Enhancement of YD• spin relaxation by the CaMn4 cluster in photosystem II detected at room temperature: A new probe for the S-cycle

The long-lived, light-induced radical YD• of the Tyr161 residue in the D2 protein of Photosystem II (PSII) is known to magnetically interact with the CaMn4 cluster, situated ∼ 30 Å away. In this study we report a transient step-change increase in YD• EPR intensity upon the application of a single laser flash to S1 state-synchronised PSII-enriched membranes from spinach. This transient effect was observed at room temperature and high applied microwave power (100 mW) in samples containing PpBQ, as well as those containing DCMU. The subsequent decay lifetimes were found to differ depending on the additive used. We propose that this flash-induced signal increase was caused by enhanced spin relaxation of YD• by the OEC in the S2 state, as a consequence of the single laser flash turnover. The post-flash decay reflected S2 → S1 back-turnover, as confirmed by their correlations with independent measurements of S2 multiline EPR signal and flash-induced variable fluorescence decay kinetics under corresponding experimental conditions. This flash-induced effect opens up the possibility to study the kinetic behaviour of S-state transitions at room temperature using YD• as a probe.