Energetics of primary and secondary electron transfer in Photosystem II membrane particles of spinach revisited on basis of recombination-fluorescence measurements

Photon absorption by one of the roughly 200 chlorophylls of the plant Photosystern II (PSII) results in formation of an equilibrated excited state (Chl200*) and is followed by chlorophyll oxidation (formation of P680+) coupled to reduction of a specific pheophytin (Phe), then electron transfer from Phe- to a firmly bound quinone (QA), and subsequently reduction of P680+ by a redox-active tyrosine residue denoted as Z. The involved free-energy differences (Delta G) and redox potentials are of prime interest. Oxygen-evolving PSII membrane particles of spinach were studied at 5 degrees C. By analyzing the delayed and prompt Chl fluorescence, we determined the equilibrium constant and thus free-energy difference between Chl200* and the [Z+,QA-] radical pair to be -0.43 +/- 0.025 eV, at 10 mu s after the photon absorption event for PSII in its S-3-state. On basis of this value and previously published results, the free-energy difference between P680* and [P680+,QA-] is calculated to be -0.50 +/- 0.04 eV, the free-energy loss associated with electron transfer from Phe to QA is found to be 0.34 +/- 0.04 eV. The given uncertainty ranges do not represent a standard deviation or likely error, but an estimate of the maximal error. Assuming a QA-/QA redox potential of -0.08 V [Krieger et al., 1995, Biochim. Biophys. Acta 1229, 193], the following redox-potential estimates are obtained: +1.25 V for P680/P680+; +1.21 V for Z/Z+ (at 10 mu s); -0.42 V for Phe-/Phe; -0.58 V for P680*/P680+. (c) 2005 Elsevier B.V. All rights reserved.