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 2 CO 3 /HCO 3 À and HCO 3 À /CO 3 2À and greater number of proton transfer sites; and (iv) multiple composition possibilities for the Mn III photo-product as Mn III (HCO 3 À ) 3 , Mn III (HCO 3 À )(CO 3
2À) and Mn III (HCO 3 À ) 2 (OH À ) (due to the high Lewis acidity of Mn III (pK o 1).
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