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 H 2CO3/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).
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry