Aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c at low temperature

Exciton theory for supramolecular chlorosomal aggregates: 1. Aggregate size dependence of the linear spectra

The interior of chlorosomes of green bacteria forms an unusual antenna system organized without proteins. The steady-spectra (absorption, circular dichroism, and linear dichroism) have been modeled using the Frenkel Hamiltonian for the large tubular aggregates of bacteriochlorophylls with geometries corresponding to those proposed for Chloroflexus aurantiacus and Chlorobium tepidum chlorosomes. For the Cf. aurantiacus aggregates we apply a structure used previously (V. I. Prokhorenko., D. B. Steensgaard, and A. R. Holzwarth, Biophys: J. 2000, 79:2105-2120), whereas for the Cb. tepidum aggregates a new extended model of double-tube aggregates, based on recently published solid-state nuclear magnetic resonance studies (B.-J. van Rossum, B. Y. van Duhl, D. B. Steensgaard, T. S. Balaban, A. R. Holzwarth, K. Schaffner, and H. J. M. de Groot, Biochemistry 2001, 40:1587-1595), is developed. We find that the circular dichroism spectra depend strongly on the aggregate length for both types of chlorosomes. Their shape changes from "type-II" (negative at short wavelengths to positive at long wavelengths) to the "mixed-type" (negative-positive-negative) in the nomenclature proposed in K. Griebenow, A. R. Holzwarth, F. van Mourik, and R. van Grondelle, Biochim: Biophys. Acta 1991, 1058:194-202, for an aggregate length of 30-40 bacteriochlorophyll molecules per stack. This "size effect" on the circular dichroism spectra is caused by appearance of macroscopic chirality due to circular distribution of the transition dipole moment of the monomers. We visualize these distributions, and also the corresponding Frenkel excitons, using a novel presentation technique. The observed size effects provide a key to explain many previously puzzling and seemingly contradictory experimental data in the literature on the circular and linear dichroism spectra of seemingly identical types of chlorosomes.

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

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.

The influence of the presence of lipid on the aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c located in adsorbed layers and monolayers

The photoacoustic spectra and time-resolved delayed luminescence spectra in the microsecond time range were measured for layers of 8,12-diethyl farnesyl bacteriochlorophyll c adsorbed on quartz supports by solvent evaporation and as Langmuir-Blodgett monolayers. Both types of model system were also investigated with the addition of lipid. The data showed a very strong influence of lipid addition on pigment aggregation. In samples with synthetic and natural lipid addition, the pigments were found to be predominantly in the monomeric and dimeric states, whereas in the same type of sample without lipid, the pigments were aggregated to a higher degree. The influence of the presence of lipid on the aggregation of bacteriochlorophyll c in monolayers and adsorbed layers may also suggest that the contact of various pigment molecules with the lipids surrounding the chlorosome may influence the formation of various pigment aggregates in vivo. The synthetic lipid L-alpha-phosphatidylcholine dipalmitoyl and the natural lipid L-alpha-phosphatidylcholine type IVS from soy beans were used. In the latter case, only adsorbed layers were investigated. Our interpretation is preliminary as only one 8,12-diethyl farnesyl bacteriochlorophyll c homologue was present in our systems.