Evidence for two tightly bound iron‐quinones in the electron acceptor complex of photosystem II

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 boundCO2the 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 atEm~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 inChlamydomonas reinhardtiipreparations which lack the gating quinone.