Analytical and structural investigation via infrared spectroscopy and density functional methods of cuprous complexes of the antioxidant tripeptide glutathione (GSH). Synthesis and characterization of a novel CuI-GSH compound

Abstract

A copper(I) compound of reduced glutathione (Glu,S;Cys,R)-(GSH) was isolated from the reaction of CuICl and zwitterionic H3GSH(+−) in acetonitrile:water medium. The analytical data are in agreement with the formula CuI(H2GSH)(H2O)2. The characterization was carried out via infrared spectroscopy and density functional (DFT) method computations, that allowed to fully optimize structures for the free ligands and for several complex molecules of the type [CuI(S,O,O-H2GSH)(OH2)] having a pseudo-tetrahedral coordination arrangement and the Gly residue deprotonated at carboxylic group. The molecules were fully optimized at DFT-B3LYP/6-31G∗∗ (BS1) and in some cases at B3LYP/6-311++G∗∗ (BS2) and the solvent treatment (water, via PCM method) was taken into account (BS1W). The comparative analyses between experimental and computed infrared spectra confirmed that the free ligand was reduced glutathione in the zwitterionic form at the Glu residue. The analysis showed also that the metal compound consisted of complex molecules of the type [CuI(S,O,O-H2GSH(Gly))str(H2O)] where the ligand had a stretched (str) arrangement assisted by a network of hydrogen bonds. The presence of a globular (glo) structure for the complex molecule that brought the Glu-COOH and Glu-NH2 groups towards the Cys-SH and the Gly-COO− donors, respectively, could not be ruled out.