A theory of electron transfer between two fixed sites by tunneling is developed. Vibronic coupling in the individual molecules produces an activation energy to transfer at high temperatures, and temperature-independent tunneling (when energetically allowed) at low temperature. The model is compared with known results on electron transfer in Chromatium and in Rhodopseudomonas spheroides. It quantitatively interprets these results, with parameters whose scale is verified by comparison with optical absorption spectra. According to this description, the separation between linking sites for electron transfer is 8-10 A in Chromatium, far smaller than earlier estimates.
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