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

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 wav; t½ 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.