Previous research from this laboratory has involved the investigation of bismuth(III) compounds with L-cysteine¹. It was found that bismuth(III) nitrate interacts extensively with L-cysteine in aqueous solutions, primarily through the strong bond between the bismuth(III) and the L-cysteine thiol group. The extent of this interaction was assessed by monitoring changes in cyclic voltammetric peak potentials, as well as UV-VIS absorption characteristics, as L-cysteine was added to the solutions. Considerable work has now been carried out on pharmaceutical medications² such as bismuth subsalicylate (BSS) and bismuth citrate (BC) at pH 1.00 and 3.00 to simulate conditions in the human stomach, as well at pH 7.40 MOPS [(3-N-morpholino)propanesulfonic acid] to simulate general physiological conditions. In addition, studies of the cysteine-containing tripeptide glutathione have also been conducted.

Results and Discussion

By comparing results at pH 1.00 and 3.00 to those at pH 7.40 for all bismuth compounds investigated, it has been found that the extent of interaction of these compounds with added L-cysteine is much less at the lower pH levels. This conclusion was reached by noting that the absorbance of the characteristic band at 340 nm, due to the bismuth-sulfur interaction, was much lower at pH 1.00 and pH 3.00 than at pH 7.40 at corresponding levels of L-cysteine addition. Such results can be explained by the higher L-cysteine protonation state at the lower pH values compared to that at pH 7.40. This higher protonation state lessens the bismuth-sulfur interaction at the lower pH values. Interesting changes were also found in the cyclic voltammograms as L-cysteine was added in these situations. For bismuth(III) citrate (BC) at pH 7.40, a well-formed absorbance band at 320 nm was formed upon L-cysteine addition, reaching a limiting absorbance value at a 2:1 L-cysteine:BC ratio. For bismuth(III) salicylate, however, the absorbance at 300 nm to 340 nm was much lower than for BC, showing that the salicylate ligand strength is somewhat less than that for the citrate ligand under these conditions. It is also planned to present the results of similar work involving glutathione as a ligand.

References

1. T. Cheek and D. Peña, J. Electrochem. Soc., 167, 155522 (2020)
2. Li, R. Wang, and H. Sun, Acc. Chem. Res., 52, 216 (2019).