Indirect evidence for structural changes coupled with Q−⋅B formation in photosystem II

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Abstract
The thermal blockage of Q−⋅ A oxidation was analysed in PS II membrane fragments by monitoring flash‐induced changes of the relative fluorescence quantum yield as a function of temperature. The results obtained reveal: (a) in dark‐adapted samples the fraction of Q−⋅ A that is not reoxidised within a time domain of 10 s after the actinic flash increases with lowering the temperature (half‐maximum effect at 250–260 K), (b) at low temperatures where Q−⋅ A generated in dark‐adapted samples remains almost completely reduced, a significant extent of Q−⋅ A reoxidation arises when samples are used that were preilluminated at room temperature by one saturating flash followed by rapid freezing before performing the experiment, and (c) the extent of Q−⋅ A that is reoxidised at 258 K exhibits a characteristic binary oscillation as a function of the number of preillumination flashes given at room temperature. Based on these data it is inferred that QB and Q−⋅ B are located at different equilibrium positions in the QB site. As a consequence the formation of Q−⋅ B is coupled with significant structural changes that require sufficient flexibility of the protein matrix. This general feature corresponds with a recently proposed model for the acceptor side reactions of anoxygenic bacteria [Stowell, M.H.B., McPhillips, T.M., Rees, D.C., Soltis, S.M., Abresch, E. and Feher, G., Science 276 (1997) 812–816].