Spectral hole burning and fluorescence line narrowing in subunits of the light-harvesting complex LH1 of purple bacteria

Spectroscopic properties of the B820 and B777 subunits of the core light-harvesting complex LH1 of purple bacteria Rhodospirillum rubrum G9 were studied by hole burning (HB) and fluorescence line narrowing (FLN) between 1.2 and 4.2 K. We have found that an equilibrium exists between the three forms B820, B777, and the native LH1-complex in the presence of the detergent n-octyl-beta-D-glucopiranoside (OG). The shift of this equilibrium was followed as a function of OG concentration by means of absorption and fluorescence spectra. Low-frequency modes at 19 cm(-1) for B820 and at 25 cm(-1) for B777 were identified by FLN. From the spectral position of these modes as a function of excitation wavelength lambda(exc) and from the homogeneous line width Gamma(hom)' as a function of lambda(exc), we conclude that "downhill" energy transfer does not take place either among B820 or among B777 subunits. The temperature dependence of Gamma(hom)', however, indicates that optical dephasing and/or spectral diffusion does occur in these subunits. The positions of side holes and antiholes, furthermore, suggest that the hole-burning mechanisms in B820 and B777 are similar, although their HB efficiencies differ by a factor of 10.