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 pr oblem. All cadmium compounds are considered toxic and are regulated by the US Environmental Protection Agency. In organisms, thiols or organic sulfhyd ryl compounds are primarily involved in mobilizing and detoxifying cadmium through the formation of cadmium-thiol complexes inside the cell. This comp lexation is also of environmental importance because a variety of thiols ar e generated by biotic and abiotic processes in anaerobic environments, such as estuarine wetlands and coastal sediments, which have increasingly becom e the targets of metal pollution. We investigated the complexation of cadmi um 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 complexe s with MPA in aqueous solutions at near-neutral pH values. While the dithio complex was highly soluble in water, the monothio complex rapidly precipit ated from solution. Based on mass spectrometric and X-ray absorption spectr oscopic data, we propose a cyclic, charge-neutralized structure for the mon othio complex, and a linear ionic structure for the dithio complex. These r esults suggest that the type of complex formed is an important determinant of the mobility of cadmium. Published by Elsevier Science B.V..