Transient absorption spectroscopy of energy-transfer and trapping processes in the reaction center complex of Chlorobium tepidum

Reaction center complexes from the green sulfur bacterium Chlorobium tepidum were examined using picosecond absorption difference spectroscopy at room temperature. The complexes include the core proteins containing the primary electron donor P840 and associated core antenna pigments and also include some of the Fenna-Matthews-Olson (FMO) antenna protein. Upon excitation at 590 nm, long-lived absorption difference changes were observed that are interpreted to arise from a combination of the excited FMO protein and the P840-containing complex. Most of the FMO protein does not transfer energy to the core in this preparation. Excitation at 840 nm directly into the core complex gave an overall excited-state decay of 35 ps, forming the long-lived charge-separated state P840⁺A₀^-and no excited FMO complex. No antenna components were observed with absorption at wavelengths longer than the primary donor, in contrast to the situation in the related reaction centers from heliobacteria and photosystem I.