An electroluminescence study of stabilization reactions in the oxygen-evolving complex of Photosystem II

The stabilization of the photosynthetic charge separation in Photosystem II by secondary reactions was studied using chlorophyll luminescence induced by electric field pulses in a suspension of preilluminated osmotically swollen chloroplasts. This 'electroluminescence' was measured as a function of the delay time between illumination and field pulse, and as a function of the number of preilluminating flashes. The result is a survey of, in principle, all stabilization and deactivation processes beyond the state Z⁺Q^- _A, which is formed within the approx. 20 μs time resolution of the method. Most of these could be identified with known secondary electron transfer reactions. A 20-fold stabilization with a half-time of 330 μs is attributed to Q^- _A reoxidation. No further stabilization at the acceptor side seemed to occur and no flash number dependence was detected, although a normal Q_B/Q^- _B oscillation was found in ultraviolet absorbance. With regard to the donor side, the data are consistent with the known S-state-dependent Z⁺ reduction times and indicate values of 9, 5 and 65 for the equilibrium constants associated with this reaction on the transitions S₁→S₂, S₂→S₃ and S₃→S₀(O₂) respectively. Z⁺ reduction was found to be electrogenic and exposed to about 5% of the membrane potential. An 0.1 s phase in S₀ is attributed to oxygen release. S₂ and S₃ are further stabilized in two phases of unknown origin with half-times of 15 ms and 0.4 s, followed by a final 20 s phase attributed to deactivation. In S₁, Z⁺ reduction was probably hidden in an unresolved fast phase present on all transitions, but in addition a 350 μs phase was found, which might be related to proton release. In nearly 20% of the Photosystem II reaction centers electron transfer beyond Q^- _A was inhibited. In these centers Z⁺ reduction appeared to take about 1 ms and charge recombination followed in phases of about 8, 80 and 800 ms half-time.