The S0 state of the water oxidizing complex in photosystem II : pH dependence of the EPR Split signal, induction and mechanistic implications

• Water oxidation in photosystem II is catalyzed by the CaMn4 cluster.   The electrons extracted from the CaMn4 cluster are transferred to   P-680(+) via the redox-active tyrosine residue D1-Tyr161 (Y-Z). The   oxidation of Y-Z is coupled to a deprotonation creating the neutral   radical Y-Z(center dot). Light-induced oxidation of Y-Z is possible   down to extreme temperatures. This call be observed as a split EPR   signal from Y-Z(center dot) in a magnetic interaction with the CaMn4   cluster, offering a way to probe for Y-Z oxidation in active PSII. Here   we have used the split S-0 EPR signal to study the mechanism of Y-Z   oxidation at 5 K in the S-0 state. The state of the hydrogen bond   between Y-Z and its proposed hydrogen bond partner D1-His190 is   investigated by varying the pH. The split S-0 EPR signal was induced by   illumination at 5 K between pH 3.9 and pH 9.0. Maximum signal intensity   was observed between pH 6 and pH 7. On both the acidic and alkaline   sides the signal intensity decreased with the apparent pK(a)s (pK(app))   similar to 4.8 and similar to 7.9, respectively. The illumination   protocol used to induce the split S-0 EPR signal also induces a mixed   radical signal in the g similar to 2 region. One part of this signal   decays with similar kinetics as the split S-0 EPR signal (similar to 3   min, at 5 K) and is easily distinguished from a stable radical   originating from Car/Chi. We suggest that this fast-decaying radical   originates from Y-Z(center dot). The pH dependence of the light-induced   fast-decaying radical was measured in the same pH range. as for the   split S-0 EPR signal. The pK(app) for the light-induced fast-decaying   radical was identical at acidic pH (similar to 4.8). At alkaline pH the   behavior was more complex. Between pH 6.6 and pH 7.7 the signal   decreased with pK(app) similar to 7.2. However, above pH 7.7 the   induction of the radical species was pH independent. We compare our   results with the pH dependence of the split S-1 EPR signal induced at 5   K and the S-0 -> S-1 and S-1 -> S-2 transitions at room temperature.   The result allows mechanistic conclusions concerning differences   between the hydrogen bond pattern around Y-Z in the S-0 and S-1 states.

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NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

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Chemistry

Kemi

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