Spectral Hole Burning and Fluorescence Line Narrowing in Subunits of the Light-Harvesting Complex LH1 of Purple Bacteria

Abstract

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

On the relation of protein dynamics and exciton relaxation in pigment–protein complexes: An estimation of the spectral density and a theory for the calculation of optical spectra

“…͑4.18͒ and ͑4.28͒ yields vibrational sidebands, in agreement with experiment. 2 A partial ordering prescription ͑POP͒ gave the best agreement with the sideband obtained in the experiment as seen in the contrast between Figs. 6 and 8.…”

“…A small difference in the two inhomogeneous widths 600 cm Ϫ1 and 500 cm Ϫ1 deduced for the pigment transition energies of the B777-complex and the B820-complex might, if real, be due to a greater exposure of the B777 pigment to the solvent, 73 as suggested in Ref. 2. The widths ͑FWHM͒ of 500 cm Ϫ1 and 600 cm Ϫ1 were determined here from the calculation of linear absorption spectra shown in Figs.…”

“…͑4.11͒ and ͑4.9͒. In the limit of delocalized protein vibrations (R c ӷR mn ) ␥ MK ϭ͉͗M ͉K͉͘ 2 ϭ␦ MK , and so no relaxation occurs, whereas for localized protein vibrations ( 2 , which can be as small as zero for localized electronic states…”

“…Fluorescence line narrowing spectra of B820 complexes are calculated and compared with the experimental data. 2 The approximations and results are discussed in Sec. V. A summary is given in Sec.…”

Exciton Dynamics in LH1 and LH2 of Rhodopseudomonas Acidophila and Rhodobium Marinum Probed with Accumulated Photon Echo and Pump−Probe Measurements

“…rubrum, it was explicitly shown that the observed holewidths depended on the time between burning and probing the hole. 51 Singlet-triplet annihilation and local heating effects have also been proposed as a possible reason for the discrepancy between the APE and HB results because in HB experiments typically higher laser power densities are used. 33 In Figure 3 we show the intensity dependence of the APE experiment on LH2 of Rps.…”