On the unidirectionality of electron transfer in reaction centers of Chloroflexus aurantiacus
摘要
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 ϕB binding site, corresponding to the bacteriochlorophyll BB site in purple bacterial RCs. Changes in primary photochemistry in QA-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 Qx and Qy optical transitions and is harder to reduce than BPheo. Isolated chemically modified RCs had a heterogeneous BPheo composition, and in approximately half of them, HA at the photochemically active A cofactor branch and ϕB (and/or HB) 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 QA-depleted RCs, (ii) the P+HB− state is formed in modified RCs via a P* → P+HB− superexchange mechanism with the P+131-OH-PheoϕB− state as a virtual intermediate, (iii) in native Cfl. aurantiacus RCs, the P* → P+ϕB− ET occurs in the Marcus inverted region, which slows down this reaction compared to P* → P+BA− and plays an important role in ensuring unidirectional charge separation through the A branch.