INHOMOGENEOUS SPECTRAL BROADENING OF THE B820 SUBUNIT FORM OF LH1

Site-selected fluorescence spectra of the B820 subunit of LH1 from -Rhodospirillum rubrum G9 were measured at temperatures between 160 K and 4.2 K. A linear correlation between excitation wavelength and maximal emission wavelength was observed across the whole Q(y) absorption band of the B820 subunits, which persists at higher temperatures. This demonstrates that (a) the Q(y) absorbance band of the B820 subunit is inhomogeneously broadened; (b) the subunit consists of a strongly interacting dimer of BChl a; (c) no energy transfer occurs among these dimers. The temperature dependence of the shape of the emission spectrum confirms that the protein forms a 'glass-like' environment for the bacteriochlorophyll pigments. After reassociating the octyl-glucoside-solubilized B820 subunits into the reassociated B873 form, the emission wavelength is independent of the wavelength of excitation at 4.2 K. This implies that the reassociated B873 form consists of a large number of interacting pigment molecules. The nature and origin of the spectral inhomogeneity in the B820 subunits is discussed.