Bacteriochlorophyll c monomers,dimers, and higher aggregates in dichloromethane,chloroform, and carbon tetrachloride

Bacteriochlorophyll c in vivo is a mixture of at least 5 homologs, all of which form aggregates in CH₂Cl₂, CHCl₃ and CCl₄. Three homologs exist mainly in the 2-R-(1-hydroxyethyl) configuration, whereas the other two homologs, 4-isobutyl-5-ethyl and 4-isobutyl-5-methyl farnesyl bacteriochlorophyll c, exist mainly in the 2-S-(1-hydroxyethyl) configuration (Smith KM, Craig GW, Kehres LA and Pfennig N (1983) J. Chromatograph. 281: 209–223). In CCl₄ the S-homologs form an aggregate of 2–3 molecules whose absorption (747 nm maximum) and circular dichroism spectra resemble those of the chlorosome. In CH₂Cl₂, CHCl₃ and CCl₄ the 4-n-propyl homolog (R-configuration) forms dimers absorbing at ca. 680 nm and higher aggregates absorbing at 705–710 nm. In CCl₄ the dimerization constant is approx. 10 µM^–1 (1000 times that for chlorophyll a). The difference between the types of aggregates formed by the 4-n-propyl and 4-isobutyl homologs is attributed to the difference between the R- and S-configurations of the 2-(1-hydroxyethyl) groups in each chlorophyll.Abbreviations BChl bacteriochlorophyll - CD circular dichroism - Chl chlorophyll - DNS data not shown - EEF 4-ethyl-5-ethyl farnesyl - iBM/EF 4-isobutyl-5-methyl/ethyl farnesyl - MEF 4-methyl-5-ethyl farnesyl - PEP 4-n-propyl-5-ethyl farnesyl     

Monomers,dimers, and tetramers of 4-n-propyl-5-ethyl farnesyl bacteriochlorophyll c in dichloromethane and carbon tetrachloride

Absorption (ABS) and circular dichroism (CD) spectra were recorded for 6 concentrations (2.0–290 M) of bacteriochlorophyll (BChl) c in each solvent. Monomer spectra were obtained by adding methanol (1:200) to each sample. The monomer showed an ABS peak and a CD trough at 664 nm in CH₂Cl₂ (ABS peak at 665 nm in CCl₄). Dimer-plus-monomer spectra were obtained by subtracting high concentration (e.g., 290 M) spectra appropriately scaled from lower concentration (e.g., 26 M) spectra. Pure dimer spectra were then obtained by subtracting monomer spectra appropriately scaled from dimer-plus-monomer spectra. The dimer showed an ABS peak at 679 nm in both CH₂Cl₂ and CCl₄ and a CD trough at ca. 670 nm in CH₂Cl₂. The optical properties of the dimer do not agree with the model for bacteriochlorophyllide d [Smith KM, Bobe FW, Goff DA and Abraham RJ (1986) J Am Chem Soc 108: 1111–1120]. Higher aggregate spectra were obtained by subtracting appropriately scaled monomer and dimer spectra from high concentration (e.g., 290 M) spectra. The aggregate showed ABS shoulders at ca. 636 and 678 nm with a peak at 702 nm in CH₂Cl₂ and at 708 nm in CCl₄; the CD spectrum in either solvent showed peaks at 638 and 679 nm with troughs at 658 and ca. 710 nm. These spectra are consistent with an excitonic interaction between 4 chromophores in the aggregate. Each of the 12 original ABS spectra was deconvoluted in terms of the appropriate monomer, dimer and aggregate spectra, and the concentrations of each component were determined. Plots of log aggregate concentration vs. log dimer concentration lay on or near a line of slope 1.9 for CH₂Cl₂ and on or near a line of slope 2.1 for CCl₄. The aggregate was thus shown to be a tetramer. The theoretical relationship between dimers and monomers (slope 2.0) was not observed in all cases.Abbreviations ABS absorbance - BChl bacteriochlorophyll - CD circular dichroism - Chl chlorophyll - DNS data not shown - PEF 4-n-propyl-5-ethyl farnesyl     

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).Abbreviations BChl bacteriochlorophyll - CD circular dichroism - EEF 4-ethyl-5-ethyl farnesyl - iBM/EF 4-isobutyl-5-methyl/ethyl farnesyl - H₂O-satd CCl₄ water saturated carbon tetrachloride - PEF 4-n-propyl-5-ethyl farnesyl     

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

The effect of temperature on the aggregation of 3^lR-8,12-diethyl farnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclohexane (1/1, v/v) was studied by means of absorption, circular dichroism and fluorescence spectroscopy. At room temperature essentially only two aggregate species, absorbing at 702 nm (A-702) and 719 nm (A-719), were present. Upon cooling to 219 K, A-702 was quantitatively converted to A-719. Further lowering of the temperature led to the stepwise formation of larger aggregates by the conversion of A-719 to aggregate species absorbing at 743 nm (A-743) and 755 nm (A-755). All absorption changes were reversible. A-719 was highly fluorescent (maximum at 192 K: 744 nm), while A-743 and especially A-755 were weakly fluorescent. Below 130 K the mixture solidified, and no major changes in the absorption spectrum were observed upon further cooling. At 45 K, however, a relatively strong emission at 775 nm was observed. Below 200 K, the absorption, fluorescence and circular dichroism spectra resembled that of the chlorosome. These results open up the possibility to study higher aggregates of BChl c as models for the chlorosome by various methods at low temperature, thus avoiding interference by thermal processes.Abbreviations A-680, A-702, A-719, A-743 and A-755- BChl c aggregates absorbing at the wavelengths indicated - BChl- bacteriochlorophyll - R[E,E] BChl c _F- the 3¹ R isomer of 8,12-diethyl BChl c esterified with farnesol (F), analogously - M- methyl - Pr- propyl - S- stearol (see Smith 1994) - CD- circular dichroism     

Antenna organization and evidence for the function of a new antenna pigment species in the green photosynthetic bacterium Chloroflexus aurantiacus

Whole cells and isolated chlorosomes (antenna complex) of the green photosynthetic bacterium Chloroflexus aurantiacus have been studied by absorption spectroscopy (77 K and room temperature), fluorescence spectroscopy, circular dichroism, linear dichroism and electron spin resonance spectroscopy. The chlorosome absorption spectrum has maxima at 450 (contributed by carotenoids and bacteriochlorophyll (BChl) a Soret), 742 (BChl c) and 792 nm (BChl a) with intensity ratios of 20:25. The fluorescence emission spectrum has peaks at 748 and 802 nm when excitation is into either the 742 or 450 nm absorption bands, respectively. Whole cells have fluorescence peaks identical to those in chlorosomes with the addition of a major peak observed at 867 nm. The CD spectrum of isolated chlorosomes has an asymmetric-derivative-shaped CD centered at 739 nm suggestive of exciton interaction at least on the level of dimers. Linear dichroism of oriented chlorosomes shows preferential absorption at 742 nm of light polarized parallel to the long axis of the chlorosome. This implies that the transition dipoles are also oriented more or less parallel to the long axis of the chlorosome. Treatment with ferricyanide results in the appearance of a 2.3 G wide ESR spectrum at g 2.002. Whole cells grown under different light conditions exhibit different fluorescence behavior when absorption is normalized at 742 nm. Cells grown under low light conditions have higher fluorescence intensity at 748 nm and lower intensity at 802 nm than cells grown under high light conditions. These results indicate that the BChl c in chlorosomes is highly organized, and transfers energy from BChl c (742 nm) to a connector of baseplate BChl B792 (BChl a) presumably located in the chlorosome baseplate adjacent to the cytoplasmic membrane.     

An infrared study of unordered poly-L-proline in CaCl 2 solutions

Infrared spectra have been obtained of poly-L -proline in aqueous CaCl₂ solutions. As the salt concentration is increased, the C?O stretching band develops a component at the frequency found in the solid state while the CH₂ bending band broadens to higher frequency. Since circular dichroism spectra indicate progressive disordering of the chain with increasing salt concentration, we associate the infrared spectral changes with the same phenomenon. Our interpretation of these changes, particularly in the CH₂ bending modes, is that disordering is associated primarily with an increase in the range of accessible C^α–C′ (?O) rotation angles rather than with the random introduction of cis imide bonds in the chain.     

Excitation energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus: A specific effect of 1-hexanol on the optical properties of baseplate and energy transfer processes

The effect of 1-hexanol on spectral properties and the processes of energy transfer of the green gliding photosynthetic bacterium Chloroflexus aurantiacus was investigated with reference to the baseplate region. On addition of 1-hexanol to a cell suspension in a concentration of one-fourth saturation, a specific change in the baseplate region was induced: that is, a bleach of the 793-nm component, and an increase in absorption of the 813-nm component. This result was also confirmed by fluorescence spectra of whole cells and isolated chlorosomes. The processes of energy transfer were affected in the overall transfer efficiency but not kinetically, indicating that 1-hexanol suppressed the flux of energy flow from the baseplate to the B806-866 complexes in the cytoplasmic membranes. The fluorescence excitation spectrum suggests a specific site of interaction between bacteriochlorophyll (BChl) c with a maximum at 771 nm in the rod elements and BChl a with a maximum at 793 nm in the baseplate, which is a funnel for a fast transfer of energy to the B806-866 complexes in the membranes. The absorption spectrum of chlorosomes was resolved to components consistently on the basis, including circular dichroism and magnetic circular dichroism spectra; besides two major BChl c forms, bands corresponding to tetramer, dimer, and monomer were also discernible, which are supposed to be intermediary components for a higher order structure. A tentative model for the antenna system of C. aurantiacus is proposed.Abbreviations A670 a component whose absorption maximum is located at 670 nm - (B)Chl (bacterio)chlorophyll - CD circular dichroism - F675 a component whose emission maximum is located at 675 nm - FMO protein Fenna-Mathews-Olson protein - LD linear dichroism - LH light-harvesting - McD magnetic circular dichroism - PS photosystem - RC reaction center     

Fluorescence properties of the light-harvesting bacteriochlorophyll protein from Rhodopseudomonas sphaeroides R-26

The light-harvesting bacteriochlorophyll-protein (BChl-protein) from Rhodopseudomonas sphaeroides, R-26 mutant, exhibits a strong optical absorption peak near 850 nm (Q_y band) and a weaker peak at 590 nm (Q_x band). This pigment-protein appears to contain two BChl molecules per subunit, and previous circular dichroism studies indicated the presence of excitonic interactions between the BChl molecules. The complex exhibits a fluorescence maximum near 870 nm at room temperature. Excitation in the Q_y region results in polarization p values that vary only from +0.12 at 820 nm to +0.14 near 900 nm. These values are appreciably smaller than that for monomeric BChl in viscous solvents (p > 0.4). By contrast, using Q_x excitation the p value is ?0.25 for the BChl-protein complex, which is close to that observed for the BChl monomer. For the BChl-protein these polarization values do not change greatly at a temperature of 90 K; however, the Stokes' shift of the fluorescence emission increases significantly over that at room temperature.     

Pigment organization of the B800–850 antenna complex of Rhodopseudomonas sphaeroides

The B800–850 antenna complex of Rhodopseudomonas sphaeroides was studied by comparing the spectral properties of several different types of complexes, isolated from chromatophores by means of the detergents lithium dodecyl sulfate (LDS) or lauryl dimethylamine N-oxide (LDAO). Fluorescence polarization spectra of the BChl 800 emission at 4 K indicated that rapid energy transfer between at least two BChl 800 molecules occurs with a rate constant of energy transfer k_ET > 3 · 10¹² s^?1. The maximal dipole-dipole distance between the two BChl 800 molecules was calculated to be 18–19 Å. The porphyrin rings of the BChl 800 molecules are oriented parallel to each other, while their Q_y transition moments are mutually perpendicular. The energy-transfer efficiency from carotenoid to bacteriochlorophyll measured in different complexes showed that two functionally different carotenoids are present associated with, respectively, BChl 800 and BChl 850. Fluorescence polarization and linear dichroism spectra revealed that these carotenoids have different absorption spectra and a different orientation with respect to the membrane. The carotenoid associated with BChl 800 absorbs some nanometers more to the red and its orientation is approximately parallel to the membrane, while the carotenoid associated with BChl 850 is oriented more or less perpendicular to the membrane. The fluorescence polarization of BChl 850 was the same for the different complexes. This indicates that the observed polarization of the fluorescence is determined by the smallest complex obtained which contains 8–10 BChl 850 molecules. The B800–850 complex isolated with LDAO thus must consist of a highly ordered array of smaller structures. On basis of these results a minimal model is proposed for the basic unit consisting of four BChl 850 and two BChl 800 and three carotenoid molecules.     

A comparative study of the optical characteristics of intact cells of photosynthetic green sulfur bacteria containing bacteriochlorophyll c,d or e

Energy transfer and pigment arrangement in intact cells of the green sulfur bacteria Prosthecochloris aestuarii, Chlorobium vibrioforme and chlorobium phaeovibrioides, containing bacteriochlorophyll (BChl) c, d or e as main light harvesting pigment, respectively, were studied by means of absorption, fluorescence, circular dichroism and linear dichroism spectroscopy at low temperature. The results indicate a very similar composition of the antenna in the three species and a very similar structure of main light harvesting components, the chlorosome and the membrane-bound BChl a protein. In all three species the Q_y transition dipoles of BChl c, d or e are oriented approximately parallel to the long axis of the chlorosome. Absorption and fluorescence excitation spectra demonstrate the presence of at least two BChl c-e pools in the chlorosomes of all three species, long-wavelength absorbing BChls being closest to the membrane. In C. phaeovibrioides, energy from BChl e is transferred with an efficiency of 25% to the chlorosomal BChl a at 6 K, whereas the efficiency of transfer from BChl e to the BChl a protein is 10%. These numbers are compatible with the hypothesis that the chlorosomal BChl a is an intermediary in the energy transfer from the chlorosome to the membrane.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - CD circular dichroism - LD linear dichroism     

Low-temperature optical properties and pigment organization of the B875 light-harvesting bacteriochlorophyll-protein complex of purple photosynthetic bacteria

Optical and structural properties of the B875 light-harvesting complex of purple bacteria were examined by measurements of low-temperature circular dichroism (CD) and excitation spectra of fluorescence polarization. In the B875 complex isolated from wild-type Rhodopseudomonas sphaeroides, fluorescence polarization increased steeply across the long-wavelength Q_y bacteriochlorophyll a (BChl) absorption band at both 4 and approx. 300 K. With the native complex in the photosynthetic membranes of Rhodospirillum rubrum and Rps. sphaeroides wild-type and R26-carotenoidless strains, this significant increase in polarization from 0.12 to 0.40 was only observed at low temperature. A polarization of ?0.2 was observed upon excitation in the Q_x BChl band. The results indicate that about 15% of the BChl molecules in the complex absorb at wavelengths about 12 nm longer than the other BChls. All BChls have approximately the same orientation with their Q_y transition dipoles essentially parallel and their Q_x transitions perpendicular to the plane of the membrane. At low temperature, energy transfer to the long-wavelength BChls is irreversible, yielding a high degree of polarization upon direct excitation, whereas at room temperature a partial depolarization of fluorescence by energy transfer between different subunits occurs in the membrane, but not in the isolated complex. CD spectra appear to reflect the two spectral forms of B875 BChl in Rps. sphaeroides membranes. They also reveal structural differences between the complexes of Rps. sphaeroides and Rhs. rubrum, in both BChl and carotenoid regions. The CD spectrum of isolated B875 indicates that the interactions between the BChls but not the carotenoids are altered upon isolation.     

Antenna organization and energy transfer in membranes of Heliobacterium chlorum

Absorption, fluorescence emission and fluorescence excitation spectra of membranes of the recently discovered photosynthetic bacterium Heliobacterium chlorum (Gest, H. and Favinger, J.L. (1983) Arch. Microbiol. 136, 11–16) showed that at 4 K at least three spectroscopically different forms of bacteriochlorophyll g (BChl g 778, BChl g 793 and BChl g 808) can be discerned in the antenna system. Efficient energy transfer occurs from the short-wave-absorbing bacteriochlorophylls to BChl g 808. Energy transfer to bacteriochlorophyll, albeit with lower efficiency (70%), also occurred from the main carotenoid, neurosporene, and from a pigment absorbing at 670 nm. The complex structure of the antenna system is also reflected by fluorescence polarization and linear and circular dichroism spectra. Significant circular dichroism was only observed for BChl g 793, and different orientations were observed for the various Q_y transition dipoles, the one of BChl g 808 making a smaller angle with the plane of the membrane than those of the other bacteriochlorophylls.     

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted Q_y absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.     

Isolation and structural determination of C8-vinyl-bacteriochlorophyll d from the bciA and bchU double mutant of the green sulfur bacterium Chlorobaculum tepidum

The mutant lacking enzymes BciA and BchU, that catalyzed reduction of the C8-vinyl group and methylation at the C20 position of bacteriochlorophyll (BChl) c, respectively, in the green sulfur bacterium Chlorobaculum tepidum, were constructed. This mutant accumulated C8-vinyl-BChl d derivatives, and a molecular structure of the major pigment was fully characterized by its NMR, mass, and circular dichroism spectra, as well as by chemical modification: (3¹ R)-8-vinyl-12-ethyl-(R[V,E])BChl d was confirmed as a new BChl d species in the cells. In vitro chlorosome-like self-aggregates of this pigment were prepared in an aqueous micellar solution, and formed more rapidly than those of (3¹ R)-8,12-diethyl-(R[E,E])BChl d isolated from the green sulfur bacterium Chlorobaculum parvum NCIB8327d synthesizing BChl d homologs. Their red-shifted Q _y absorption bands were almost the same at 761 nm, and the value was larger than those of in vitro self-aggregates of R[E,E]BChl c (737 nm) and R[V,E]BChl c (726 nm), while the monomeric states of the former gave Q _y bands at shorter wavelengths than those of the latter. Red shifts by self-aggregation of the two BChl d species were estimated to be 110 nm and much larger than those by BChls c (75 nm for [E,E] and 64 nm for [V,E]).     

Absorption and circular dichroism spectra of CuCl2 complexed with poly(S-carboxymethyl-L-cysteine) and poly(S-carboxyethyl-L-cysteine)

The interaction of CuCl₂ with poly(S-carboxymethyl-L -cysteine) (poly[Cys(CH₂COOH)]) and poly(S-carboxyethyl-L -cysteine) (poly[Cys(C₂H₄COOH)]) were studied by absorption spectra and circular dichroism (CD). On mixing CuCl₂ with polypeptide solutions, absorption bands appeared at 320–325 nm in both polypeptides, and at 255–260 nm in the case of poly[Cys(CH₂COOH)]. A stable bound species was formed in the case of poly[Cys(CH₂COOH)], since the apparent molar absorption coefficient of the bound species did not depend on the mixing ratio. From the absorption data, it was inferred that Cu²⁺ ions were complexed with the side chains, most probably with sulfur atoms and carboxyl groups. Induced optical activities were observed for the two polypeptides. The CD spectra of poly[Cys(CH₂COOH)] + CuCl₂ gave simpler aspects than those of poly[Cys(C₂H₄COOH)] + CuCl₂.     

Purification, characterization and crystallization of the core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

A light-harvesting-reaction center (LH1-RC) core complex has been highly purified from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. The bacteriochlorophyll (BChl) a molecules in the LH1 exhibit a Q_y transition at 914 nm, more than 25 nm red-shift from those of its mesophilic counterparts. The LH1-RC complex was isolated in a monomeric form as confirmed by sucrose density gradient centrifugation, blue native PAGE and size-exclusion chromatography. Four subunits (L, M, H and a tetraheme cytochrome) in RC and two polypeptides (α and β) in LH1 were identified. Spirilloxanthin was determined to be the predominant carotenoid in the core complex. The purified core complex was highly stable, no significant change in the LH1 Q_y transition was observed over 10 days of incubation at room temperature in dark. Circular dichroism spectrum of the LH1 complex was characterized by low intensity and nonconservative spectral shape, implying a high symmetry of the large LH1 ring and interaction between the BChl a and carotenoid molecules. A dimeric feature of the BChl a molecules in LH1 was revealed by magnetic circular dichroism spectrum. Crystals of the core complex were obtained which diffracted X-rays to about 10 Å.     

Physical-chemical studies on the role of the metal ions in concanavalin A.

Substitution of Co²⁺ for Mn²⁺ in concanavalin A generates characteristic circular dichroism and magnetic circular dichroism spectra which are strongly affected by the concentration of Ca²⁺. With three equivalents of Ca²⁺ per protomer of [(Co²⁺)Con A], no spectral effects of addition of α-methyl-d-glucopyranoside can be demonstrated. With one equivalent of Ca²⁺, however, α-methyl-d-glucopyranoside alters the circular dichroism and magnetic circular dichroism spectra in a manner identical to that produced by adding further equivalents of Ca²⁺. Under these same conditions the higher molecular weight carbohydrates, trehalose and melezitose, cause no spectral alterations in the regions investigated.The magnetic circular dichroism spectrum of [(Co²⁺)Con A] is characterized by a negative peak centered at 510 nm (θ/gauss = ?0.28 °) and a pronounced shoulder at 462 nm (θ/gauss = ? 0.16 °). Comparison of this spectrum to that of Co(H₂O)₆²⁺ indicates that the transition metal ion exhibits octahedral geometry in solution and maintains this geometry in its interaction with carbohydrate moieties.Circular dichroism experiments in the far ultraviolet region indicate a change in secondary (presumed β) structure upon interaction of Apo Con A with Mn²⁺ consistent with a more ordered arrangement. Unlike Mn²⁺, cobalt alone will not induce these secondary changes until Ca²⁺ is added. Kinetic analysis, using a mannan light scattering assay, indicates that [(Mn²⁺)Con A] and [(Co²⁺)Con A] will slowly recover cross-linking function in the absence of Ca²⁺, suggesting that the role of the metal in S₂ is to accelerate a conformational change leading to binding or effector function.Overall, the data are consistent with a suggestion by Cuatrecasas (1973) that α-methyl-d-glucopyranoside binds to a locus different from the membrane binding (or agglutination) site. Nevertheless, there are strong conformational interactions between these two sites, since α-methyl-d-glucopyranoside will elute Con A from membrane surfaces.     

The organization of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus and the structural role of carotenoids and protein

The organization of bacteriochlorophyll c (BChl c) molecules was studied in normal and carotenoid-deficient chlorosomes isolated from the green phototrophic bacterium Chloroflexus aurantiacus. Carotenoid-deficient chlorosomes were obtained from cells grown in the presence of 60 µg of 2-hydroxybiphenyl per ml. At this concentration, BChl c synthesis was not affected while the formation of the 5.7 kDa chlorosome polypeptide was inhibited by about 50% (M. Foidl et al., submitted). Absorption, linear dichroism and circular dichroism spectroscopy showed that the organization of BChl c molecules with respect to each other as well as to the long axis of the chlorosomes was similar for both types of chlorosomes. Therefore, it is concluded that the organization of BChl c molecules is largely independent on the presence of the bulk of carotenoids as well as of at least half of the normal amount of the 5.7 kDa polypeptide. The Stark spectra of the chlorosomes, as characterized by a large difference polarizability for the ground- and excited states of the interacting BChl c molecules, were much more intense than those of individual pigments. It is proposed that this is caused by the strong overlap of BChl c molecules in the chlorosomes. In contrast to individual chlorophylls, BChl c in chlorosomes did not give rise to a significant difference permanent dipole moment for the ground- and excited states. This observation favors models for the BChl c organization which invoke the anti-parallel stacking of linear BChl c aggregates above those models in which linear BChl c aggregates are stacked in a parallel fashion. The difference between the Stark spectrum of carotenoid-deficient and WT chlorosomes indicates that the carotenoids are in the vicinity of the BChls.     

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted Q_y absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.     

Horse liver alcohol dehydrogenase derivatives containing nickel(II) and cobalt(lI) in the noncatalytic metal binding site

The noncatalytic zinc in horse liver alcohol dehydrogenase was selectively replaced by nickel(II). This novel species, Zn(c)₂Ni(n)₂⁴ horse liver alcohol dehydrogenase (where c denotes the catalytic and n denotes the noncatalytic site) was compared to Zn(c)₂Co(n)₂ horse liver alcohol dehydrogenase with respect to its absorption, circular dichroism and magnetic circular dichroism spectra, as well as its magnetic moment. For Zn(c)₂Co(n)₂ horse liver alcohol dehydrogenase (prepared according to refs. 1 and 2) the extinction coefficients were redetermined in the UV, visible and near-infrared region and the molar ellipticities in the range 300-800 nm. The average magnetic moment was determined by the NMR method as 4.5-5.0 B.M. The results confirm a tetrahedral structure in the zinc-cobalt enzyme. In contrast, the spectroscopic data and the zero magnetic moment support a planar geometry for the nickel(Il) bound in the noncatalytic site. Zn(c)₂Ni(n)₂ horse liver alcohol dehydrogenase is very temperature-sensitive and precipitates after short exposure to room temperature. Stored in the cold it has the same activity as the native enzyme. The results indicate that the protein is flexible in the loop region binding the noncatalytic metal ion and that it may retain catalytic activity even in a partially distorted conformation.