No abstract

Polarized and magic angle two-color femtosecond spectroscopy was used to study B800-850 antenna complexes of the photosynthetic purple bacteria Rhodopseudomonas acidophila (Rps. acidophila), Rhodobacter sphaeroides (Rb. sphaeroides), and Chromatium tepidum (Chr. tepidum) and the B800-820 complex of Chromatium vinosum at room temperature. As was earlier found for Chr. tepidum, in the B800-850 complexes of Rps. acidophila and Rb. sphaeroides the bleaching signal of B850 was found to be several times larger than that of B800, indicating strong exciton interactions between the bacteriochlorophylls (BChls) in the B850 aggregate. Depolarization of the B850 excited state was found to occur within our time resolution of 80 fs. In all species, B800 to B850 or B820 transfer took place with a time constant of 0.7 to 0.9 ps. Depolarization studies indicated a transfer time of 1.5 ps between B800 molecules in Rps. acidophila. In Chr. tepidum, B800 depolarizes 2 to 4 times slower, dependent on the wavelength of excitation. Our results indicate that the double band structure of B800 of the latter organism is due to two separate pools of BChls, rather than dimeric exciton interaction. Upon excitation of the B800-820 complex of Chr. vinosum at 795 nm, the B820 absorbance difference spectrum shifted with time to the red by 20 nm, indicating that B820 is spectrally very heterogeneous. A 2 ps downhill energy transfer process within the B820 band is assigned to energy transfer between aggregated B800-820 complexes. Assuming that the B800-820 complex is similar to B800-850, we propose that the large spectral heterogeneity of B820 does not occur within individual B800-820 complexes.

The reaction center (RC) of photosynthetic bacteria is a membrane protein complex that promotes a light-induced charge separation during the primary process of photosynthesis. In the photosynthetic electron transfer chain, the soluble electron carrier proteins transport electrons to the RC and reduce the photo-oxidized special-pair of bacteriochlorophyll. The high-potential iron-sulfur protein (HiPIP) is known to serve as an electron donor to the RC in some species, where the c-type cytochrome subunit, the peripheral subunit of the RC, directly accepts electrons from the HiPIP. Here we report the crystal structures of the RC and the HiPIP from Thermochromatium (Tch.) tepidum, at 2.2-Å and 1.5-Å resolution, respectively. Tch. tepidum can grow at the highest temperature of all known purple bacteria, and the Tch. tepidum RC shows some degree of stability to high temperature. Comparison with the RCs of mesophiles, such as Blastochloris viridis, has shown that the Tch. tepidum RC possesses more Arg residues at the membrane surface, which might contribute to the stability of this membrane protein. The RC and the HiPIP both possess hydrophobic patches on their respective surfaces, and the HiPIP is expected to interact with the cytochrome subunit by hydrophobic interactions near the heme-1, the most distal heme to the special-pair. In photosynthetic purple bacteria, the electron transfer reactions of photosynthesis are performed by the following three components: the photosynthetic reaction center (RC), the cytochrome (Cyt) bc 1 complex, and the soluble electron carrier protein. First, the RC promotes the light-induced charge separation across the plasma membrane, which results in the oxidation of the special-pair and the reduction of the quinone to the quinol. The quinol then leaves the RC and moves to the Cyt bc 1 complex through the quinone pool of the plasma membrane. Second, the Cyt bc 1 complex reoxidizes the quinol to the quinone, and the released electrons are transferred to the soluble electron carriers. Third, the soluble electron carriers transport the electrons to the RC through the periplasmic space. Finally, the photo-oxidized special-pair is reduced by the soluble electron carriers, and the RC comes back to the initial state. In the course of the oxidation and the reduction of the quinones, the transmembrane electrochemical gradient of the protons is formed, and its energy is used to produce ATP by ATP synthase.Thermochromatium (Tch.; formerly Chromatium) tepidum is a purple sulfur bacterium originally isolated from the hot springs in Yellowstone National Park (1, 2) and belongs to the ␥-subclass. Tch. tepidum is a thermophilic bacterium and can grow at the highest temperature of all known purple bacteria. The optimum growth temperature is 48-50°C, the maximum temperature 58°C. The RC from Tch. tepidum is stable up to 70°C in chromatophore and to 48°C in detergent-micelle (3). The RC is the first membrane protein whose three-dimensional structure has been determined at an atomic resolution (4, 5), and the...

Bacteriochlorophyll (BChl) c was extracted from Chloroflexus aurantiacus and purified by reverse-phase high-pressure liquid chromatography. This pigment consists of a complex mixture of homologues, the major component of which is 4-ethyl-5-methylbacteriochlorophyll c stearyl ester. Unlike previously characterized BChls c, the pigment from C. aurantiacus is a racemic mixture of diastereoisomers with different configurations at the 2a chiral center. Diluting a concentrated methylene chloride solution of BChl c with hexane produces an oligomer with absorption maxima at 740-742 and at 460-462 nm. Both the absorption spectrum and the fluorescence emission spectrum (maximum at 750 nm) of this oligomer closely match those of BChl c in chlorosomes. Further support for this model comes from the ability of alcohols, which disrupt BChl c oligomers by ligating the central Mg atom, to convert BChl c in chlorosomes to a monomeric form when added in low concentrations. The lifetime of fluorescence from the 740 nm absorbing BChl c oligomer is about 80 ps. Although exciton quenching might be unusually fast in the in vitro BChl c oligomer because of its large size and/or the presence of minor impurities, this result suggests that energy transfer from the BChl c antenna in chlorosomes must be very fast if it is to be efficient.

Cross polarization/magic angle spinning (CP/MAS)(13)C (solid state high resolution) NMR spectra were observed for chlorosomes and BChlc aggregates. Similarity of both kinds of spectra (except for some signals assignable to proteins and lipids in chlorosomes) indicates that BChlc's in chlorosomes are present just as in synthetic BChlc aggregates. Chemical shifts for C13(1) carbonyl and C3(1) hydroxylethyl carbons indicate hydrogen bonding between them. Comparison of solution and solid state(13)C NMR chemical shifts shows the five coordinated nature of BChlc aggregates. Some chemical shift differences were attributable to ring currents shifts. Their comparisons with calculated ring current shift values predicted structures for the aggregates. Cross polarization dynamics of the CP/MAS(13)C NMR signals explored dynamic and structural nature of the BChlc aggregates.

Magnetic circular dichroism (MCD) and absorption spectra have been measured on three intact photosynthetic pigments with the chlorin ring as macrocycle: chlorophyll a, bacteriochlorophyll c and d, in various hydrophilic organic solvents. The MCD intensity of a Qy(0-0) transition for the Mg chlorin derivative was sensitive to the coordination state of the central Mg atom by the solvent molecules. The coordination number has been characterized in terms of the relationship between the ratio of Qy(0-0) MCD intensity to its dipole strength (B/D) and the difference in energies of Qx(0-0) and Qy(0-0) transitions. This relationship depends not only on the coordination number of the magnesium (Mg) atom but also on the coordination interaction of the solvent molecules to the Mg atom, and can clarify the spectroscopic change of chlorosomes by alcohol treatment. We propose that the correlation between the MCD intensity of Qy(0-0) transition and the energy difference can be used as a new measure for determining the coordination number of the Mg atom and for estimating the interaction strength of the Mg atom with solvent molecules.

Intact farnesyl (3 R)-bacteriochlorophyll (BChl) c in carbon tetrachloride forms a stable dimer at room temperature characterized by two resonances resolved for each individual proton in the NMR spectrum and by a long wavelength shift of the Q y absorption band to 710 nm. All the proton resonances are precisely assigned on the basis of two-dimensional H−C and H−H correlation experiments. Authentic farnesyl acetate is used for assistance in the assignment. Extensive nuclear Overhauser effects (NOE) are observed, from which distances between intermolecular proton pairs are evaluated. Geometry of the macrocycles determined from the distance information and refined by a molecular mechanics program is found to clearly explain the observed complexation shifts. Strong intermolecular NOE signals observed for 10-H/201-H and 10-H/21-H exclude a face-to-face arrangement but support an antiparallel “piggy-back” conformation for the BChl c dimer. Farnesyl protons do not show significant complexation shifts, and it is suggested that the farnesyl side chain may adopt a folding-back conformation with most of the group fluctuating around the periphery of the macrocycle in a restricted motion. The two-dimensional exchange experiment demonstrates that molecules in the dimer experience slow exchange between the two nonequivalent configurations with an exchange rate constant of about 1.8 s-1. Finally, the stereochemical effect of chirality at the 31 position on the aggregation behavior and possible relationships among 680, 710, and 740 nm species are discussed.

A self-assembled monolayer of l-cysteine was prepared on Au(111) under potential control. Cyclic voltammetry and in situ electrochemical scanning tunneling microscopy were employed to investigate the molecular adsorption and adlayer structure in perchloric acid solution. The molecules chemically adsorb on Au(111) and form a well-defined adlayer. A new structure of (4 × √7)R19° was observed in the double-layer region. A phase transition at the positive potential region resulted in the appearance of a disordered layer. A structural model is proposed to interpret the molecular registry with Au(111) substrate.