Thermodynamics of the charge recombination in photosystem II.

The temperature dependence of the electric field-induced chlorophyll luminescence in photosystem II was studied in Tris-washed, osmotically swollen spinach chloroplasts (blebs). The system II reaction centers were brought in the state Z(+)P(+)-QA (-)QB (-) by preillumination and the charge recombination to the state Z(+)PQAQB (-) was measured at various temperatures and electrical field strengths. It was found that the activation enthalpy of this back reaction was 0.16 eV in the absence of an electrical field and diminished with increasing field strength. It is argued that this energy is the enthalpy difference between the states IQA (-) and I(-)QA and accounts for about half of the free energy difference between these states. The redox state of QB does not influence this free energy difference within 150 μs after the photoreduction of QA. The consequences for the interpretation of thermodynamic properties of QA are discussed.