Species‐dependence of the redox potential of the primary quinone electron acceptor Q A in photosystem II verified by spectroelectrochemistry

The redox potentials E m (Q A /Q A −) of the primary quinone electron acceptor Q A in oxygen‐evolving photosystem II complexes of three species were determined by spectroelectrochemistry. The E m (Q A /Q A −) values were experimentally found to be −162 ± 3 mV for a higher plant spinach, −171 ± 3 mV for a green alga Chlamydomonas reinhardtii and −104 ± 4 mV vs. SHE for a red alga Cyanidioschyzon merolae . On the basis of possible deviations for the experimental values, as estimated to differ by 9–29 mV from each true value, plausible causes for such remarkable species‐dependence of E m (Q A /Q A −) are discussed, mainly by invoking the effects of extrinsic subunits on the delicate structural environment around Q A .