The reduced S<sub>-1</sub> and S<sub>-2</sub> oxidation states of the O<sup>2</sup>-evolving complex of photosystem II: An EPR microwave power saturation study

Abstract

Progressive microwave power saturation (P1/2) measurements have been performed on the tyrosine D radical (YD •) of photosystem II (PSII) in order to examine its relaxation enhancement by the oxygen-evolving complex (OEC) poised to the reduced S−1 and S−2 oxidation states by NO treatment. Analysis of the power saturation curves showed that the S−1 oxidation state of the OEC does not enhance the relaxation of YD •: it therefore possesses a diamagnetic ground state. In contrast, the Mn(II)-Mn(III) multiline electron paramagnetic resonance (EPR) signal characteristic of the S−2 oxidation state of the OEC was shown to provide a relaxation enhancement pathway for YD •, however less efficient relative to the one provided by the S2-state multiline EPR signal. We also examined the YD • relaxation enhancement characteristics of the EPR-silent oxidation state produced after brief (1–5 min) dark incubation at 0°C of a PSII sample poised to the EPRactive S−2 state. This EPR-silent oxidation state denoted as “0°C incubation” state was shown to possess remarkably similar P1/2 values with the EPR-active S−2 state in the overall examined temperature range (6–20 K). In addition, these values remained unchanged after successive cycles of the OEC between the EPR-active S−2 state and the “0°C incubation” state. The data presented in this work point to the conclusion that the “0°C incubation” state is indeed an S−2 oxidation state with half-integer spin.