A new look at the primary charge separation in bacterial photosynthesis

A theory for the primary charge separation in photosynthetic bacteria is presented. We propose that this reaction may lie in a regime that is different from the traditional nonadiabatic (golden rule) regime. In the context of the primary charge separation, the assumption of nonadiabaticity implies that vibrational relaxation in (BChl)₂⁺(BPh)^- is much faster than the electron-transfer rate from (BChl)₂* to BPh (i.e., much faster than a picosecond). Instead, we propose that vibrational relaxation in the charge-separated state might well compete with the rate of initial electron transfer. To describe such a regime, we abandon the nonadiabatic theory and suggest that in the case of this reaction, the vibronic mixings between initial and final vibronic states are of the same order of magnitude as the vibronic widths of the final states. When this is true, the transfer rate competes with relaxation in the final vibronic manifold. We show that the proposed regime is plausible for the primary charge separation since it predicts reasonable values for the relevant parameters of the reacting system (vibronic mixings, widths, etc.), and it is consistent with several recent experiments. It also explains the robustness of the primary rate to changes in the energy gap, temperature, and initial excitation in P*I.