Bacteriochlorophyll Aggregates Research Articles

Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.

FT-IR and near-infrared FT-Raman study of aggregation of bacteriochlorophyll c in solutions: evidence for involvement of the ester group in the aggregation

Ultraviolet-visible (UV-Vis) absorption, Fourier-transform infrared (FT-IR), and near-infrared (NIR)-excited FT-Raman spectra have been measured for bacteriochlorophyll c (BChl-c) in acetone, tetrahydrofuran (THF), pyridine-d5, carbon disulfide (CS2), and water-saturated carbon tetrachloride (CCl4) to investigate its aggregation in vitro. The UV-Vis absorption spectra can be classified into two groups. Group I (acetone, THF, and pyridine-d5 solutions) gives a spectrum with a Qy band around 665 nm while group II (CS2 and water-saturated CCl4 solutions) shows a spectrum typical of BChl-c aggregates with a broader red-shifted Qy band. All the NIR-FT-Raman spectra, which are preresonant with the Qy band, are very close to those of chlorophyll a (Chl-a) measured in the corresponding solutions. Bands due to a C = O stretching mode of free and strongly hydrogen-bonded 13(1)-keto carbonyl groups appear near 1685 and 1645 cm-1, respectively. In contrast to the FT-Raman spectra, FT-IR spectra of the pyridine-d5 solution and group II are largely different from those of Chl-a in the corresponding solutions, suggesting that BChl-c forms quite different types of aggregates. It is clear from the IR spectra that the ester carbonyl group plays an important role in the aggregation for the pyridine-d5 and group II solutions. Of particular note is that bands due to C = O stretching modes of the ester group are observed at 1733, 1719, and 1705 cm-1 in the spectrum of BChl-c in water-saturated CCl4.(ABSTRACT TRUNCATED AT 250 WORDS)

Exciton dynamics in circular aggregates: application to antenna of photosynthetic purple bacteria

A theoretical model of exciton dynamics in circular molecular aggregates of light-harvesting bacteriochlorophyll of photosynthetic bacteria is proposed. The spectra and anisotropy of photoinduced absorption changes in the femto- and picosecond time domain are under its scope. The excited state of aggregate was treated due to the standard exciton theory, taking into account a pigment inhomogeneity. Dephasing processes via the exciton-phonon interactions were described by means of the Haken-Strobl equation. It was shown that only two exciton levels are dipole-allowed in the case of homogeneous circular aggregate. The pigment inhomogeneity results in the appearance of several weak transitions to higher exciton levels. It was proposed that the minor band (B896) in an absorption spectrum of the B875 complex as well as the similar minor band in spectra of B800-850 complex correspond to electron transition from the ground to the lowest exciton level, whereas the major band corresponds to transition to the higher exciton level. The proposed model shows the subpicosecond decay of anisotropy at the short-wavelength side of absorption band and a high degree of anisotropy at the long-wavelength side, even at high temperatures.

Bacteriochlorophyll aggregates in positively charged micelles

Micellar complexes were prepared from bacteriochlorophyll a and bacteriopheophytin a with the cationic detergents, cetyltrimethyl ammonium bromide and cetylpyridinium chloride. These complexes have spectroscopic properties (absorption, circular dichroism) which are very different from the ones formed with non-ionic detergents like Triton X-100, and also with anionic detergents. Bacteriochlorophyll a forms two complexes: One is blue-shifted and has excitonically coupled Q y transitions. The second one is extremely red-shifted. The unusual properties are suggested to result from interactions of the positively charged head-group of the detergent with the tetrapyrrole.

Aggregation of bacteriochlorophyll c homologs to dimers, tetramers, and polymers in water-saturated carbon tetrachloride

Three homologs of BChl c, 2-(R)-(1-hydroxyethyl)-4-n-propyl-5-ethyl-farnesyl BChl c (PEF-BChl c), 2-(R)-(1-hydroxyethyl)-4-ethyl-5-ethyl-farnesyl BChl c (EEF-BChl c), and 2-(S)-(1-hydroxyethyl)-4-isobutyl-5-methyl/ethyl-farnesyl BChl c (iBM/EF-BChl c), formed aggregates in water-saturated carbon tetrachloride (H2O-satd CCl4). The water content was about 100 times higher than that of the dried CCl4 previously used. Absorption spectra were recorded for 8 concentrations for the three homologs of BChl c and were deconvoluted in terms of standard spectra of monomer, dimer, tetramer and polymer (747-nm aggregate, Olson and Pedersen (1990) Photosynthe Res 25: 25). PEF- and EEF-BChl c formed dimers (680 nm maximum) and tetramers (705-710 nm maximum), but iBM/EF-BChl c formed polymers. Inhibition of dimer formation by water faciliated the study of the initial stages of the polymerization of BChl c. When the logarithm of polymer concentration was plotted versus the logarithm of the monomer concentration for iBM/EF-BChl c, the initial slope was 30±10 and indicated the cooperation of 20-40 BChl c molecules to form a polymer from a seed. Circular dichroism spectra of the polymers with positive and negative bands at 743 and 760 nm, respectively, were similar to those for chlorosomes (Brune et al. (1990) Photosynth Res 24: 253).

A synthetic zinc chlorin aggregate as a model for the supramolecular antenna complexes in the chlorosomes of green bacteria

UV—visible spectra of synthetic zinc chlorins in non-polar organic solvents indicate a supramolecular structure similar to the aggregates of bacteriochlorophylls- c in the extramembranous antenna complexes of green photosynthetic bacteria.

Cooperative polymerization of photosynthetic pigments in formamide-water solution

The aggregation of bacteriochlorophyll a and bacteriopheophytin a into large oligomers with maximum optical absorption at 860 nm was studied in a 3:1 (vol/vol) formamide/water solution, using optical absorption spectroscopy and electron microscopy. The aggregation is cooperative and proceeds according to two equilibrium constants. Initially, two pigment molecules form a "seed" that absorbs at approximately 860 nm. The equilibrium constant, K(a), governing this reaction equals 1.3 x 10(3) M(-1) in the case of bacteriochlorophyll a (due to experimental limitations, K(a) for bacteriopheophytin a could not be determined). The addition of monomers to aggregates consisting of two or more units is governed by an equilibrium constant, K(b), equal to 2.2 x 10(6) M(-1) for bacteriochlorophyll a and approximately 10(9) M(-1) for bacteriopheophytin a. The enthalpy and entropy changes that drive the bacteriochlorophyll oligomer formation are -9.25 and approximately 0.0 kcal/mol, respectively. Above a threshold concentration, the amount of oligomers remains constant but their length continues to increase. Each oligomer appears to consist of dimers that are associated by hydrophobic interactions among their alcohol residues, forming long strands. Single strands presumably coil into helices that are seen as cylinders. The bacteriochlorophyll a oligomers form cylinders with a constant diameter of 150 A and an average length of 2,000 A (at 1.5 x 10(-5) M bacteriochlorophyll a). These cylinders contain 200-250 bacteriochlorophyll a dimers. The bacteriopheophytin oligomers coil into wider cylinders ( approximately 400 A in diameter) which contain approximately 600-700 bacteriopheophytin a dimers. In both cases, the separation between the dimers is approximately 20 A. At such distances, the dipolar interactions among adjacent dimers are negligible and do not affect the optical absorption of each individual pair. Therefore, the optical absorption of these pairs can be a tool for investigating the absorption pattern of photosynthetic pigments in vivo.

Co-ordination and aggregation of bacteriochlorophyll a : an n.m.r. and electronic absorption study

A detailed analysis is presented of the aggregation and co-ordination equilibria available to bacteriochlorophyll a, and n.m.r. spectroscopic titration experiments are proposed for their investigation. The results show that bacteriochlorophyll a is dimeric in wet benzene, five-co-ordinate monomeric in acetone or ether solution with one bound solvent molecule, and six-co-ordinate monomeric in pyridine or methanol solution with two bound solvent molecules. The second derivatives of the electronic absorption spectra show separate resolved peaks for five- and six-co-ordinate species near 575 and 600 nm respectively.

Aggregation of chlorophyll in nonpolar solvents from molecular weight measurements

The state of aggregation of chlorophylls a and b, pyrochlorophyll a, bacteriochlorophyll, pheophytin a and pyropheophytin a in a variety of nonpolar solvents has been deduced from molecular weight measurements made by vapor-phase osmometry. In carbon tetrachloride and benzene solution, chlorophyll a and pyrochlorophyll a exist predominately as dimers over a wide concentration range, although there is evidence for oligomer formation in concentrated solutions. In aliphatic or cycloaliphatic hydrocarbon solvents, chlorophyll a can form oligomers with aggregation numbers as large as 20. In even the most dilute cyclohexane solutions studied, chlorophyll a exists as a tetramer. Chorophyll b and bacteriochlorophyll aggregate more strongly. The basic unit for these tow chlorophyll is the trimer which has a strong tendency to form hexamer and higher oligomers in aliphatic hydrocarbon solvents. Infrared spectra for solutions of chlorophyll of known states of aggregation are presented.

Action de solvants basiques sur le spectre d'absorptiou des complexes pigmentes extraits de la bactérie pourpre Rhodospirillum rubrum

Abstract Absorption spectrum of Rhodospirillum rubout pigment complexes in basic solvents. — The adsorption and aggregation of bacteriochlorophyll are studied in various polar solvents, at different concentrations. It is shown that 3 absorption bands exist in these conditions, duplicating 3 of the absorption bands found in vivo: the 772 nin, the 830 nm and the 890 nm‐bands. The 800 nm‐band cannot be formed in vitro. The relations between the respective concentrations of the 3 states of bacteriochlorophyll found In vitro are calculated and discussed.

An analysis of the relations between fluorescence and photochemistry during photosynthesis

The dynamics of transfer and trapping of singlet excitation energy in photosynthetic systems have been analysed in terms of several models of the pigmented tissue. The formulations of Joliot and of Vredenberg and Duysens have been derived as special examples of a more general case. Published data on the relation between the yield of chlorophyll (or bacteriochlorophyll) fluorescence and the state of the energy-trapping molecules have been compared with theoretical predictions in an effort to decide what conclusions are afforded about the architecture of the photosynthetic tissue. It is concluded that in photosynthetic bacteria the migration of a singlet excitation quantum in the light harvesting bacteriochlorophyll aggregate extends over at least seven trapping molecules of the type P870, P985, etc.