Time-resolved Mn Kα emission reveals early redox dynamics in the S3 to S0 transition of the photosystem II Kok cycle

Photosystem II (PS II) catalyzes the light-driven oxidation of water to O2. Kok cycle describes this four-electron process in the oxygen-evolving complex (Mn4Ca cluster) of PS II. PSII catalytic cycle has been analyzed with time-resolved methods such as time-resolved crystallography, optical pump-probe and X-ray spectroscopies to identify the key mechanistic features of the process. The consensus is that O-O bond is forming in the S3 to S0 transition of the Kok cycle, but the mechanistic details of this process remain unconfirmed. Time-resolved X-ray emission spectroscopy (TR-XES) allows the analysis of the Kok cycle where spectroscopic changes potentially reflect the O-O bond formation. A new spectrometer was commissioned for optimized detection of the Mn Kα lines. S0-S1-S2-S3 states of the Kok cycle were probed, including 200 and 500 μsec time points in the S3 to S0 transition. The Mn Kα1 spectral line is sensitive to the Mn oxidation state via the line width and position. The line width measured in PS II is consistent with Mn4Ca cluster MnIII and MnIV oxidation states. Line position shifts to low energy during oxidation in the S1-S2-S3 transitions and to higher energy for all states after S3. Results have shown spectral changes as early as 200 μsec in the S3-to-S0 transition, in agreement with earlier reports of the Mn Kβ TR-XES. These were attributed to significant changes in the electronic structure of the Mn centers consistent with the reduction of the Mn ion from IV to III oxidation state, which are likely to accompany the O-O bond formation.