We have extended the radical pair theory to treat systems of membrane-bound radicals with g tensor anisotropy. Analysis of the polarized electron paramagnetic resonance (EPR) signals of P700+, originating from photosystem I of higher plants, in terms of the radical pair mechanism provides information about the sequence of early electron acceptors. To account for the orientation dependence of the line shape and integrated area of this polarized signal, we propose the electron transfer sequence to be P700 leads to A1 leads to X leads to Fd(A, B), where A1 is a small organic molecule (possibly chlorophyll), X is the acceptor species observed recently in low-temperature EPR studies, and Fd(A, B) are the ferredoxin iron-sulfur centers A and B. Our calculations provide information about the life-times of A1-, and X-, and their exchange interactions with P700+. We also find supporting evidence for the orientation of X- in the thylakoid membrane reported recently by G. C. Dismukes and K. Sauer (Biochim. Biophys. Acta. 504:431–445.).
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