SPECTRAL FORMS AND ORIENTATION OF BACTERIOCHLOROPHYLLS c AND α IN CHLOROSOMES OF THE GREEN PHOTOSYNTHETIC BACTERIUM Chloroflexus aurantiacus

Spectral forms of bacteriochlorophyll (Bchl) in chlorosomes were analyzed by linear dichroism, circular dichroism (CD), and deconvolution of these spectra. Isolated chlorosomes were embedded in polyacrylamide gels and compressed unidirectionally (along the x-axis) while allowing the gel to stretch in another direction (along the z-axis). The chlorosomes were aligned three-dimensionally due to their flat oblong shape; the longest axis was presumed to parallel the z-axis, its shortest axis was presumed to parallel the x-axis, and the intermediate-length axis was presumed to parallel the y-axis. Degrees of polarization (A_I? A₁)/(A_I+ A₁) of Bchl c and a measured from the y-axis with linearly polarized light were significantly different from those measured from the x-axis. Deconvolution of spectra into components revealed the presence of two major forms of Bchl c with peaks at 744 nm and 727 nm. The degrees of polarization of the 744 and 727 nm spectral forms were 0.76 and 0.59 from the y-axis and 0.48 and 0.39 from the x-axis, respectively. The degrees of polarization of Bchl a794 were –0.21 from the y-axis and 0.12 from the x-axis. These values indicate that the direction of the Q_y transition moment of Bchl c744 is almost completely parallel to the longest axis of chlorosomes and that of Bchl c727 is also nearly, but slightly less so, parallel to the longest axis of the chlorosomes. The Q_y transition moment of the baseplate Bchl a peak at 794 nm is nearly perpendicular to the longest axis and parallel to the shortest axis: that is, it is perpendicular to the associated membrane plane in the cell. These alignments of Bchl transition moments in chlorosomes were lost after suspending the chlorosomes in a solution saturated with 1-hexanol accompanying a shift in the peak position from 742 nm to 670 nm. The alignment recovered after the hexanol concentration was decreased. The presence of two major spectral forms of Bchl c was supported by the deconvolution of CD spectra and absorption spectra.     

CHLOROPHYLL a FLUORESCENCE OF GONYAULAX POLYEDRA GROWN ON A LIGHT-DARK CYCLE AND AFTER TRANSFER TO CONSTANT LIGHT

Abstract— The chlorophyll a fluorescence properties of Gonyaulax polyedra cells before and after transfer from a lightdark cycle (LD) to constant dim light (LL) were investigated. The latter display a faster fluorescence transient from the level ‘I’ (intermediary peak) to ‘D’ (dip) to ‘P’ (peak) than the former (3 s as compared to 10 s), and a different pattern of decline in fluorescence from ‘I’ to ‘D’ and from ‘P’ to the steady state level with no clearly separable second wave of slow fluorescence change, referred to as ‘s' (quasi steady state)→‘M’ (maximum) →‘T’ (terminal steady state). The above differences are constant features of cells in LD and LL, and are not dependent on the time of day. They are interpreted as evidence for a greater ratio of photosystem II/photosystem I activity in cells in LL. After an initial photoadaptive response following transfer from LD to LL, the cell absorbance at room temperature and fluorescence emission spectra at 77 K for cells in LL and LD are comparable. The major emission peak is at 685–688 nm (from an antenna Chl a 680, perhaps Chl a-c complex), but, unlike higher plants and other algae, the emission bands at 696–698 nm (from Chl a_II complex, Chl a 685, close to reaction center II) and 710–720 nm (from Chl a₁, complexes, Chl a 695, close to reaction center I) are very minor and could be observed only in the fluorescence emission difference spectra of LL minus LD cells and in the ratio spectra of DCMU-treated to non-treated cells. Comparison of emission spectra of cells in LL and LD suggested that, in LL, there is a slightly greater net excitation energy transfer from the light-harvesting peridinin-Chl a (Chl a 670) complex, fluorescing at 675 nm, to the other antenna chlorophyll a complex fluorescing at 685–688 nm, and from the Chl a., complex to the reaction center II. Comparison of excitation spectra of fluorescence of LL and LD cells, in the presence of DCMU, confirmed that cells in LL transfer energy more extensively from the peridinin-Chl a complex to other Chl a complexes than do cells in LD.     

CHLOROPHYLL-PROTEINS AND REACTION CENTER PREPARATIONS FROM PHOTOSYNTHETIC BACTERIA,ALGAE AND HIGHER PLANTS*,‡

Abstract— The use of sodium dodecyl sulfate to dissociate photosynthetic membranes followed by standard fractionation techniques yields chlorophyll-proteins and reaction center complexes with molecular weights of 500,000 or less. Much about the structure and function of photo-synthetic units in vivo can be deduced from the properties of the isolated complexes. The Bchl-protein from green bacteria is approximated by an incompletely filled sphere ? 80 Â in diameter consisting of four identical subunits. The five Bchl molecules in each subunit are 14 to 20Â apart. The related Chl a-proteins from a blue-green alga and various eukaryotic plants may have similar structural characteristics. The Chl a-protein from a blue-green alga contains one molecule of P700 per 60–90 Chl a molecules. The quantum requirement for P700 oxidation is 2.6 or less. The midpoint potential in various preparations ranges from 0.38 V to 0.42 V. Green algae and higher plants yield a Chl a-protein similar to that from the blue-green alga; in addition they yield another Chl-protein (mol. wt. = 2–3×10⁴) which contains an equal amount of Chl a and Chl b. These two Chl-proteins account for most of the chlorophyll in these organisms. Two photosynthetic bacteria (Rhodopseudomonas viridis and Chromatium) yield protein complexes containing Bchl, carotenoid, and bound cytochromes. The reaction center complex from R. viridis contains P960 (E_m, ₈= 0.39 to 0.42 V), cytochrome 558 (E_m,8= 0.33 V) and cytochrome 553 (E_m,7=— 0.02 V). Quantum requirements for P960 and C558 oxidation are ?2.2 and 3.0, respectively. Complex A from Chromatium contains Bchl 890, P883, cytochrome 556 (E_m,8= 0.34 V) and cytochrome 552 (E_m,7=?0.04 V). The quantum requirement for C556 oxidation is about 15. Both high- and low-potential cytochromes can donate electrons to the reaction center chlorophyll present in either complex. This fact supports the idea that only one kind of photochemical reaction center functions in photosynthetic bacteria. An hypothesis about the nature of the photosynthetic unit in purple bacteria is outlined.     

STUDIES OF CHLOROPHYLL-CHLOROPHYLL AND CHLOROPHYLL-LIGAND INTERACTIONS BY VISIBLE ABSORPTION AND INFRARED SPECTROSCOPY AT LOW TEMPERATURES

Abstract— A comparison of the visible absorption and infrared spectra of various chlorophyll-chlorophyll (Chl) and Chi-nucleophile aggregates at room temperature and at low temperatures has been made. The IR data provide structural information indispensable for the interpretation of the visible spectra. As a necessary preliminary, it is shown that Chl a solutions in nonpolar solvents can be prepared by appropriate drying techniques that contain at a conservative estimate ≤ 3 mol % of water (i.e. Chl a/H₂O > 30:1). Very dry solutions of Chl a or Pyrochl a(≥ 10 mM) in toluene or methylcyclohexane-isopentane solution show only slight changes in visible spectra on cooling to 77 K. From IR, additional Chl-Chl aggregation occurs on cooling in methylcyclohexane-isopentane but not to a significant extent in toluene. Dilute (10 μM) solutions of Chl a or Pyrochl a in nonpolar solvents form a new absorption peak near 700 nm at low temperatures, which we attribute to traces of water in the solvent or other residual nucleophiles not removed during the Chl purification. Addition of stoichiometric amounts of water increases the size of the ?700 nm peak even in dilute Chl solutions. Chlorophyll a, Pyrochl a, but not pheophytin a are shown to interact with nucleophiles of the general type RXH (where R= H or alkyl, and X = O, N, or S). Such nucleophiles can coordinate to the Mg atom of one Chl molecule by lone pairs on O, N, or S, and hydrogen bond to oxygen donor functions in another Chl molecule. A ?0.1 M solution of Chl a or Pyrochl a in toluene containing 1.5 equivalents of ethanol is converted almost entirely to a species absorbing at ?700 nm at 77 K. Infrared spectroscopy shows conclusively that it is the keto C=O function that is involved in the cross-linking by hydrogen bonding, a conclusion supported by the observation that Pyrochl a forms a very similar red-shifted species at low temperatures, despite the absence of a carbomethoxy C=O function. n-Butylamine and ethanethiol interact in much the same way as does ethanol to form species red shifted to ?700 nm. A variety of possible structures for the low temperature forms is discussed, and the use of these red shifted species as paradigms for photoreaction center Chl is described.     

Biphasic fluence-response curves and derived action spectra for light-induced absorbance changes in Phycomyces mycelium

The photoresponses of Phycomyces, including phototropism and photocontrol of sporangiophore development, are mediated primarily by blue and UV light. Recent results on these two responses indicated a subsidiary role for green light. We have measured in vivo light-induced absorbance changes (LIAC) in mycelial samples of a caroteneless (carB) strain to compare the effectiveness of UV, blue, and green light. In the dual-wavelength kinetic mode of the spectrophotometer, measuring wavelengths of 445 and 470 nm were chosen, because green light produces substantial absorbance changes between these two wavelengths. Fluence-response curves were measured for 13 wavelengths between 365 and 530 nm, and for variable exposure times between 0.5 and 320 s. With one exception (365 nm), the curves were biphasic. The low fluence component was generally sigmoidal with an abrupt rise. The high fluence component failed to reach saturation for the fluences tested (less than 70 μmol m⁻² s⁻¹). Using the inferred threshold fluences of these two components as criterion effects, we obtained two action spectra. For the low fluence component, the action spectrum showed major peaks at 394, 450, and 530 nm and a minor peak at 416 nm. The high fluence component action spectrum showed very little sensitivity in the blue region. The major sensitivity was in the near UV, and a relatively small peak appeared in the green part of the spectrum at 507 nm. The biphasic character of the fluence-response curves suggests that two photosystems are responsible for the absorbance changes. The low fluence photosystem is sensitive mainly to blue and UV light and may thus represent a physiological blue-light photoreceptor. The high fluence photosystem is clearly not of this type. It (and perhaps the low fluence system as well) may mediate some of the subsidiary physiological effects of green light.     

MAGNETIC CIRCULAR DICHROISM OF BACTERIOCHLOROPHYLL a IN SOLUTION AND IN A PROTEIN

Abstract— The magnetic circular dichroism (MCD) (300–850nm) of the bacteriochlorophyll (Bchl) a -protein from the green photosynthetic bacterium Prosthecochloris aestuarii 2 K is qualitatively similar to the MCD of Bchl a in methanol and ether solution. This result implies that the transition dipole of the lowest energy electronic transition (near 800 nm) is roughly perpendicular to the transition dipole of the next higher electronic band (near 600nm) for Bchl a molecules in the protein just as it is for molecules in solution. This result provides no support for the recent proposal that interactions with the protein rotates the direction of the transition dipoles of the 800nm band of all the Bchl a molecules in the protein by 90°. While a rotation of the 800nm transition dipoles cannot be rigorously excluded, it would be necessary for the postulated perturbation to rotate the transition dipoles of both the 800 and 600nm bands by 90°. In a broader sense, any postulated perturbation would have to be shown to leave both the absorption spectrum and the MCD largely unaffected. MCD is a more sensitive test than absorption spectroscopy for perturbations of electronic states and changes in the relative orientation of transitions, because it depends on both the magnitudes and directions of at least two electric and one magnetic transition dipole.     

Theoretical Study of Fluorescence Spectra Utilizing the pKa Values of Acids in Their Excited States

Assignment of the fluorescence spectrum of firefly luciferin in aqueous solutions was achieved by utilizing not only emission energies but also theoretical absorption spectra and relative concentrations as estimated by pK_a values. Calculated Gibbs free energies were utilized to estimate pK_a values. These pK_a values were then corrected by employing the experimental results. It was previously thought that the main peak near 550 nm observed in the experimental fluorescence spectra at all pH values corresponds to emission from the first excited state of the luciferin dianion [Ando et al. (2010) Jpn. J. Appl. Phys. 49, 117002–117008]. However, we found that the peak near 550 nm at low pH corresponds to emission from the first excited state of the phenolate monoanion of luciferin. Furthermore, we found that the causes of the red fluorescence at pH 1–2 are not only the emission from phenol monoanion but also the emission from the protonated species at nitrogen atom in the thiazoline ring of dianion.     

ASSOCIATION OF CHLOROPHYLL WITH AMIDES ON PLASTICIZED POLYETHYLENE PARTICLES—I. N,N-DIMETH YLM YRISTAMIDE*

Abstract— Model systems have been prepared in which chlorophyll a (Chl a) and N.N-dimethylmyristamide (DMMA) are adsorbed together in various ratios to particles of polyethylene swollen with undecane. The adsorption is performed by equilibrating the particles with methanol-water solutions of increasing water content. Absorption spectra of the coated particles in viscous suspenions show sharp well-marked bands over much of the composition range examined. With the aid of second derivative spectra. the red absorption band has been resolved into three components. at 661.5. 674 and 680 nm. Fluoresccnce spectra have also been resolved into their principal components with some assistance from comparison with spectra of Chl in undecane solution containing DMMA. At room temperature (295 K) the resolvable components are of monomeric Chl at 670 nm. and of associated species at 681 and 725 nm. Fluorescence at 77 Kis of similar intensity but is distributed differently. in favor of longer-wave components. Corresponding to the 295 K components are emission bands at 675, 683–5 and 735 nm. Othcr components appear under certain conditions: at 695–700 nm when the Chl and DMMA conccntrations are both high, and at 705 nm whcn the ratio of DMMA to Chl is low. If DMMA is absent or at low concentration, much of the Chl exists as an aggregated form absorbing near 741 nm and fluorescing weakly near 760 nm at 77 K. Adsorption isotherms indicate some degree of cooperativity in the binding of Chl and DMMA to the particles. The photoreduction of p-dinitrobenzene by hvdrazobenzene. scnsitized by these particles, has been demonstrated     

Photophysical Characterization of the Naturally Occurring Dioxobacteriochlorin Tolyporphin A and Synthetic Oxobacteriochlorin Analogues

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium‐containing culture known as HT‐58‐2. Tolyporphins A–J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1‐pentanol, 2‐butanone, ethanol, methanol, N,N‐dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time‐resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited‐state photophysical parameters across the solvents. The long‐wavelength (Q_y) absorption band of tolyporphin A appears at ~680 nm and is remarkably narrow (full‐width‐at‐half‐maximum ~7 nm). The position of the long‐wavelength (Q_y) absorption band of tolyporphin A (~680 nm) more closely resembles that of chlorophyll a (662 nm) than bacteriochlorophyll a (772 nm). The absorption spectra of tolyporphins B–I, K (which were available in minute quantities) are also reported in methanol; the spectra of B–I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.     

Iron(II), cobalt(III), nickel(II) and copper(II) chelates of furan and thiophene azo-oximes

Summary Complexes of furan and thiophene azo-oximes with iron(II), cobalt(III), nickel(II) and copper(II) have been prepared and characterised. Iron(II), cobalt(III) and copper(II) complexes are diamagnetic in the solid state. The diamagnetism of the copper(II) chelates is suggestive of antiferromagnetic interaction between two copper centres.¹H n.m.r. spectral data suggest atrans-octahedral geometry for the tris-chelates of cobalt(III). Nickel(II) complexes are paramagnetic, in contrast to the diamagnetism of the analogous complexes of arylazooximes. The electronic spectra are suggestive of octahedral geometry for the iron(II), cobalt(III) and nickel(II) complexes, andD _4h -symmetry for copper(II). Infrared data indicate N-bonding of the oximino-group to the metal ions.     

THE EFFECT OF PERIPHERAL SUBSTITUTION ON THE BATHOCHROMIC SHIFT OF THE Qy TRANSITION OF BACTERIOCHLOROPHYLL DIMERS; in vitro MODELS OF THE PROTEIN EFFECT ON THE SPECTRUM OF PIGMENT CENTERS IN THE LIGHT-HARVESTING COMPLEXES*

Abstract— The effect of chemical modifications in the side groups of the isocyclic ring V on the formation, optical absorption and circular dichroism of bacteriochlorophyll (Bchl) dimers was examined in a mixture of formamide and water containing TritonX–100 and variable amounts of pyridine. Substitution of the carbomethoxy group in the C13² position with a hydrogen atom, had no effect on the dimerization constant but increased the shift of the Q_y transition by 1000 cm^-1 with respect to the native Bchl. Substitution of the C13 hydrogen atom with OH decreased the shift of the Q_y transition by 400 cm^-1. The similarity between the spectra of the modified Bchl dimers and Bchl dimers in vivo indicates that protein binding to the side groups at Bchl dimers may profoundly affect the energy of their Q_y transition but have minor effects on the Q_y transitions of the monomelic Bchl.     

ACTION SPECTRA FOR THE ABSORBANCE CHANGE AT 880 nm AND FOR P870 FLUORESCENCE FROM A PHOTOSYNTHETIC REACTION CENTER*

Abstract— Photosynthetic reaction centers isolated from blue-green mutant strain R-26 of Rhodopseudomonas spheroides were used to study the action spectra for the light-induced absorption changes at 880 nm, and for the P870 fluorescence. These spectra indicated that a pigment, possibly bacteriopheophytin, with an absorption maximum at 757 nm is an intrinsic component of the reaction center and transfers energy to P870 with relatively high efficiency.     

Spectroscopical UV–Vis implications of an intramolecular η2–Mg coordination in bacteriochlorophyll–a from the Fenna–Matthews–Olson complex

Chlorophylls and their related compounds prominently feature a Mg²⁺ ion in the center of a porphyrine, with an intermolecular fifth coordination usually observed to place the ion out of the macrocyclic plane. Herein, we assess the role of a potential intramolecular η²–(C = C)Mg interaction and compare it to the intermolecular coordination from the Hystidine groupt to Mg²⁺ for Bacterichlorophyll–a (Bchl–a), the main photosynthetic pigment in the Fenna–Matthews–Olson complex present in green and purple bacteria. The influence of this fifth coordination on the UV‐Vis spectroscopy (CAM‐B3LYP/cc‐pVDZ), and the concomitant change in geometry around Mg in Bchl–a from planar to pyramidal is assessed by the quantum theory of atoms in molecules based non–covalent interactions scheme and through energetic analysis via natural bond orbital population methods at the M06‐2X/cc‐pVDZ and compared to the reference multi–hapto compound, magnesocene, Cp₂Mg.     

Synthesis and luminescence of polymeric metal complexes based on 1,10-phenanthroline and 8-hydroxyquinoline

A ligand containing different coordination groups, 5-([1,10]phenanthroline-[4,5-f]imidazo-2-yl)-8-hydroxyquinoline (PhenI8Q) has been synthesized and two corresponding polymeric metal complexes Cu(II) (1) and Zn(II) (2) were prepared by coordination polymerization of the ligand with copper(II) and zinc(II) halides, respectively. The ligand was characterized by ¹H-NMR, ¹³C-NMR, and Fourier transform-infrared (FT-IR) and its corresponding polymeric metal complexes 1 and 2 were characterized by FT-IR, UV-Vis, elemental analysis, thermal gravimetric analysis, and conductivity measurements. The absorption spectra and luminescence of the ligand, 1, and 2 were investigated by UV-Vis and fluorescence spectroscopy at room temperature. Compared with the ligand, the fluorescence spectra of the polymeric metal complexes exhibit blue shifts in dimethyl sulfoxide (DMSO) solution and bathochromic shifts in the solid state. Complexes 1 and 2 emit blue light with emission maximum (λ _{f max}) at 449 and 431 nm in DMSO solution and at 485 and 484 nm in the solid state, respectively.     

PHYCOBILIPROTEINS: COMPARISON OF SOLUTION AND SINGLE CRYSTAL FLUORESCENCE FOR C-PHYCOCYANIN AND B-PHYCOERYTHRIN*

Abstract— Trimeric and hexameric solution forms of C-phycocyanin (CPC) from the cyanophyte Agme-nellum quadruplicatum have been isolated and their spectral properties compared to those obtained from single crystals. Although the absorbance peak of a suspension of small C-phycocyanin crystals is red-shifted only 7 nm relative to the solution forms, the single crystal fluorescence is red-shifted 60 nm relative to the solution forms. The crystal fluorescence spectrum exhibits a single peak at LD_max= 708 nm when excited at 514.5 or 530.9 nm and two peaks (LD_max= 661 and 708 nm) when excitation occurs at 568.2 nm. Fluorescence depolarization measurements indicate that extensive energy transfer could occur for both solution and crystal forms with the latter being dependent upon the relative orientation of the crystal with respect to the excitation dipole. Similar results were obtained with B-phycoerythrin (BPE) from the red alga Porphyridium cruentum where the single crystal fluorescence is red-shifted =50nm relative to the solution spectra with two peaks (LD_max= 583 and 617 nm) observed whose relative intensities are dependent on the excitation wavelength (LD_max– 514.5 and 530.9 nm). Single crystal fluorescent lifetimes exhibited considerable shortening relative to that observed for the solution forms. The implications of these results are discussed with respect to the possible relationships of the crystalline structures to the assembly forms present within phycobilisomes.     

Phase equilibria in the SrS-Ga2S3 system

In the SrS-Ga₂S₃ system, there exist two individual compounds: SrGa₂S₄ (a = 2.084 nm, b = 2.050 nm, c = 1.220 nm; congruent melting at 1530 K) and Sr₂Ga₂S₅ (a = 1.253 nm, b = 1.203 nm, c = 1.117 nm; peritectic melting at 1330 K); both are orthorhombic. We discovered a compound of composition Sr₄Ga₂S₇; this compound crystallizes in cubic system with the unit cell parameter a = 0.6008 nm, space group Pa3, and decomposes by a solid-phase reaction at 870 K. Eutectic compositions are 42 and 73 mol % Ga₂S₃; eutectic melting temperatures are 1210 and 1170 K, respectively. The SrS solubility in γ-Ga₂S₃ at 1070 K reaches 4 mol %.     

Syntheses,crystal structures,electrochemical studies,and antioxidant activities of zinc(II) and copper(II) complexes with bis(2-benzimidazolyl) aniline derivatives

Two ligands, bis(benzimidazol-2-ylmethyl) aniline (bba) and bis(N-methyl-benzimidazol-2-ylmethyl) aniline (Mebba), and their transition metal complexes [Zn(bba)(Br)₂]·2DMF (1) and [Cu(Mebba)(Br)₂]·2DMF (2) have been synthesized and characterized by elemental analyses, molar conductivities, UV–vis spectra, IR spectra, NMR spectroscopy, and X-ray crystallography. The structure around Zn(II) can be described as distorted tetrahedral. Complex 2 can be described as distorted trigonal bipyramidal. Cyclic voltammograms of 2 indicate a quasireversible Cu²⁺/Cu⁺ couple. Additionally, the antioxidant activities of the free ligands and their complexes were determined by the superoxide and hydroxyl radical scavenging methods in vitro. Complexes 1 and 2 possess potent hydroxyl radical scavenging activity and better than standard antioxidants such as vitamin C and mannitol. Complex 2 possesses excellent superoxide radical activity.     

Theoretical Study of Firefly Luciferin pKa Values—Relative Absorption Intensity in Aqueous Solutions

Ground‐state vibrational analyses of firefly luciferin and its conjugate acids and bases are performed. The Gibbs free energies obtained from these analyses are used to estimate pK_a values for phenolic hydroxy and carboxy groups and the N–H⁺ bond in the N‐protonated thiazoline or benzothiazole ring of firefly luciferin. The theoretical pK_a values are corrected using the experimental values. The concentrations of these chemical species in solutions with different pH values are estimated from their corrected pK_a values, and the pH dependence of their relative absorption intensities is elucidated. With the results obtained we assign the experimental spectra unequivocally. Especially, the small peak near 400 nm at pH 1–2 in experimental absorption spectra is clarified to be due to the excitation of carboxylate anion with N‐protonated thiazoline ring of firefly luciferin. Our results show that the pK_a values of chemical species, which are contained in the aqueous solutions, are effective to assign experimental absorption spectra.     

SITE-DIRECTED MUTAGENESIS OF THE LH2 LIGHT-HARVESTING COMPLEX OF Rhodobacter sphaeroides: CHANGING βLys23 TO Gin RESULTS IN A SHIFT IN THE 850 nm ABSORPTION PEAK

The present study describes the construction of a Rhodobacter sphaeroides light-harvesting (LH2) mutant in which the charged residue βSLys23 is changed by site-directed mutagenesis to a Gin residue, and the characterization of the resulting mutant complex by a range of spectroscopic techniques. In the 77 K absorption spectrum of the mutant, the peak equivalent to the 850 nm peak in the wild-type membrane is blue-shifted by approximately 18 nm to 837 nm; except for this blue-shift, the 77 K. fluorescence excitation and emission spectra and the circular dichroism spectrum of the mutant are very similar to the equivalent spectra from the wild-type membranes, suggesting that the mutation βLys23 → Gin probably does not cause any major changes in the conformation or aggregation state of these membranes. Possible causes of the 18 nm blue-shift in the absorption spectrum are discussed.     

Concentration-controlled Zn(II) compounds constructed from N-tosyl-L-glutamic acid and 1,10-phenanthroline

By controlling the concentration of the reaction system, two zinc(II) complexes, [Zn₂(tsgluO)₂(phen)₂]_n (1) and [Zn₂(tsgluO)₂(phen)₂(H₂O)₂] (2) (H₂tsgluO = N-tosyl-L-glutamic acid, phen = 1,10-phenanthroline), have been synthesized under hydrothermal conditions and characterized by elemental analysis, infrared spectra, thermogravimetric analysis, and single-crystal X-ray diffraction. Complexes 1 and 2 both crystallize in the triclinic space group P-1. Complex 1 exhibits a 1-D double-chain structure. Complex 2 has a dinuclear structure which is extended by hydrogen-bonding interactions to form a 2-D supramolecular network. The structural difference indicates that the concentration plays a crucial role in modulating structures of coordination complexes. The two compounds also show intense fluorescence at room temperature.