Oxidation-reduction potential dependence of low-temperature photoreactions of chloroplast Photosystem II |
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Authors: | Alan J Bearden Richard Malkin |
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Institution: | 1. Donner Laboratory, University of California, Berkeley, Calif. 94720 U.S.A.;2. Department of Cell Physiology, University of California, Berkeley, Calif. 94720 U.S.A. |
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Abstract: | The light-induced free-radical signal of Photosystem II (observed after illumination at 77 °K) has been studied in chloroplasts as a function of the oxidation-reduction potential established prior to freezing. The intensity of the light-induced signal is unchanged in the potential region of +590 mV to +760 mV. At higher potential (+850 mV), there is a 30% decrease in signal intensity. The light-induced signal decreases to zero in the low-potential region, with a midpoint potential of +475 mV. These results are considered in terms of a Photosystem II reaction-center complex in which the light-induced free-radical signal arises from the oxidized form of the reaction-center chlorophyll, and this chlorophyll molecule is capable of being reduced at liquid-nitrogen temperature by a secondary electron donor which has a midpoint oxidation-reduction potential of +475 mV. |
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