Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

Abstract

In photosynthesis, photosystem II evolves oxygen from water by the accumulation of photooxidizing equivalents at the oxygenevolving complex (OEC). The OEC is a Mn4CaO5 cluster, and its sequentially oxidized states are termed the Sn states. The darkstable state is S1, and oxygen is released during the transition from S3 to S 0. In this study, a laser flash induces the S1 to S 2 transition, which corresponds to the oxidation of Mn(III) to Mn(IV). A broad infrared band, at 2,880 cm-1, is produced during this transition. Experiments using ammonia and 2H2O assign this band to a cationic cluster of internal water molecules, termed "W 5 +." Observation of theW5 + band is dependent on the presence of calcium, and flash dependence is observed. These data provide evidence that manganese oxidation during the S1 to S2 transition results in a coupled proton transfer to a substrate-containing, internal water cluster in the OEC hydrogen-bonded network.