AbstractLow temperature photosystem 2 emission (F685, F695) of chloroplasts may originate from a small pool of pigments. However, the excitons reaching the reaction center have been generated in a much larger pool of chlorophylls constituting the almost non-fluorescent light-harvesting complex. Based upon structural information an interpretation for the molecular origin of the F695 emission of variable yield was proposed [FEBS Lett. (1982) 147, 17–20]: in reaction centers having the quinone acceptor reduced, a charge recombination occurring between the primary donor P680+ and Phe− (the pheophytin primary acceptor) can generate a singlet excited state of Phe which deactivates by emitting F695. Here, an analogous process is discussed for F685 with the emission occurring either from P680 directly or from the small pool of core antenna chlorophylls surrounding the reaction center. Furthermore, the presence of F695 in the low temperature emission spectra of dark-adapted chloroplasts leads us to propose that charge recombination also takes place in open reaction centers when the quinone acceptor is oxidized. In this case the short lifetime (130 ± 20 ps) observed for the singlet exciton in the intact membrane suggests that the rate-limiting step in conditions of active photosynthesis is more probably determined by the stabilization of the negative charge on the quinone than by either the rate of energy transfer among antenna or the rate of trapping by the reaction center
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