EXCITED-STATES AND TRAPPING IN REACTION-CENTER COMPLEXES OF THE GREENSULFUR BACTERIUM PROSTHECOCHLORIS-AESTUARII

The excited states of bacteriochlorophyll (BChl) a were studied by pum p-probe transient absorption spectroscopy in reaction center core (RCC ), Fenna-Matthews-Olson (FMO) and FMO-RCC complexes of the green sulfu r bacterium Prosthecochloris aestuarii. Excitation at 790 or 835 nm re sulted in rapid equilibration of the energy between the BChl a molecul es of the RCC complex: within 1 ps, most of the excitations had relaxe d to the lowest energy level (835 nm), as a result of strong interacti ons between the BChls. Excitation of chlorophyll a 670 resulted in ene rgy transfer to BChl a with a time constant of 1.2 ps, followed by the rmal equilibration. Independent of the wavelength of excitation, the d ecay at 835 nm could be fitted with a time constant of about 25 ps, co mparable 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 MI, bu t the results upon 790 nm excitation were quite different. Again an eq uilibrium was rapidly reached, but now most of the excitations remaine d 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 bleac hing 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 c omplex and that the efficiency of energy transfer from FMO to the reac tion center core is low.