Energy relaxation within the B850 absorption band of the isolated light-harvesting complex LH2 from Rhodopseudomonas acidophila at low temperature

Energy relaxation on the bacteriochlorophyll B850 aggregate of the isolated light-harvesting antenna complex LH2 of Rhodopseudomonas acidophila at 7 K was examined by means of two-color femtosecond absorption spectroscopy. Upon excitation at 860 nm, at the blue side of the B850 absorption maximum, two kinetic components were observed in the subpicosecond time region: a major component of 100 fs and a minor one of 700 fs. The spectrum of the first component was typical for energy relaxation in an excitonically coupled system. The same two components were observed in the position of the isosbestic wavelength of the absorbance difference spectrum, which shifted to the red by 4 nm with time. Anisotropy decay to a final value of 0.07 mainly occurred within our time resolution of 70 fs. This corresponds to a homogeneous line width of B850 of at least 80 cm(-1). In addition, a weak slow phase in the anisotropy decay of 700 fs with an amplitude of 5-10% of the total decay was observed. In the absorbance kinetics, time constants of 100 and 700 fs were also observed upon excitation at 880 nm, which is at the red side of the absorption band, but now the spectrum only shifted by about 0.5 nm with time. In this case the decay of the anisotropy occurred within the time resolution to a final value of 0.12-0.14. The final position of the difference spectrum upon excitation at 880 nm was red-shifted by 1 nm with respect to that excited at 860 nm.