Oxidation‐reduction potential dependence of reaction centre triplet formation in the isolated D1/D2/cytochrome b‐559 photosystem II complex

Formation of a spin‐polarized triplet on illumination at low temperature is thought to be characteristic of charge separation in photosynthetic reaction centres. The formation of a spin‐polarized triplet was confirmed in the D1/D2/cytochrome b‐559 photosystem II reaction centre preparation. The oxidation‐reduction potential range in which triplet formation occurs was determined. Triplet formation was observed at potentials between 400 and −530 mV. Loss of triplet formation, at more negative potentials, is suggested to reflect chemical reduction of the pheophytin acceptor. At potentials more oxidised than 400 mV, illumination results in formation of a radical at g = 2.0025 with a 0.75 mT linewidth. We propose that this is the result of oxidation of P680 coupled to reduction of an electron acceptor, possibly non‐haem iron. The results confirm that none of the other electron acceptors normally found in photosystem II are functional in this preparation.