Electrochemical and Spectroscopic Investigations of L-Glutathione with Bismuth Pharmaceuticals

Abstract

Introduction The electrochemical and spectroscopic investigation of bismuth(III) compounds with L-cysteine has been carried out in this laboratory since our initial publication.1 Cyclic voltammetry has shown that the interaction of L-cysteine with bismuth nitrate, bismuth salicylate, and bismuth citrate increases the solubility of these compounds by complexation with L-cysteine, thereby increasing the reduction current for the bismuth complexes formed. The appearance of a UV-VIS spectroscopic band at 340 nm indicates the formation of a Bi-S bond, the sulfur being supplied by the L-cysteine. These investigations have been conducted at pH 1.00 and 3.00 to simulate conditions in the human stomach and at pH 7.40 MOPS [(3-N-morpholino)propanesulfonic acid] to simulate general physiological conditions, in particular the intestinal tract. In this presentation, continuing studies of L-glutathione, a tripeptide incorporating L-cysteine, under these conditions shall be discussed. This work is applicable to the important field of bismuth pharmaceuticals as a means of understanding the interaction of these compounds under various conditions in the human digestion system.2 Results and Discussion Initial studies of L-glutathione with bismuth nitrate, bismuth salicylate, and bismuth citrate have been carried out under conditions similar to those of our initial work.1 It has been found that the general characteristics of the cyclic voltammograms for L-glutathione (GSH) are similar to those for L-cysteine; however, the slower rate of complexation of GSH with these bismuth compounds is evident in the changes in voltammetric features in successive sweeps. In the case of GSH addition to bismuth citrate in pH 7.4 MOPS buffer, for instance, the first sweep shows reduction of the bismuth:GSH complex at -0.71 V vs Ag/AgCl at a gold electrode, whereas a reduction process at -0.60 V is observed on the second sweep. Similar results are obtained at glassy carbon. The bismuth:GHS complex is destroyed during the reduction process as a bismuth film forms on the electrode surface. The following sweep, then, is indicative of the complexation environment in solution at that point. The bismuth ion, restored in the stripping step, apparently does not have time in the few seconds after the stripping process to fully form a complex with GSH. The observed reduction potential around -0.60 V is due to the reduction of another bismuth complex, perhaps with citrate. Further investigations are in progress to more fully describe this situation.References G. T. Cheek and D. Peña, J. Electrochem. Soc., 167, 155522 (2020)H. Li, R. Wang, and H. Sun, Acc. Chem. Res., 52, 216 (2019).