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

In photosynthesis, photosystem II evolves oxygen from water by the accumulation of photooxidizing equivalents at the oxygen-evolving complex (OEC). The OEC is a Mn4CaO5 cluster, and its sequentially oxidized states are termed the S-n states. The dark-stable state is S-1, and oxygen is released during the transition from S-3 to S-0. In this study, a laser flash induces the S-1 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 (H2O)-H-2 assign this band to a cationic cluster of internal water molecules, termed W-5(+). Observation of the W-5(+) band is dependent on the presence of calcium, and flash dependence is observed. These data provide evidence that manganese oxidation during the S-1 to S-2 transition results in a coupled proton transfer to a substrate-containing, internal water cluster in the OEC hydrogen-bonded network.