On the unidirectionality of electron transfer in reaction centers of Chloroflexus aurantiacus

A characteristic feature of the reaction center (RC) of the filamentous phototrophic bacterium Chloroflexus (Cfl.) aurantiacus is the presence of а bacteriopheophytin molecule (BPheo; H) at the ϕ_B binding site, corresponding to the bacteriochlorophyll B_B site in purple bacterial RCs. Changes in primary photochemistry in Q_A-depleted Cfl. aurantiacus RCs caused by chemical exchange of BPheo molecules with 13¹-deoxo-13¹-hydroxy-pheophytin a (13¹-OH-Pheo) molecules were investigated using femtosecond transient absorption difference spectroscopy. In vitro, 13¹-OH-Pheo has higher energy Q_x and Q_y optical transitions and is harder to reduce than BPheo. Isolated chemically modified RCs had a heterogeneous BPheo composition, and in approximately half of them, H_A at the photochemically active A cofactor branch and ϕ_B (and/or H_B) at the normally inactive B branch were replaced by 13¹-OH-Pheo. Global analysis indicates that in this RC fraction, ion pairs consisting of P⁺ and 13¹-OH-Pheo^- are not populated by electron transfer (ET) from the primary electron donor excited state P^*, and significant (~ 35%) photoinduced B-branch charge separation occurs. The data are discussed using a model in which (i) the introduction of 13¹-OH-Pheo alters the ET energetics in Q_A-depleted RCs, (ii) the P⁺H_B^- state is formed in modified RCs via a P^* → P⁺H_B^- superexchange mechanism with the P⁺13¹-OH-Pheo_ϕB^- state as a virtual intermediate, (iii) in native Cfl. aurantiacus RCs, the P^* → P⁺ϕ_B^- ET occurs in the Marcus inverted region, which slows down this reaction compared to P^* → P⁺B_A^- and plays an important role in ensuring unidirectional charge separation through the A branch.