The oxygen flash yield and the kinetics of Chl a + ii (P-680 + ) reduction have been measured under repetitive excitation as a function of pH between pH 4.0 and pH 9.0 in oxygen-evolving PS ii particles from Synechoco...
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The oxygen flash yield and the kinetics of Chl a + ii (P-680 + ) reduction have been measured under repetitive excitation as a function of pH between pH 4.0 and pH 9.0 in oxygen-evolving PS ii particles from Synechococcus sp. (i) The optimum of oxygen yield is observed between pH 6.5 and pH 7.5. The inhibition in the acidic pH region is reversible and can be described by a monoprotic binding site with a p K value of about 4.5. In the alkaline pH region the inhibition is half maximal at pH 8.3 and might be described by the titration of three binding sites or more. The loss of oxygen evolution at pH 9.0 is caused by reversible inhibition and irreversible inactivation. (ii) Between pH 4.0 and pH 7.5 the fraction of Chl a + ii decaying in the nanosecond time range and the oxygen yield follow the same pH dependence. (iii) Both in Photosystem ii centers reversibly inhibited at low pH and in Photosystem ii centers inactivated at high pH, Chl a + ii is reduced by a donor Z, different from the normal immediate donor D 1 or a modified state of D 1 , and, in part, by back reaction. (iv) Below pH 5.0, the decay in the nanosecond range can be explained by the existence of two phases with t 1 2 ≈ 42 ns and t 1 2 ≈ 300 ns (ratio of amplitudes, 1.3:1). A reduction phase with t 1 2 = 20 ns that is the major phase around the pH optimum is not observed below pH 5.0.
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