Primary charge separation in Chloroflexus aurantiacus reaction centers at room temperature: ultrafast transient absorption measurements on Q_A-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_A⁻) at ~ 1035 nm, which reflects the dynamics of the charge-separated state P⁺B_A⁻. It was found that in Q_A-depleted RCs containing native bacteriopheophytin a (BPheo) molecules at the H_A and H_B binding sites, the decay of P^* to form the P⁺H_A⁻ state contains a fast (4 ps; relative amplitude 70%) and a slow (13 ps; relative amplitude 30%) kinetic components. The B_A⁻ 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^* → P⁺B_A⁻ → P⁺H_A⁻ 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^* → P⁺H_A⁻ ET (contribution of 30%). Similar femtosecond TA measurements on Q_A-depleted-Pheo_A-modified RCs, in which the charge separation energetics was changed by replacing BPheo H_A with plant pheophytin a, suggest the presence of a P^* population where P⁺H_A⁻ formation can occur via a thermally activated two-step ET process.