Glutathione and N-acetylcysteinylglycine: Protonation and Zn2+ complexation

The speciation study of the Zn2+/glutathione (GSH, H3G) and Zn2+/N-acetylcysteinylglycine (NAcCG, H2L) was performed in aqueous solution by means of potentiometry and ESI mass spectrometry. The ligand N-acetylcysteinylglycine was synthesized by protection/activation strategies. 1H NMR data for the Zn2+/NAcCG system at different pH were also collected, to gain insight in the coordination modes for the ligand. The information collected for the NAcCG model ligand were used to propose the structure in solution for the Zn2+/GSH complexes. Dinuclear complexes of GSH with Zn2+, which have never been proposed previously in the literature, were identified in solution and a model of their structure was proposed. Moreover, the Zn2+ promoted deprotonation of the cysteinyl peptidic NH with formation of five membered (S,NCys-) chelating rings was evidenced. The speciation study of the ternary Zn2+/GSH/NAcCG system was also performed, showing that the Zn2+ does not bind preferentially to GSH in presence of NAcCG. The 1H NMR protonation studies of both GSH and NAcCG were also performed, and a novel proton dissociation microconstant calculation procedure has been proposed and applied to GSH equilibria.