Temperature dependence of P680⁺ reduction in O₂‐evolving PS II membrane fragments at different redox states Si of the water oxidizing system

The electron transfer kinetics from Z to P680 + was analyzed as a function of temperature in the range of 248< T < 295 K by measuring absorption changes induced at 830 nm by a laser flash train in dark adapted O 2 evolving PS II membrane fragments from spinach. It was found: (i) that the kinetics of P680 + reduction and their dependence on the redox state S i of the catalytic site of water oxidation are only slightly affected by temperature within the physiological range of 270< T <295 K. (ii) In the dark relaxed state S, the electron transfer from Z to P680 + exhibits an activation energy of the order of 10 kJ/mol in 248< T <295 K. (iii) In the 2nd and subsequent flashes of the train the ability for a stable charge separation between P680 + and Q − A , markedly decreases below −10°C. This phenomenon is assumed to be due to a strong effect of temperature on the electron transfer from Q − A to Q B . The results are briefly discussed in relation to possible effects of structural changes in the D‐1/D‐2 polypeptide complex on the reaction coordinate of electron transfer steps in PS II.