Electronic couplings and energy transfer dynamics in the oxidized primary electron donor of the bacterial reaction center

It has been known for 30 years that the oxidized special pair radical cation P⁺ is as efficient as the neutral ground-state species P in quenching excitation from the neighboring accessory bacteriochlorophylls B_L and B_M, but the mechanism for this process has remained elusive. Indeed, simple treatments based on application of standard Förster theory to the most likely acceptor candidate fails by 5 orders of magnitude in the prediction of the energy transfer rates to P⁺. We present a qualitative description of the electronic energy transfer (EET) dynamics that involves mixing of the strongly allowed transitions in P⁺ with a manifold of exotic lower-energy transitions to facilitate EET on the observed time scale of 150 fs. This description is obtained using a three-step procedure. First, multireference configuration-interaction (MRCI) calculations are performed using the semiempirical INDO/S Hamiltonian to depict the excited states of P⁺. However, these calculations are qualitatively indicative but of insufficient quantitative accuracy to allow for a fully a priori simulation of the EET and so, second, the INDO results are used to establish a variety of scenarios, empirical parameters that are then fitted to describe a range of observed absorption and circular dichroism data. Third, EET according to these scenarios is predicted using a generalized Förster theory that uses donor and acceptor transition densities, which together account for the large size of the chromophores in relation to the interchromophore spacings. The spectroscopic transitions of P⁺ that facilitate the fast EET are thus unambigously identified.