Primary charge separation in Chloroflexus aurantiacus reaction centers at room temperature: ultrafast transient absorption measurements on Q-depleted preparations with native and chemically modified bacteriopheophytin composition.

The initial electron transfer (ET) processes in reaction centers (RCs) of Chloroflexus (Cfl.) aurantiacus were studied at 295 K using femtosecond transient absorption (TA) difference spectroscopy. Particular attention was paid to the decay kinetics of the primary electron donor excited state (P) and the formation/decay of the absorption band of the monomeric bacteriochlorophyll a anion (B) at ~ 1035 nm, which reflects the dynamics of the charge-separated state PB. It was found that in Q-depleted RCs containing native bacteriopheophytin a (BPheo) molecules at the H and H binding sites, the decay of P to form the PH state contains a fast (4 ps; relative amplitude 70%) and a slow (13 ps; relative amplitude 30%) kinetic components. The B absorption band at ~ 1035 nm was detected only for the fast component. Based on global analysis of the TA data, the results are discussed in terms of the presence of two P populations: in one, P decays in 4 ps via a dominant two-step activationless P → PB → PH ET with a contribution of 70% to the overall primary charge separation process, and in the other, P decays in 13 ps via a one-step superexchange P → PH ET (contribution of 30%). Similar femtosecond TA measurements on Q-depleted-Pheo-modified RCs, in which the charge separation energetics was changed by replacing BPheo H with plant pheophytin a, suggest the presence of a P population where PH formation can occur via a thermally activated two-step ET process.