SINGLET-SINGLET ANNIHILATION AND LOCAL HEATING IN FMO COMPLEXES

Energy transfer in a trimeric Bchl a containing FMO pigment-protein co mplex from the green sulfur bacterium Chlorobium tepidum has been stud ied by means of picosecond transient absorption spectroscopy under hig h-excitation conditions. At room temperature the excited state absorpt ion spectrum of the FMO complex was found to be similar to that of non interacting Bchl molecules in solution, which suggests that the influe nce of exciton coupling on the spectroscopic properties of the FMO com plex at room temperature is not substantial. Analysis of the excited s tate relaxation kinetics in singlet-singlet annihilation conditions sh ows that the energy transfer from the excited monomer to another excit ed monomer is independent of the oxidation-reduction state of the comp lex and is slower than the intermonomer excitation migration rate. The difference spectrum at 77 K resembles the absorption spectrum, showin g three exciton subbands. In addition to the singlet-singlet annihilat ion, the 7 ps rate of which is similar to that at room temperature, an d to the intrinsic exciton decay, which is also temperature independen t, energy redistribution between exciton states with a mean time of 26 ps is evident. This redistribution is explained as being due to local heating/cooling kinetics stimulated by the excitation pulses.