Temperature dependence of the oxidation kinetics of TyrZ and TyrD in oxygen-evolving photosystem II complexes throughout the range from 320K to 5K.

Abstract The photo-induced oxidation of Tyr Z and Tyr D by P680 •+ , that involves both electron and proton transfer (PCET), has been studied in oxygen-evolving photosystem II from Thermosynechococcus elongatus . We used time-resolved absorption spectroscopy to measure the kinetics of P680 •+ reduction by tyrosine after the first flash given to dark-adapted PS II as a function of temperature and pH. The half-life of Tyr Z oxidation by P680 •+ increases from 20 ns at 300 K to about 4 μs at 150 K. Analyzing the temperature dependence of the rate, one obtains a reorganization energy of about 770 meV. Between 260 K and 150 K, the reduction of P680 •+ by Tyr Z is increasingly replaced by charge recombination between P680 •+ and Q A •− . We propose that the driving force for Tyr Z oxidation by P680 •+ decreases upon lowering the temperature. Tyr Z oxidation cannot be excluded in a minority of PS II complexes at cryogenic temperatures. Tyr D oxidation by P680 •+ with a half-life of about 30 ns was observed at high pH. The pH dependence of the yield of Tyr D oxidation can be described by a single protonable group with a pK of approximately 8.4. The rate of Tyr D oxidation by P680 •+ is virtually identical upon substitution of solvent exchangeable protons with deuterons indicating that the rate is limited by electron transfer. The rate is independent of temperature between 5 K and 250 K. It is concluded that Tyr D donates the electron to P680 •+ via P D2 .