Photochemistry in the isolated Photosystem II reaction-centre core complex.

The photochemistry of the isolated Photosystem II reaction-centre core from pea and the green alga Scenedesmus was examined by e.s.r. Two types of triplet spectrum were observed in addition to the spin-polarized reaction-centre triplet previously identified. The additional triplet formed on continuous illumination at 4.2 K was attributed to a monomeric phaeophytin molecule. The second triplet, which was stable in the dark at 4.2 K following illumination, was assigned to the radical pair Donor+I-. This provides evidence that an electron donor to chlorophyll P680 is present on the polypeptide D1-polypeptide D2-cytochrome b-559 core complex.