In photosynthetic reaction centers from purple bacteria (pbrcs) and photosystem ii (psii), the photoinduced charge separation is terminated by an electron transfer between the. Transfer of the first electron to the primary (qa) and secondary (qb) quinone electron. Kato y, nagao r, noguchi t (2016) redox potential of the terminal quinone electron acceptor qb in photosystem ii reveals the mechanism of electron transfer regulation.