Primary charge separation in native and plant pheophytin a-modified reaction centers of Chloroflexus aurantiacus: Ultrafast transient absorption measurements at low temperature.

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the B binding site were monitored as indicators of the formation and decay of the PB state, respectively (P is the primary electron donor, a dimer of bacteriochlorophyll a molecules). Global analysis of the TA data indicated the presence of at least two populations of the P excited state, which decay by distinct means, forming the state PH (H is a photochemically active bacteriopheophytin a molecule). In one population (~65 %), P decays in ~2 ps with the formation of PH via a short-lived PB intermediate in a two-step ET process P → PB→ PH. In another population (~35 %), P decays in ~20 ps to form PH via a superexchange mechanism without producing measurable amounts of PB. Similar TA measurements performed on chemically modified RCs of Cfl. aurantiacus containing plant pheophytin a at the H binding site also showed the presence of two P populations (~2 and ~20 ps), with P decaying through PB only in the ~2 ps population. At 100 K, the quantum yield of primary charge separation in native RCs is determined to be close to unity. The results are discussed in terms of involving a one-step P → PH superexchange process as an alternative highly efficient ET pathway in Cfl. aurantiacus RCs.