Oxidation-reduction properties of the electron acceptors of Photosystem II I. Redox titration of the flash-induced carotenoid band shift, of C550 and of the variable fluorescence yield in spinach chloroplasts

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Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential ( E m,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, Q H. A second titration wave is observed at low potential ( E m,7.5 ⋍ − 240 mV ) and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at − 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, Q L) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Q β is indeed characterized by an E m,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with Q L.