bacteriochlorophylls and pyridine

Bacteriochlorophyll f (BChl f) is a photosynthetic pigment predicted nearly 40 years ago as a fourth potential member of the Chlorobium chlorophyll family (BChl c, d, and e). However, this pigment still has not been found in a naturally occurring organism. BChl c, d, and e are utilized by anoxygenic green photosynthetic bacteria for assembly of chlorosomes--large light-harvesting complexes that allow those organisms to survive in habitats with extremely low light intensities. Recently, using genetic methods on two different strains of Chlorobaculum limnaeum that naturally produce BChl e, two research groups produced mutants that synthesize BChl f and assemble it into chlorosomes. In this study, we present detailed investigations on spectral and dynamic characteristics of singlet excited and triplet states of BChl f with the application of ultrafast time-resolved absorption and fluorescence spectroscopies. The studies were performed on isolated BChl f in various solvents, at different temperatures, and on BChl f-containing chlorosomes in order to uncover any unusual or unfavorable properties that stand behind the lack of appearance of this pigment in natural environments.

Topics: Bacteriochlorophylls; Chlorobium; Photolysis; Pyridines; Solvents; Spectrometry, Fluorescence

We show that resonant impulsive excitation of the Qy absorption band of bacteriochlorophyll a (BChl) launches a rapidly damped (gamma < 200 fs) ground-state coherent wave-packet motion that arises from intermolecular modes with clustered solvent molecules. Femtosecond pump-probe, dynamic-absorption signals were obtained at room temperature with BChl solutions in pyridine, acetone, and 1-propanol. The vibrational coherence observed in the 0-800-fs regime is modeled in the time domain by two (or three, in the case of 1-propanol) modulation components with asymmetric, inhomogeneously broadened line shapes and frequencies in the 100-200-cm(-1) range. The mean frequency of the vibrational coherence exhibits at least a quadratic dependence on the dipole moment of the solvent molecules and a y-intercept in the 100-cm(-1) regime. This trend is modeled by an expression for the natural frequency of a "6-12" potential composed of attractive terms from van der Waals forces and a repulsive term from the exchange (Pauli exclusion) force. The model suggests that comparable contributions to the potential are provided by the dipole-dipole and London dispersion interactions. These results support the hypothesis that the low-frequency vibrational modes in the 100-cm(-1) regime that are coupled to the light-driven charge-separation reactions in the reaction center from purple bacteria are derived from intermolecular vibrational modes between the chromophores and the surrounding protein medium.

Topics: 1-Propanol; Absorption; Acetone; Bacteriochlorophyll A; Biophysics; Chemistry, Physical; Models, Statistical; Molecular Conformation; Oscillometry; Pyridines; Solvents; Spectrophotometry; Temperature; Time Factors

Acaryochloris marina is a unique photosynthetic prokaryote containing chlorophyll(Chl)-d as a major photoactive pigment (over 95%). The molecular structure of Chl-d is proposed as the 3-formyl analog of Chl-a. However, the stereochemistry of Chl-d at the 13(2)-, 17- and 18-positions has not yet been established unambiguously. In the first part of this paper, we describe the determination of their stereochemistries to be 13(2)-(R)-, 17-(S)- and 18-(S)-configurations by using 1H-1H NOE correlations in 1H-NMR and circular dichroism spectra as well as chemical modification of Chl-a to produce stereochemically defined Chl derivatives. In the second part of the paper, we report a facile synthesis of a self-aggregative Chl by modifying isolated Chl-d. Since Chl-d was characterized by its reactive 3-formyl group, the formyl group was reduced with t-BuNH2BH3 to afford the desirable Chl, 3-deformyl-3-hydroxymethyl-pyrochlorophyll-d (3(1)-OH-pyroChl-d). The synthetic 3(1)-OH-pyroChl-d molecules spontaneously self-organized to form well-ordered aggregates in a non-polar organic solvent. The self-aggregates are a good model of major light-harvesting antenna systems of green photosynthetic bacteria, chlorosomes, in terms of the following three findings. (1) Both the red-shifted electronic absorption band above 750 nm and its induced reverse S-shape CD signal around 750 nm were observed in 0.5% (v/v) THF-cyclohexane. (2) The stretching mode of the 13-carbonyl group was downshifted by about 35 cm(-1) from the wavenumber of its free carbonyl. (3) The self-aggregates were quite stable on titration of pyridine to the suspension, in comparison with those of natural chlorosomal bacteriochlorophyll-d possessing the 3-(1-hydroxyethyl) group.

Topics: Bacteriochlorophylls; Chlorobi; Chlorophyll; Chromatography, High Pressure Liquid; Cyanobacteria; Molecular Conformation; Pyridines; Sensitivity and Specificity; Spectrum Analysis; Stereoisomerism; Titrimetry

We present the first observations of vibrational coherence in the 10-220-cm-1 region from bacteriochlorophyll a (BChl) in solution. A distinction can be made for the first time between BChl's intramolecular normal modes and intermolecular modes between BChl and solvent. The results show that the low-frequency vibrations that accompany the initial electron-transfer reaction from the paired BChl primary electron donor, P, in photosynthetic reaction centers arise predominantly from intramolecular modes of histidine-ligated BChl macrocycles. The results also suggest that polar-solvent interactions can significantly perturb the electronic properties of BChl in a manner that might have important functional consequences.

Topics: Bacteriochlorophyll A; Fourier Analysis; Photosynthesis; Pyridines; Spectrum Analysis

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.

Topics: Bacterial Proteins; Bacteriochlorophylls; Dimethyl Sulfoxide; Methanol; Pyridines; Spectrometry, Fluorescence

The transient absorption anisotropy spectrum of bacteriochlorophyll a (BChl a) in pyridine was measured in the wavelength interval 550-850 nm, 1 ps after optical excitation with a 792-nm femtosecond light pulse. In the wavelength region of Q(y) absorption and stimulated emission (775-825 nm), the anisotropy was found to be close to the theoretically expected value (0.4) for a two-level system. In the wavelength region 650-750 nm, where the transient absorption signal is dominated by excited state absorption, the anisotropy is reduced to approximately 0.18. Anisotropy kinetics were measured at several wavelengths and found to be constant within the time window 0-5 ps, showing that no internal dynamics of the BChl a molecule change the anisotropy on the time scale of tens of picoseconds.

Topics: Bacteriochlorophylls; Fluorescence Polarization; Pyridines

The function and specific structural aspects of the tryptophan-rich sensory protein (TspO) of Rhodobacter sphaeroides 2.4.1 were studied using site-directed mutagenesis involving some 17 different amino acids. The choice of these amino acids changes was dictated from an analysis of the TspO family of proteins derived from the data bases. These studies demonstrated the importance of several highly conserved tryptophan residues in the sensory transduction pathway involving TspO through the proposed binding of an intermediate(s) in the tetrapyrrole biosynthesis pathway. These studies also revealed that the substitution of one or several of the amino acid residues dramatically affected, either directly or indirectly, the levels of TspO in the membranes of R. sphaeroides. Mounting evidence is presented suggesting that TspO normally forms a dimer within the bacterial outer membrane, and the dimer form of TspO may be the active form for TspO function. Because our earlier studies provided us with a functional framework within which to view these amino acid substitutions, we are able to suggest a preliminary model for TspO structure-function. Not only do these studies tell us more about TspO, but they also shed light on the TspO homologue, the drug-binding component of the mitochondrial peripheral benzodiazepine receptor. Mounting evidence draws numerous parallelism between these proteins and supports the significance of using TspO as a model for the structure and function of the mitochondrial protein.

Topics: Amino Acid Sequence; Animals; Bacterial Outer Membrane Proteins; Bacterial Proteins; Bacteriochlorophylls; beta-Galactosidase; Blotting, Western; Carotenoids; Cross-Linking Reagents; Dimerization; Membrane Proteins; Models, Biological; Molecular Sequence Data; Mutagenesis, Site-Directed; Phenotype; Photosynthesis; Plasmids; Point Mutation; Protein Structure, Secondary; Pyridines; Pyrroles; Receptors, GABA-A; Rhodobacter sphaeroides; Sequence Homology, Amino Acid; Spectrophotometry; Structure-Activity Relationship; Tetrapyrroles; Time Factors; Transcription, Genetic