Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to the production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe) a. However, the reported values for Phe a are still controversial. Here, we measured the redox potential of Phe a under physiological conditions (pH 7.0; 25°C) in two cyanobacteria with different special pair chlorophylls (Chls): Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chl a, and Acaryochloris marina MBIC 11017, whose special pair for PS II consists of Chl d. We obtained redox potentials of −536 ± 8 mV for Synechocystis sp. PCC 6803 and −478 ± 24 mV for A. marina on PS II complexes in the presence of 1.0 M betaine. The difference in the redox potential of Phe a between the two species closely corresponded with the difference in the light energy absorbed by Chl a versus Chl d. We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V for Synechocystis sp. PCC 6803 (P680) and A. marina (P713), respectively. This clearly indicates conservation in the properties of water-oxidation systems in oxygenic photosynthetic organisms, irrespective of the special-pair chlorophylls.hotosynthesis mediates the conversion of solar light energy to chemical-bond energy through multistep reactions. Two photosystems (PSs) are present in oxygenic photosynthetic organisms, and these two PSs function cooperatively to capture light energy and drive electron flow. PS II supplies an energy source (i.e., an electron) by water oxidation, and PS I supplies a highly reduced compound, NADPH, to reduce CO 2 to carbohydrates.Reaction processes in the electron transfer system in photosynthesis are governed by two major factors: the relative geometry and the redox potentials of the electron transfer components. The molecular environment supplied by the amino acid matrix of the components will give a supplemental effect(s). Crystal structures of PS II complexes at atomic resolution have been reported from several laboratories (1-4). Thus, with the exception of some inconsistencies in the water-oxidation reaction system, an essential part of the primary charge separation machinery has been characterized (5). The electron transfer mechanisms, in contrast,have not yet been clarified in most cases.Pheophytin (Phe) a is the primary electron acceptor in PS II (6-8), although the primary electron donor of PS II is still controversial [P680 or accessory chlorophyll (Chl) a] (9, 10). These two are not in disagreement with respect to the nature of the primary charge separation, but they differ in the value of rate constants and the question of "transfer to the trap limited" or "trap-limited reaction" (5). In this report, we used the term the special pair instead of the primary electron...