Abstract
The redox potential of the primary electron acceptor, pheophytin a (Ph), is an important physicochemical parameter for charge separation in Photosystem II, because this potential should be a base point for speculations of the potential of the primary donor P680 inducing a strong power enough for water oxidation, which has not been measured. However, the reported values of Ph redox potential should not be necessarily reliable, though Klimov et al. reported -610 mV vs. SHE and an additional measurement was performed about 30 years ago. Possible problems in this reliability should be due to a titration technique; further they performed measurements at pH 11.0, different from physiological condition, to enhance a reductive power of titrants. In the present study, we have tried to measure the Ph redox potential at pH 6.5 by spectroelectrochemistry, leading us to find the potential higher by ca. 100 mV than the reported values.
