EXCITON DYNAMICS IN FMO BACTERIOCHLOROPHYLL PROTEIN AT LOW-TEMPERATURES

A time response over almost 5 decades (from 10(-13) to about 10(-8) s) to a (sub)picosecond laser pulse excitation has been observed in the Fenna, Matthews, and Olson (FMO) antenna protein trimer. The FMO prote in is unique in having a fine-structured bacteriochlorophyll a Q(y) ex citon absorption spectrum over the whole investigated temperature rang e between 6 and 160 K. As measured by a two-color pump-probe different ial absorption, the population decay of the exciton states of seven st rongly coupled bacteriochlorophyll a molecules in a protein monomer is the dominant dynamical process in the subpicosecond time domain. The through-band scattering takes a few picoseconds and depends only weakl y on temperature, probably because of a low density of exciton states. At low temperatures, evidence for a slow pico-nanosecond relaxation p rocess has also been obtained via time-dependent red-shift and broaden ing of the exciton emission spectrum. Two nonexclusive tentative inter pretations to this effect have been provided. The phenomenon may be du e to exciton solvation in the surrounding protein and water-glycerol m atrix or/and due to slow scattering of closely spaced bacteriochloroph yll a exciton states in a protein trimer. The shape of the excited-sta te absorption spectrum (arising from transitions between singly and do ubly excited exciton states) and its oscillator strength has been roug hly estimated from the analysis of the pump-probe spectrum. The spectr um peaks at around 805 nm and is less featured compared to the ground- state absorption spectrum. Both spectra have comparable strength.