pH and Temperature Dependence of Tyrosine Z^・ Decay Kinetics in Tris-Treated PSII Particles Studied by Time-Resolved EPR

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The decay kinetics of tyrosine Z^・ (Y_z^・) in Tris-treated Photosystem II particles were measured by time-resolved EPR at different pH values (pH 5.5 to 7.5) between 230 and 297 K. Y_z^・ induced by laser flashes decayed in a biphasic way; t_<1/2> values were about 100 ms in the fast phase and about 1 s in the slow one, respectively, at room temperature. The fast phase was attributed to a recombination of charges on Y_z^・ and Q_A^-. The activation energies for the reaction of Y_z^・ with Q_A^- between 245 and 297 K have been estimated to be more than 38 kJ mol^<-1> at pH (>6.5) and less than 32 kJ mol^<-1> at pH (<6.0), respectively. The activation energy gap between high pH (>6.5) and low pH (< 6.0) ranges was found to be ΔE=4.4 kJ mol^<-1>. Judging from the fact that pH 6.2 appears to be the border between the high and low pH regions, we suggest that the kinetics of Y_z^・ is strongly influenced by the dissociation of the nearby histidine residue.
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