Species-dependence of the redox potential of the primary quinone electron acceptor QA in photosystem II verified by spectroelectrochemistry

The redox potentials Em(QA/QA−) of the primary quinone electron acceptor QA in oxygen-evolving photosystem II complexes of three species were determined by spectroelectrochemistry. The Em(QA/QA−) 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 Cyanidioschyzonmerolae. 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 Em(QA/QA−) are discussed, mainly by invoking the effects of extrinsic subunits on the delicate structural environment around QA.