Primary charge separation in Chloroflexus aurantiacus reaction centers at room temperature: ultrafast transient absorption measurements on QA-depleted preparations with native and chemically modified bacteriopheophytin composition

• Published in: Photosynthesis research 2025-02, Vol.163 (1), p.1-15
• Main Authors: Zabelin, Alexey A., Kovalev, Vyacheslav B., Khristin, Anton M., Khatypov, Ravil A., Shkuropatov, Anatoly Ya
• Format: Article
• Language: English
• Subjects: Bacteriochlorophyll; Biochemistry; Biomedical and Life Sciences; Electron transfer; Electrons; Life Sciences; Phaeophytin; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Reaction centers; Spectroscopy
• ISSN: 0166-8595; 1573-5079; 1573-5079
• DOI: 10.1007/s11120-024-01122-5

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.