Fluorescence polarization and low-temperature absorption spectroscopy of a subunit form of light-harvesting complex I from purple photosynthetic bacteria

Abstract

Measurements of polarized fluorescence and CD were made on light-harvesting complex 1 and a subunit form of this complex from Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodobacter capsulatus. The subunit form of LH1, characterized by a near-infrared absorbance band at approximately 820 nm, was obtained by titration of carotenoid-depleted LH1 complexes with the detergent n-octyl beta-D-glucopyranoside as reported by Miller et al. (1987) [Miller J. F., Hinchigeri, S. B., Parkes-Loach, P. S., Callahan, P. M., Sprinkle, J. R., & Loach, P. A. (1987) Biochemistry 26, 5055-5062]. Fluorescence polarization and CD measurements at 77 K suggest that this subunit form must consist of an interacting bacteriochlorophyll a dimer in all three bacterial species. A small, local decrease in the polarization of the fluorescence is observed upon excitation at the blue side of the absorption band of the B820 subunit. This decrease is ascribed to the presence of a high-energy exciton component, perpendicular to the main low-energy exciton component. From the extent of the depolarization, we estimate the oscillator strength of the high-energy component to be at most 3% of the main absorption band. The optical properties of B820 are best explained by a Bchl a dimer that has a parallel or antiparallel configuration with an angle between the Qy transition dipoles not larger than 33 degrees. The importance of this structure is emphasized by the results showing that core antennas from three different purple bacteria have a similar structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Exciton (De)Localization in the LH2 Antenna of Rhodobacter sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit

“…3 This could then implicate that the large Stark spectrum of LH1 arises from the "real" R BChl 2 dimer. Notice that in the earlier studies of the B820 subunit by means of CD, polarized fluorescence, and singlet minus triplet spectroscopies, 45,46,53 the interaction energy within this subunit was estimated as M ) 230 cm -1 and the site inhomogeneity as 2M at 77 K and 3M at room temperature, i.e., 460 and 690 cm -1 , respectively, in very reasonable agreement with our results.…”

Modification of a hydrogen bond to a bacteriochlorophyll a molecule in the light-harvesting 1 antenna of Rhodobacter sphaeroides.

“…The difference in the maximum absorption of the aTyr43 (865 nm) and aPhe43 (853 nm) mutants in the DD13 deletion background, at room temperature, is 12 nm and, assuming the residues produce similar structural perturbations, and hence blue shifts, this may represent the extent of the red shift that could be attributed to the H bond. It is clear from the in vitro work (16,17,22,41,42) When looking at the aligned sequences of LH1 and LH2 complexes it is apparent that aTrp43 is one ofthe most highly conserved residues in LH1 and that the equivalent residue in LH2 is Tyr44. From this work we have strong evidence that this Trp is H-bonded to one of the bound bchls, giving rise to a component of the 1640-cm-1 peak, which is a feature of all LH1 Raman spectra; thus it is probable that this H-bond arrangement exists in all LH1 complexes.…”

Excitons in a photosynthetic light-harvesting system: A combined molecular dynamics, quantum chemistry, and polaron model study

“…The latter value is obtained when a computationally determined value of C = 64.38Å 3 eV [7], which corresponds to a transition dipole moment of 11 D, is rescaled to match the experimental value 6.3 D [26] of the transition dipole moment. For simplicity we assume that the coupling between neighboring (and all other) BChls is given by W ij (t), even though the short distance between neighboring BChls does not justify limitation to only the leading term of a multipole expansion.…”

“…We can now determine the absorption rate for radiation by employing this expression instead of ˆ µ(t) in (26). In doing so we consider the time average (denoted by an overbar) over a cycle of the radiation field and use exp(±iωt) = 0.…”