The interaction of hydrophilic thiols with cadmium: investigation with a simple model, 3-mercaptopropionic acid

Environmental contamination by cadmium is widely recognized as a serious problem. All cadmium compounds are considered toxic and are regulated by the US Environmental Protection Agency. In organisms, thiols or organic sulfhydryl compounds are primarily involved in mobilizing and detoxifying cadmium through the formation of cadmium–thiol complexes inside the cell. This complexation is also of environmental importance because a variety of thiols are generated by biotic and abiotic processes in anaerobic environments, such as estuarine wetlands and coastal sediments, which have increasingly become the targets of metal pollution. We investigated the complexation of cadmium by 3-mercaptopropionic acid (MPA), using it as a simple model to better understand the potential pathways of cadmium transformations by hydrophilic thiols. Our results show that cadmium forms primarily 1:1 and 1:2 complexes with MPA in aqueous solutions at near-neutral pH values. While the dithio complex was highly soluble in water, the monothio complex rapidly precipitated from solution. Based on mass spectrometric and X-ray absorption spectroscopic data, we propose a cyclic, charge-neutralized structure for the monothio complex, and a linear ionic structure for the dithio complex. These results suggest that the type of complex formed is an important determinant of the mobility of cadmium.