Dipole Strengths of the Qy(0,0) Bacteriochlorophyll c Transition¶

Abstract

The absorption spectra of bacteriochlorophyll (BChl) c solutions in two mixtures of two solvents (acetonitrile with pyridine and dimethylsulfoxide with methanol) exhibiting different refractive indices were measured and deconvoluted into Gaussian components. The refractive index of mixed solvents was changed by the change in the ratio of the volumes of the liquids used in the mixture. Using the Qy(0,0) band half widths and absorption coefficient, based on the simplified formula proposed by Knox, the dipole strengths of the Qy(0,0) BChl c transition for various values of solvent refractive index were calculated and compared with values given by Knox and Spring. For both investigated combinations of two liquids, the dependence of Qy(0,0) transition dipole strength of the BChl c on solvent refractive index was almost linear. The slopes of the lines obtained from the experimental absorption bands were different for two investigated solvent mixtures. More accurate linear dependence and similar slopes of lines for both solvent mixtures were obtained using half widths and absorption coefficients of the Gaussian components of Qy(0,0) transition. It is explained by the superposition of the contributions from other electronic and vibronic transitions of BChl c monomers or possibly also from transitions of the pigments involved in some complexes with solvent molecules in the absorption region investigated. The results show that the formula proposed by Knox can be successfully applied also for BChl c, after elimination of the overlapped contributions from the other transitions, by applying Gaussian analysis to select only contribution from Qy(0,0) pigment transition.

Probing the Spatial Organization of Bacteriochlorophyll c by Solid-State Nuclear Magnetic Resonance

“…30 In this work, we used a BChl c dipole strength of 16.4 D 2 . 31 Furthermore, we assumed equal site energies for all the BChls in the cluster (E n = 0) and assumed that the transition dipoles of the Q y excitations were aligned along the direction shown in Figure 1. We then calculated the full exciton Hamiltonian H as an N × N matrix, followed by diagonalizing H numerically to obtain eigenstates that are responsible for light absorption:…”

Pigment Analysis of “ Candidatus Chlorothrix halophila,” a Green Filamentous Anoxygenic Phototrophic Bacterium

“…The extinction coefficient of BChl a in acetone is 69.0 mM Ϫ1 cm Ϫ1 at 772 nm (28), and the extinction coefficient of BChl c in acetone is 64.0 mM Ϫ1 cm Ϫ1 at 662 nm (2,15). MS. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) MS and tandem MS were used to determine the molecular masses of individual pigments obtained from the HPLC analysis (Applied Biosystems Mass Spec Voyager DE-STR).…”

“…However, BChl c is different from BChl a because it has an oxidized ring B and the methoxycarbonyl group is not attached at C-13 2 on ring E (3). Since the spectroscopic properties of all BChls result primarily from the bacteriochlorin head group, reduction of ring B causes the absorption spectrum of BChl a to be redshifted to a longer wavelength compared to the spectrum of BChl c. In the Q y region of the spectrum, BChl a has maximum absorption at 772 nm in acetone (28), while BChl c has maximum absorption at 662 nm in organic solvents (2,15). Both C. tepidum and C. aurantiacus contain both BChl a and BChl c (23,31).…”

“…In Chlorobium limicola, a green sulfur bacterium, up to six different BChls c and six different BChls d are present (10). Although they have different alkyl substituents at C-8 and C-12 and different esterifying alcohols at C-17 2 , these BChl c homologs exhibit nearly indistinguishable absorption characteristics in solution, although there are spectral differences in the intact chlorosomes (7,10,24). BChl c has some special characteristics, including the absence of the bulky carboxymethyl group at C-13 2 that allows tight packing of BChls to form large oligomers and the presence of the hydroxyethyl substituent at C-3, which together play major roles in pigment oligomerization and chlorosome formation (3).…”