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 protein is unique in having a fine-structured bacteriochlorophyll a Q_y exciton absorption spectrum over the whole investigated temperature range between 6 and 160 K. As measured by a two-color pump-probe differential absorption, the population decay of the exciton states of seven strongly coupled bacteriochlorophyll a molecules in a protein monomer is the dominant dynamical process in the subpicosecond time domain. The throughband scattering takes a few picoseconds and depends only weakly on temperature, probably because of a low density of exciton states. At low temperatures, evidence for a slow pico-nanosecond relaxation process has also been obtained via time-dependent red-shift and broadening of the exciton emission spectrum. Two nonexclusive tentative interpretations to this effect have been provided. The phenomenon may be due to exciton solvation in the surrounding protein and water-glycerol matrix or/and due to slow scattering of closely spaced bacteriochlorophyll a exciton states in a protein trimer. The shape of the excited-state absorption spectrum (arising from transitions between singly and doubly excited exciton states) and its oscillator strength has been roughly estimated from the analysis of the pump-probe spectrum. The spectrum peaks at around 805 nm and is less featured compared to the ground-state absorption spectrum. Both spectra have comparable strength.