Excitation Energy Transfer from Carotenoid to Bacteriochlorophyll in the Photosynthetic Purple Bacterial Reaction Center of Rhodobacter Sphaeroides

The role of carotenoid in singlet excitation energy transfer in the purple bacterial reaction center of Rb. sphaeroides has been studied using femtosecond transient absorbance spectroscopy techniques at room temperature. The carotenoid S₀-to-S₂ transition band was directly excited at 490 nm and the transient spectral changes were recorded in the visible and near-infrared spectral regions with subpicosecond time resolution. This result was compared with transient spectral changes observed following direct excitation of the bacteriochlorophyll dimer of the reaction center (P) at 860 nm. With 490-nm excitation, a subpicosend relaxation process from the carotenoid S₂-to-S₁ state is observed, followed by energy transfer to the monomer bacteriochlorophylls. Excitation energy is further transferred to P and then is utilized for the formation of the charge separated state P⁺H_A^-. The majority of the excitation energy from the carotenoid is transferred to bacteriochlorophyll from its S ₁ state and occurs with a time constant of 2.2 ps. The overall energy transfer efficiency between the excited carotenoid and bacteriochlorophyll is estimated to be greater than or equal to 75%. The carotenoid in the purple bacterial reaction center is in a 15-cis-configuration, which differs from the all-trans configuration of carotenoids found in the vast majority of antenna systems. The efficient singlet excitation energy transfer observed in this study suggests that the configuration difference is not a key factor in determining the efficiency of carotenoid-to-bacteriochlorophyll singlet excitation energy transfer.