Inhomogeneous spectral broadening of the B820 subunit form of LH1

Abstract

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.