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. Up on excitation at 860 nm, at the blue side of the B850 absorption maximum, t wo kinetic components were observed in the subpicosecond time region: a maj or 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 sy stem. The same two components were observed in the position of the isosbest ic wavelength of the absorbance difference spectrum, which shifted to the r ed by 4 nm with time. Anisotropy decay to a final value of 0.07 mainly occu rred 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 i n the anisotropy decay of 700 fs with an amplitude of 5-10% of the total de cay 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 w ith 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 differ ence spectrum upon excitation at 880 nm was red-shifted by 1 nm with respec t to that excited at 860 nm.