Primary charge separation process in Chloroflexus aurantiacus reaction centers

The primary processes in photosynthesis occur in pigment-protein complexes called reaction centers. The Chloroflexus aurantiacus (C. aurantiacus) reaction center, the smallest functionally active reaction center which has been isolated so far, is composed of only two protein subunits. As in the reaction centers of purple bacteria, the primary donor P is dimer of bacteriochlorophyll (Bchl). In contrast, the different pigment composition (3 Bchl:3 Bphe) together with spectroscopic data suggests that the accessory Bchl B_M on the inactive M branch in reaction centers from purple bacteria has been replaced by a bacteriopheophytin (Bphe) in C. aurantiacus reaction centers. The transient absorption spectrum at 10 K, measured 500 fs after direct excitation of P at 870 nm, reveals the inhomogeneous behavior of the 800-nm absorption band with the instantaneous shift of a transition located at 811 nm. An attempt to rationalize these data is described.