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.
Keywords: EPR; ESR; Mn cluster; Oxygen-evolving complex; Photosystem II.
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