Temperature dependence of the formation of the g ~ 5 EPR signal in the oxygen evolving complex of photosystem II

Abstract

The temperature dependence of the formation of the g ~ 5 S₂ state electron paramagnetic resonance (EPR) signal in photosystem II (PSII) was investigated. The g ~ 5 signal was produced at an illumination above 200 K. The half inhibition temperature of the formation of the g ~ 5 EPR signal was approximately 215 K. The half inhibition temperature is close to that of the transition from the S₂ state-to-S₃ state in the untreated PSII, and not to that of the transition from S₁ state -to-S₂ state in the untreated PSII. The upshift of the half inhibition temperature of the transition from the S₁ state -to-S₂ state (g ~ 5) reflects the structural change upon transition from the S₁ state to the S₂ state. The activation energy of the g ~ 5 state formation was estimated as 40.7 ± 4.4 kJ/mol, which is comparable to the reported activation energy for the S₂ formation in the untreated PSII. The activation enthalpy and entropy were estimated to be 39.0 ± 4.4 kJ/mol and − 103 ± 19 J/mol K at 210 K, respectively. Based on these parameters, the formation process of the g ~ 5 state is discussed in this study.