High-resolution site-selection fluorescence-and hole-buming spectroscopy were used to study energy transfer in two LH2 light-harvesting complexes of purple bacteria: the B800-850 complex of isolated Rb. sphaeroides and the B800-820 complex of Rps. ucidophilu, at 1.2 K. Fluorescence spectra, hole widths, and hole depths were measured as a function of excitation wavelength A, , , within the B800 band. For A,,, Z 798 nm, fluorescence line-narrowing is observed and the energy-transfer times (t = 2.5 and 2.0 ps for B800-850 and B800-820, respectively) are independent of A, , , . In this spectral region only interband B800 -B850 (B820) energy transfer takes place. For 780 nm 5 A, , , 5 798 nm, the fluorescence bands are broad and the transfer time, obtained from hole widths extrapolated to zero buming-flurence density, decreases toward the blue side of B800. In this wavelength region competition occurs between B800 -B850 (B820) and B8OO -B800 "downhill" energy transfer. For A, , , 5 780 nm, the broad fluorescence bands, with maxima at Aem -805 nm, become independent of A, , , and intruband B800 -. B800 transfer combined with excited-state vibrational relaxation are the dominant processes. The spectral distribution of the most-red absorbing pigments within the B800 band, which transfer energy exclusively from B800 to B850 (B820), was determined from the depth of the hole versus Aexc. The results indicate that one-third of the B800 pigments transfer their energy only to B850 (B820), from which it is concluded that the minimal functional LH2-unit consists of at least three B800 pigments and six B850 pigments, in addition to carotenoids.

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--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 λ exc and from the homogeneous line width Γ′ hom as a function of λ exc , we conclude that "downhill" energy transfer does not take place either among B820 or among B777 subunits. The temperature dependence of Γ′ 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. IntroductionLight-harvesting (LH) complexes in purple bacteria are responsible for the efficient collection of sun light and the transfer of excitation energy to the reaction center (RC) in photosynthesis. The primary charge separation occurs in the RC and leads to the subsequent conversion of excitation energy into a chemically useful form. Most purple bacteria contain two types of LH complexes: the LH1 core complex surrounding each RC and peripheral LH2 complexes which absorb slightly further to the blue and transfer the excitation energy to LH1. 1 Both the LH1 and LH2 complexes have ringlike structures 2,3 containing a basic repeating subunit. This subunit consists of a dimer of bacteriochlorophyll-a (BChl-a) molecules bound to two helical polypeptides (R and ), each comprising ∼50 amino acids 4 with a protein mass of approximately 6 kDa. The circular arrangement of the reconstituted LH1 in Rhodospirillum rubrum G9 is built up from 16 subunits. It has an outer diameter of 116 Å and an inner diameter of 68 Å, probably enclosing the RC. 2 The LH1 complex (B873) can be reversibly dissociated into its constituent subunits by detergent titration. 5,6 The detergent, n-octyl--D-glucopyranoside (OG), forms micelles which enclose either the dimeric subunit, called B820 (if ∼1.2% OG is used) or the B777 monomer subunit (when ∼5% OG is used). Reassociation of the subunits yields an aggregate with a spectrum nearly identical to that of LH1. 5,7 Although the absorption spectrum and the fluorescence lifetime of B777 are similar to those of BChl-a in solution, 8,9

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