The EPR signal of the iron-quinone electron acceptor of photosystem II in higher plant chloroplasts is normally difficult to observe. If the preparation is washed with formate to remove bound CO 2 the signal becomes larger. Using formate-washed photosystem II particles from peas the redox potential of the ironquinone complex has been determined. Two waves are observed in the titrations at E m ~0 and ~ −250 mV. The ability to reduce the pheophytin intermediary electron acceptor by illumination at 200 K is largely associated with the −250 mV step. It is suggested that there are two tightly bound quinones in the reaction centre and that the low-potential component is Qa. It is unlikely that the high-potential component is the gating quinone (Qb) as it is not displaced by DCMU and is present in Chlamydomonas reinhardtii preparations which lack the gating quinone.
