Proton Translocation via Tautomerization of Asn298 During the S(2)–S(3) State Transition in the Oxygen-Evolving Complex of Photosystem II

[Image: see text] In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y(Z)) mediates electron transfer between the Mn(4)CaO(5) cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S(i) (i = 0–4). In contrast to lower S-states (S(0), S(1)), in higher S-states (S(2), S(3)) of the Mn(4)CaO(5) cluster, Y(Z) cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S(1) → S(2) transition. However, oxidation of Y(Z) by illumination of S(2) at 77–190 K followed by rapid freezing and charge recombination between Y(Z)(•) and the plastoquinone radical Q(A)(•–) allows trapping of an S(2) variant, the so-called S(2)(trapped) state (S(2)(t)), that is capable of forming Y(Z)(•) at cryogenic temperature. To identify the differences between the S(2) and S(2)(t) states, we used the S(2)(t)Y(Z)(•) intermediate as a probe for the S(2)(t) state and followed the S(2)(t)Y(Z)(•)/Q(A)(•–) recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H(2)O and D(2)O. The results show that while S(2)(t)Y(Z)(•)/Q(A)(•–) recombination can be described as pure electron transfer occurring in the Marcus inverted region, the S(2)(t) → S(2) reversion depends on proton rearrangement and exhibits a strong kinetic isotope effect. This suggests that Y(Z) oxidation in the S(2)(t) state is facilitated by favorable proton redistribution in the vicinity of Y(Z), most likely within the hydrogen-bonded Y(Z)–His190–Asn298 triad. Computational models show that tautomerization of Asn298 to its imidic acid form enables proton translocation to an adjacent asparagine-rich cavity of water molecules that functions as a proton reservoir and can further participate in proton egress to the lumen.