Changes in the redox potential and catalase activity of Mn2+ ions during formation of Mn-bicarbonate complexes.
Changes in the redox potentials of Mn2+ ions and Mn-bicarbonate complexes were studied due to their possible participation in the photosynthetic oxidation of water in plant photosystem 2 and in H2O2 decomposition. Electrochemical oxidation of Mn2+ ions was demonstrated by voltammetry on a platinum electrode in 0.1 M LiClO4 solution at a potential of 1.19 V (vs NHE). When NaHCO3 was added, the oxidation peak of Mn2+ ions disappeared. New oxidation peaks appeared at 0.92 V and 0.63 V which corresponded to the oxidation of Mn(HCO3)+ and Mn(HCO3)2, respectively, generated by complex formation between Mn2+ and HCO3- ions. The effect of the Mn(2+)-bicarbonate complex on H2O2 redox decomposition was studied. It was shown that the addition of MnSO4 to the H2O2 solution did not affect the H2O2 oxidation peak height thereby indicating the absence of H2O2 decomposition by Mn2+ ions. At the same time, subsequent addition of NaHCO3 resulted in the disappearance of the oxidation peaks of both H2O2 and Mn2+. As at pH 7 the thermodynamic potential of H2O2 reduction is 1.1 V and the Mn2+ oxidation potential is 1.19 V, the redox reaction between them is hampered. Formation of the Mn(2+)-bicarbonate complex shifted the Mn2+ oxidation potential to 0.63 V, thereby inducing the decomposition of H2O2. It is suggested that the decrease in the Mn2+ oxidation potential resulting from the formation of the bicarbonate complex determines the bicarbonate capability to enhance the Mn2+ ability to donate electrons for PS 2 reaction centres.