Protection of Endogenous Thiols against Methylmercury with Benzimidazole-Based Thione by Unusual Ligand-Exchange Reactions

Organomercurials, such as methylmercury (MeHg+), are among the most toxic materials to humans. Apart from inhibiting proteins, MeHg+ exerts its cytotoxicity through strong binding with endogenous thiols cysteine (CysH) and glutathione (GSH) to form MeHgCys and MeHgSG complexes. Herein, it is reported that the N,N-disubstituted benzimidazole-based thione 1 containing a N-CH2CH2OH substituent converts MeHgCys and MeHgSG complexes to less toxic water-soluble HgS nanoparticles (NPs) and releases the corresponding free thiols CysH and GSH from MeHgCys and MeHgSG, respectively, in solution by unusual ligand-exchange reactions in phosphate buffer at 37 degrees C. However, the corresponding N-substituted benzimidazole-based thione 7 and N,N-disubstituted imidazole-based thione 3, in spite of containing a N-CH2CH2OH substituent, failed to convert MeHgX (X=Cys, and SG) to HgS NPs under identical reaction conditions, which suggests that not only the N-CH2CH2OH moiety but the benzimidazole ring and N,N-disubstitution in 1, which leads to the generation of a partial positive charge at the C2 atom of the benzimidazole ring in 1:1 MeHg-conjugated complex of 1, are crucial to convert MeHgX to HgS NPs under physiologically relevant conditions.