Abstract
O2 evolving is carried out by Mn-cluster in Photosystem (PS) II. In the enzyme cycle a Mn-cluster goes through five sequential redox states denoted Sn, where n varies from 0 to 4, upon the absorption of four successive photons. In the S4 state two molecules of water are oxidized, the S0-state is regenerated, and O2 is released. The S0 and S2 states are paramagnetic with S=1/2, giving rise to a EPR multiline signal that can be detected at low temperature.
In order to characterize the nature of S0-state, CW and pulsed EPR have been performed in S0-enriched PS II membrane. The S0-enriched membranes were prepared by 3 flash illumination of the S1-enriched membranes after YD* radical was reduced chemically. S0-multiline signal was observed at 5 K. By illuminating the S0-enriched membrane, we observed disappearance of S0-state signal under light. After turning off the light, the S0-state signal appeared again. The reversible change cannot be ascribed to the effect of the NIR light, but the oxidation of manganese cluster.
