Energy Relaxation within the B850 Absorption Band of the Isolated Light-Harvesting Complex LH2 from Rhodopseudomonas Acidophila at Low Temperature

Abstract

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, ...