Abstract
The redox potential of the primary plastoquinone electron acceptor QA, Em(QA/QA-), in photosystem (PS) II is one of the important physicochemical parameters for discussing the energetics within PS II. Though many studies for the measurement of Em(QA/QA-) have been reported, however, non-negligible experimental errors and scattering values among researches have prevented us discussing the energetics in detail. In this work, Em(QA/QA-) in an oxygen-evolving PS II complex from a thermophilic cyanobacterium Thermosynechococcus elongatus was determined to be -140 mV vs. SHE (with a error range of 2 mV) by thin-layer cell spectroelectrochemistry for the first time. The Em(QA/QA-) value obtained here together with the recently determined redox potential of pheophytin (Phe) a (Kato et al., 2009) yields -330 to -370 mV for the free energy change by electron transfer from Phe a- to QA, and provides a renewed picture for the energetics on the electron acceptor side in PS II.
