Electron acceptors of photosystem 2 in the cyanobacterium Phormidium laminosum

Photosystem 2 preparations with very high rates of oxygen evolution from the thermophilic cyanobacterium Phormidium laminosum have been studied by EPR spectrometry. In the presence of DCMU the g = 1.82 signal of the iron—quinone electron acceptor (Q) can be observed. It is proposed that DCMU is necessary to disrupt a magnetic interaction, between the semiquinone forms of Q and the secondary acceptor B, which otherwise prevents detection of the Q−Fe signal. A doublet EPR signal arising from magnetic interaction between Q−Fe and the reduced intermediary electron acceptor pheophytin (I−), and a spin‐polarized triplet signal assumed to arise from the back reaction between I− and P680+ can also be seen. Preliminary redox titrations of Q reduction have been carried out, indicating E m ⋍ 0 mV.