Excited States and Trapping in Reaction Center Complexes of the Green Sulfur Bacterium Prosthecochloris aestuarii

The excited states of bacteriochlorophyll (BChl) a were studied by pump−probe transient absorption spectroscopy in reaction center core (RCC), Fenna-Matthews-Olson (FMO) and FMO−RCC complexes of the green sulfur bacterium Prosthecochloris aestuarii. Excitation at 790 or 835 nm resulted in rapid equilibration of the energy between the BChl a molecules of the RCC complex: within 1 ps, most of the excitations had relaxed to the lowest energy level (835 nm), as a result of strong interactions between the BChls. Excitation of chlorophyll a 670 resulted in energy transfer to BChl a with a time constant of 1.2 ps, followed by thermal equilibration. Independent of the wavelength of excitation, the decay at 835 nm could be fitted with a time constant of about 25 ps, comparable to the 30 ps measured earlier with membrane fragments, which is ascribed to trapping in the reaction centers. Similar results were obtained with the FMO−RCC complex upon excitation at 835 or 670 nm, but the results upon 790 nm excitation were quite different. Again an equilibrium was rapidly reached, but now most of the excitations remained within the FMO complex, with a maximum bleaching at 813 nm, the same as observed in the isolated FMO. Even after 100 ps there was no bleaching at 835 nm and no evidence for charge separation. We conclude that there is no equilibration of the energy between the FMO and the RCC complex and that the efficiency of energy transfer from FMO to the reaction center core is low.