Anisotropic magnetic interactions in the primary radical ion-pair of photosynthetic reaction centers

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Abstract
The quantum yield of triplets formed by ion-pair recombination in quinone-depleted reaction centers [of photosynthetic bacteria] depended on their orientation in a magnetic field. This new effect is expected to be a general property of radical pair reactions in the solid state. For 0 < H [magnetic field strength] < 1000 G (Gauss), the quantum yield anisotropy is caused by anisotropic electron dipole-electron dipole or nuclear hyperfine interactions, or both. For high fields it is dominated by the anisotropy of the difference g-tensor in the radical ion-pair. The magnitude and sign of the contribution of each interaction depended on the values of the principal components of each anisotropic tensor and on the geometric relationship of the principal axes of each tensor to the transition dipole moment used to detect the yield. A detailed formalism is presented relating these quantities to the observed yield anisotropy. The expected magnitude of each anisotropic parameter is discussed. The field dependence of the yield anisotropy is consistent with these values for certain reaction center geometries.