Water Oxidation by Photosystem II: H2O−D2O Exchange and the Influence of pH Support Formation of an Intermediate by Removal of a Proton before Dioxygen Creation

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Abstract
Understanding the chemistry of photosynthetic water oxidation requires deeper insight into the interrelation between electron transfer (ET) and proton relocations. In photosystem II membrane particles, the redox transitions of the water-oxidizing Mn complex were initiated by nanosecond laser flashes and monitored by absorption spectroscopy at 360 nm (A360). In the oxygen evolution transition (S3 + hν → S0 + O2), an exponential decrease in A360 (τO2 = 1.6 ms) can be assigned to Mn reduction and O2 formation. The corresponding rate-determining step is the ET from the Mn complex to a tyrosine radical (YZox). We find that this A360 decrease is preceded by a lag phase with a duration of 170 ± 40 μs (τlag at pH 6.2), indicating formation of an intermediate before ET and O−O bond formation and corroborating results obtained by time-resolved X-ray spectroscopy. Whereas τO2 exhibits a minor kinetic isotope effect and negligible pH dependence, formation of the intermediate is slowed significantly both in D2O (τlag increase of ∼140% in D2O) and at low pH (τlag of 30 ± 20 μs at pH 7.0 vs τlag of 470 ± 80 μs at pH 5.5). These findings support the fact that in the oxygen evolution transition an intermediate is created by deprotonation and removal of a proton from the Mn complex, after YZox formation but before the onset of electron transfer and O−O bond formation.

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