Oxidation potentials and electron donation to photosystem II of manganese complexes containing bicarbonate and carboxylate ligands

The oxidation potentials of Mn^II in aqueous solutions of bicarbonate, formate, acetate and oxalate are reported as a function of concentration and compared to the rate of photooxidation of these solutions by the Mn-depleted water-oxidizing complex of photosystem II (apo-WOC-PSII) from peas. Although all the carboxylate species lower considerably the oxidation potential of Mn^II, only bicarbonate stimulates the electron transfer from Mn^II to apo-WOC-PSII. On the basis of the electrochemical data it is proposed that the unique capability of Mn-bicarbonate complexes to be photooxidized by PSII could be due to four possible reasons: (i) significantly larger decrease in the oxidation potential of Mn^II (down to 0.52 V); (ii) electroneutrality of the functional electron transfer complex; (iii) the more favorable energetics reflected in the two pK_a values for H ₂CO₃/HCO₃^- and HCO₃ ^-/CO₃^2- and greater number of proton transfer sites; and (iv) multiple composition possibilities for the Mn^III photo-product as Mn^III(HCO₃^-)₃, Mn^III(HCO₃^-)(CO₃^2-) and Mn^III(HCO₃^-)₂(OH^-) (due to the high Lewis acidity of Mn^III (pK < 1).