1.0 and pH 7.4. UV–vis spectroscopic investigations support interaction of Bi(III) and L-cysteine to form a 1:2 Bi(III): Cys complex in pH 7.4 MOPS buffer. L-cysteine addition to solutions of the pharmaceutical bismuth(III) salicylate was found to alter the voltammetric behavior of the salicylate complex. These results, especially at pH 1.0, are relevant to understanding the interaction of various cysteine-containing proteins in the human digestive system with bismuth pharmaceuticals and may help guide future explorations of bismuth formulations.