Thermodynamics of electron transport in Photosystem I studied by electric field-stimulated charge recombination

The kinetics of chlorophyll luminescence induced by externally applied electric-field pulses in osmotically swollen thylakoid systems are known to consist of a fast and a slow phase, attributed to charge recombinations in Photosystem I and II, respectively (Symons, M., Korenstein, R. and Malkin, S. (1986) Biochim. Biophys. Acta 806, 305-310). We confirm this assignment and show that conditions can be created under which the phases can be studied separately. A previously unresolved 80 μs decay of the precursor of Photosystem II electroluminescence is attributed to manganese oxidation. After illumination the precursor of Photosystem I electroluminescence decayed biexponentially with a major component with a half-time of 70 ms and a minor component with a half-time of about 15 ms at room temperature. The same kinetics were observed in the absorbance difference at 700 nm and in 'normal' Photosystem I luminescence. On the basis of these findings the Photosystem I electroluminescence is attributed to charge recombination between the oxidized primary donor P⁺-700 and the reduced electron acceptor F_A^- (70 ms component) and possibly F_B^-. The enthalpy of activation of the first reaction was determined at 460 ± 30 meV. Field-induced absorbance changes showed that charge recombination occurred in up to 20% of the reaction centers. It is further shown that in Photosystem I, in contrast to Photosystem II, the luminescence yield is enhanced by an external electric field with at least a factor of 150 at higher field strengths. The results are discussed in terms of a thermodynamical model. It is concluded that in PS I the primary charge separation, P-700A₀ → P⁺-700A^-₀, as well as subsequent electron transport to F_A and F_B are electrogenic.