Abstract
The redox potential of pheophytin a, Em(Ph/Ph-), is one of the important physicochemical parameters for discussing the energetics within PSII; however, previous values are not consistent with energetic relationships estimated from kinetic analyses. This situation should be due to problems of titration employed traditionally for measuring Em(Ph/Ph-). In view of this, we have established spectroelectrochemical conditions overcoming the problems, and obtained a value of -505 mV for Em(Ph/Ph-) using PSII isolated from Thermosynechococcus elongatus (Kato et al. 2009). In this work, we investigated the influence of difference in the core proteins D1 of PSII on Em(Ph/Ph-), because T. elongatus has three D1 genes. The measurement for D1:1, which is expressed under normal cultivation conditions, yielded -522 mV; this result means the wild type maintains more negatively by 17 mV than that obtained by the previous measurement in D1:3, which is similar to the higher plant type.