Proton Translocation via Tautomerization of Asn298 During the S2–S3 State Transition in the Oxygen-Evolving Complex of Photosystem II

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or YZ) mediates electron transfer between the Mn4CaO5 cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states Si (i = 0–4). In contrast to lower S-states (S0, S1), in higher S-states (S2, S3) of the Mn4CaO5 cluster, YZ cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S1 → S2 transition. However, oxidation of YZ by illumination of S2 at 77–190 K followed by rapid freezing and charge recombination between YZ• and the plastoquinone radical QA•– allows trapping of an S2 variant, the so-called S2trapped state (S2t), that is capable of forming YZ• at cryogenic temperature. To identify the differences between the S2 and S2t states, we used the S2tYZ• intermediate as a probe for the S2t state and followed the S2tYZ•/QA•– recombination kinetics at 10 K using time-resolved electron paramagneti...