Voltammetric and Spectroscopic Investigations of L-Cysteine Interactions with Bismuth Ions

Abstract

Bismuth compounds have been widely used for many years as treatments for gastric conditions.1 The electrochemical activity of bismuth suggests that voltammetric studies can provide some insights into interactions of bismuth with naturally occurring ligands found in the digestive system. Of particular interest is L-cysteine, the sulfhydryl side chain of which can interact with bismuth(III) ions.1 Previous work in this laboratory has characterized the electrochemical behavior of L-cysteine in aqueous systems,2 and the present work involves the effect of L-cysteine on the voltammetric behavior of bismuth(III) nitrate, bismuth(III) citrate, and bismuth(III) salicylate. In order to simulate the pH level of gastric contents, studies have been carried out in 0.100 M HCl and 0.100 M HNO3 using the techniques of cyclic voltammetry and square wave voltammetry. Voltammetric scans have been carried out at both glassy carbon and boron-doped diamond electrodes in order to avoid the reduction of protons, which would occur at platinum and gold electrodes. It was found that L-cysteine addition greatly increases the solubility of bismuth(III) nitrate in 0.100 M HNO3, as seen by an increase in the bismuth reduction current and also in the subsequent stripping current of the bismuth deposit. Bismuth(III) salicylate was found to be soluble at the 1 mM level in 0.100 HNO3, and L-cysteine additions produced potential shifts in the voltammetric processes for the bismuth(III) system. These results support the complexation of bismuth(III) salicylate by L-cysteine, even under these very acidic conditions. Similar studies for bismuth(III) citrate were also carried out. Finally, the electrochemical behavior of bismuth(III) nitrate in pH 7.4 MOPS3 buffer was also investigated. It was found that bismuth(III) nitrate (nominal 1.0 mM) was essentially insoluble in this medium; however, additions of L-cysteine produced voltammetric currents typical of the 1 mM concentration level. Initial reduction of bismuth(III) was followed by bismuth stripping peaks at more positive potentials. This behavior supports the complexation of bismuth(III) ion by L-cysteine at pH 7.4. Further evidence of this complexation was found by UV-VIS studies of bismuth(III) nitrate in pH 7.4 MOPS buffer with incremental additions of L-cysteine. An absorbance band at 325 nm appeared as L-cysteine was added, the absorbance of which remained constant after the 1:2 Bi3+: L-cysteine point. These results indicate formation of Bi(Cys)2 under these conditions. As a whole, the present studies indicate the ability of bismuth(III) compounds to interact with L-cysteine over a range of pH levels.