PHOTODEMETHYLATION OF METHYLMERCURY BY SUNLIGHT-GENERATED SINGLET OXYGEN

Monomethylmercury (MeHg) is a neurotoxin that poses significant risks to human health due to its persistence in aquatic ecosystems and bioaccumulation in food webs. Sunlight demethylation is an important component of the mercury cycle that maintains MeHg at low concentrations in natural waters. Rates of photodemethylation, however, can vary drastically between different bodies of waters for reasons that are largely unknown. Here, we show that photodemethylation occurs through sunlight sensitization of dissolved natural organic matter (NOM) and that the rate of demethylation depends on water composition and constituents that bind CH₃Hg⁺ ions. We demonstrate that singlet state oxygen (¹O₂), which is generated by sunlight sensitization of chromophoric NOM, is capable of demethylating MeHg at rates similar to field observations. However, these rates apply only to MeHg species bound to thiol-containing organic ligands such as glutathione, mercaptoacetate, and humic substances. In contrast, CH₃HgCl complexes are unreactive towards ¹O₂. Photochemical experiments indicate that binding of CH₃Hg⁺ to electron-dense thiolates lowers the excitation energy of the C-Hg bond, making the bond susceptible to attack by electrophiles such as ¹O₂. Our results provide an explanation to why photodemethylation is rapid in NOM-rich freshwater lakes, and relatively slow in coastal marine waters where CH₃HgCl complexes dominate methylmercury speciation.