Redox pathways in the carnosine-manganese system mediated by semiquinone radicals

The interaction of carnosine histidine containing dipeptide with the semiquinone radical of 3,4Dihydroxybenzoic dianion in dimethyl sulfoxide solution under aerobic conditions was investigated in the present study. The interaction of dipeptide-manganese system with the same semiquinone was also studied under the same conditions. The metal-binding and redox chemistry of these systems were investigated using electronic, ESR 1H-NMR spectroscopies and cyclic voltammetry. The in-situ formal semiquinone radicals (extensively studied in previous works) were significantly modified upon addition of carnosine, as evidenced by the redistribution of the visible absorption bands and the change of the redox couples of the catecholic product. Subsequent influence of manganese ions induced further spectral and electrochemical changes, consistent with metal centered oxidation and formation of ligand-stabilized manganese species. Paramagnetic broadening in the 1H-NMR spectra supported the presence of higher-valent manganese intermediates in solution. The combined data support a cooperative redox pathway in which semiquinone species mediate both peptide oxidation and manganese centered electron transfer. These results provide chemical insight into the understanding of multicomponent systems usually present in nature.