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

Interaction of higher S-states of the OEC with reductants: Is S₃ a radical containing state? Nikolaos Ioannidis^*,1, Vassili Petrouleas¹, ¹ Institute of Materials Science, Athens, Greece

ABSTRACT- The nature of the S₂ to S₃ state transition, i.e. Mn-centered oxidation versus ligand-centered oxidation (and thus radical formation), is a matter of an ongoing debate and of high importance when considering possible mechanisms for the water oxidation reaction performed by Photosystem II. We tried to probe the nature of the S₃ state by comparing the reactivity exhibited by the S₃ and S₂ states towards various small reductants. Our results indicate that nitric oxide (which is a radical) and the sulfur dioxide radical (the product of dithionite monomerization) react faster with the S₃ state. On the contrary, hydroxylamine and hydrogen peroxide reduce faster the S₂ state. We propose that in the S₃ state reduction, an outer sphere mechanism is operative and the reductant transfers one electron to the Mn cluster via a Mn ligand, which has radical character. In the S₂ state reduction, the reductant may form an inner sphere complex with the Mn cluster and reduce it to the S₁ state by one electron and one proton transfer.

KEY WORDS: oec, epr, S₃ state, reductants