PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Oxidation potentials and functional interaction with Photosystem II of manganese complexes containing bicarbonate and carboxylate ligands. Yuri Kozlov¹, Konstantin Tikhonov¹, Jyotishman Dasgupta², Sergey Zharmukhamedov¹, Alevtina Kazakova¹, Charles Dismukes^*,2, Vyacheslav Klimov¹, ¹ Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region, RUSSIA² Princeton University, Department of Chemistry, Hoyt Laboratory, Princeton, New Jersey, USA

ABSTRACT- 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 considerably lower the oxidation potential of Mn^II, only bicarbonate but not carboxylate ligands (acetate, formate) 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) the significantly larger decrease in the oxidation potential of Mn^II (down to 0.52 V), ii) the electroneutrality of the functional electron transfer complex, iii) the more favorable energetics reflected in the two pKA values H₂CO₃/HCO₃^-/CO₃^2- and greater number of proton transfer sites, and iv) multiple 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)).

KEY WORDS: water oxidation, photosystem II , bicarbonate, manganese, redox potential