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

The oxidation potentials of MnII 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 MnII, only bicarbonate stimulates the electron transfer from MnII 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 MnII (down to 0.52 V); (ii) electroneutrality of the functional electron transfer complex; (iii) the more favorable energetics reflected in the two pKa values for H2CO3/HCO3− and HCO3−/CO32− and greater number of proton transfer sites; and (iv) multiple composition possibilities for the MnIII photo-product as MnIII(HCO3−)3, MnIII(HCO3−)(CO32−) and MnIII(HCO3−)2(OH−) (due to the high Lewis acidity of MnIII (pK < 1).