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ABSTRACT: The resonance Raman spectra of aminoazobenzene derivatives in acidic solution are investigated. The marked spectral changes observed on decreasing pH are related to transformations between azo and quinoid forms. The pKa value of 4-amidino, 4′-diméthylaminoazobenzene obtained from intensity measurements is in good agreement with the value obtained from visible absorption measurements. Comparison of resonance Raman spectra of some substituted aminoazobenzene derivatives and the quinoid form of an hydroxyazobenzene derivative enable a more complete assignment of the bands observed between 900 and 1700 cm−1 to be made. All the spectra are rather similar and the various ring substituents are not resonance Raman active. The influence of the NHN vibration on resonance Raman spectra of the quinoid forms is demonstrated using deuterium substitution. In strong acid solution a second protonation occurs; a tautomeric equilibrium between azo and quinoid forms of aminoazobenzene derivatives is demonstrated using different exciting laser lines.
Journal of Raman Spectroscopy 04/2005; 11(2):131 - 137. · 3.09 Impact Factor
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ABSTRACT: Resonance Raman spectra of azobenzene and some hydroxyazobenzene derivatives, in strong acid solution (HCI 6 M), are analysed. All the observed bands between 900 and 1700 cm−1 can be assigned to vibrational modes of the aromatic rings and the azo group, and give evidence for a trans azo type structure both for azobenzene and the para substituted hydroxyazobenzene derivatives studied. Resonance Raman spectra of six substituted 2-(4′- hydroxyphenylazo)-benzoic acid derivatives in strong acid solutions are investigated. The spectral features are rather different from the above studied hydroxyazobenzene derivatives, but can be explained on the basis of an azo type structure. A strong perturbation effect on the NN stretching mode is demonstrated using deuterium substitution. From the complexity of frequency shifts obtained on deuterium substitution, vibrational coupling between NH bending and many other vibrational modes is evident. A tentative assignment of the observed resonance Raman bands is proposed.
Journal of Raman Spectroscopy 04/2005; 11(3):209 - 214. · 3.09 Impact Factor
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ABSTRACT: Resonance Raman spectra of twenty parasubstituted azobenzene derivatives are compared, enabling a more complete assignment of bands to be made. Since all the spectra are rather similar, the various ring substituents are not resonance Raman active, and all the observed bands can be assigned to vibrational modes of the aromatic rings and to the azo group itself. Frequency shifts in the PhN stretching region are opposite to those expected based on delocalization along the PhNNPh moiety. From the complexity of band pattern around the NN stretching region and from the above described frequency shifts, several types of vibrational coupling are evident. On the basis of the band assignments, some useful conclusions can be reached for interpreting the observed spectral changes when azobenzene derivatives are used as chromophore probes in biochemical system studies.
Journal of Raman Spectroscopy 04/2005; 8(2):81 - 87. · 3.09 Impact Factor
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ABSTRACT: A nanosecond transient absorption spectroscopy study of flavone performed with a 248 nm pump radiation has been investigated with the support of a chemometric treatment: SIMPLISMA. The experimental spectra obtained in various solvent with a pump-probe delay lower than about 2 micros are in quite good concordance with those already presented in the literature. Nevertheless after about 10 micros, the spectrum pattern significantly evolves as a function of time particularly for the methanolic solution. A qualitative analysis together with a SIMPLISMA chemometric treatment of the experimental data allowed to elucidate and characterise two interdependent transient species in the alcoholic medium: the lowest T1 triplet state of flavone and the ketyl radical forming by H-abstraction reaction from the solvent. In cyclohexane and acetonitrile, the same species seem to be produced in the studied time-scale but the radical form is generated with variable quantum yield depending on the solvent polarity. The pure spectrum and the photochemical kinetics of each reaction intermediate could have been determined with the help of the second derivative SIMPLISMA calculation procedure.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 11/2002; 58(12):2633-45. · 2.10 Impact Factor
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ABSTRACT: Complex formation between aluminium and quercetin (Q) in methanol was studied by the combined use of spectroscopic measurements and quantum chemical calculations. Quercetin presents in its structure three possible chelating sites in competition. UV-visible spectroscopy has showed the successive formation of two complexes of stoichiometry Al(III):Q of 1:2 and 2:1, respectively. The first site involved in the complex formation process is the 3-hydroxychromone and the second one is the ortho-dihydroxyl group. Semiempirical treatment, using the AM1 hamiltonian, permitted calculation of the structural modifications engendered by the ligand through chelation of one then two aluminium ions. The electronic and vibrational spectra have been calculated with the same method in order to compare them to the experimental spectra and so confirm the involved chelating sites. The simulated electronic spectra obtained from the complex models are in good agreement with the experimental UV-visible absorption spectra. In the same way the vibrational spectra of the complexes validate the proposed complex formation mechanism. The pH influence on the complexes stoichiometry and on the preferentially occupied chelating sites has been also investigated.
Journal of Inorganic Biochemistry 10/2002; 92(1):19-27. · 3.35 Impact Factor
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ABSTRACT: Complex formation between aluminium chloride and 3'4'-dihydroxyflavone (3'4'diOHF) in methanol has been studied by UV-visible and Raman spectroscopies combined with quantum chemical calculations. Job's method of continuous variation and the molar ratio method were applied to ascertain the stoichiometry composition of the chelate in pure methanol. A 1:1 complex was indicated by both the methods. Geometry optimizations of free and complexed molecules by AMI and DFT methods show that structural modifications of the ligand, induced by complexation, are minor, and are localized on the chelating site. The good agreement between experimental and theoretical electronic spectra of both 3'4'diOHF and complex confirm the structural models. The great similarities between Raman spectra of the free and complexed form constitute an another proof of the absence of pronounced electronic and geometric changes, and notably demonstrate that the quinoidal form induced by the deprotonation of the two hydroxyl groups does not participate in the 3'4'diOHF complex structure. Whereas no complexation occurs in acidic medium, complexes of high stoichiometry are formed in alkaline medium. (Al(3'4'diOHF)2)- and (Al(3'4'diOHF)3)3- species are observed in methanol in the presence of sodium acetate or sodium methanoate.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 04/2001; 57(3):591-602. · 2.10 Impact Factor
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ABSTRACT: The FT-infrared and Raman microscopy spectra of the 2',6'-dimethoxyflavone and its 1:1 complex with formic acid in solid state have been recorded and analysed. Some vibrational components appear as specific to the cis-rotamer of formic acid in the crystalline sample, especially the CH group stretching vibration feature. The broad and intense infrared absorption observed in the range 3400-1900 cm(-1) and assigned to the hydrogen bonded OH group stretching vibration exhibits the characteristic ABC structure of strong hydrogen bonded complexes. This ABC pattern corroborates previous X-ray crystallographic data showing that cis-formic acid is strongly hydrogen bonded to the flavonic compound. The inclusion complex is quite unstable and the infrared spectrum clearly shows that formic acid disappears after a period of a few months. In order to get some information on the stability criterions of the intermolecular hydrogen bonded complex, semiempirical AM1 calculations have been investigated. The comparison of the calculated heats of complexation (deltacH) for chelates involving the cis- and trans-conformers of formic acid suggests that the reaction of hydrogen bonding complexation with the cis-rotamer is surely favoured.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 12/2000; 56A(12):2439-50. · 2.10 Impact Factor
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ABSTRACT: 3-Hydroxyflavone (3HF), which is the simplest molecule of the flavonol class, possesses chelating properties towards Al(III). Spectrophotometric methods have shown that the 3HF molecule forms an Al(3HF)2 complex in pure methanol. The structure of this complex, obtained by quantum semi-empirical AM1 method, indicated that complexed 3HF adopts a pyronium form. Structural and electronic modifications induced by chelation are illustrated by the important frequency shifts observed between free and complexed 3HF FT-Raman spectra and by the chemical shifts variations in the 13C NMR spectra of the two species. Complexes with the same stoichiometry were formed when AcO- or MeO- are present in the medium. However, in acidic medium the chelate composition is Al2(3HF).
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 04/2000; 56(4):829-39. · 2.10 Impact Factor
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ABSTRACT: Resonance Raman spectroscopy and quantum chemical calculations were used to investigate the molecular origin of the large redshift assumed by the electronic absorption spectrum of astaxanthin in alpha-crustacyanin, the major blue carotenoprotein from the carapace of the lobster, Homarus gammarus. Resonance Raman spectra of alpha-crustacyanin reconstituted with specifically 13C-labeled astaxanthins at the positions 15, 15,15', 14,14', 13,13', 12,12', or 20,20' were recorded. This approach enabled us to obtain information about the effect of the ligand-protein interactions on the geometry of the astaxanthin chromophore in the ground electronic state. The magnitude of the downshifts of the C==C stretching modes for each labeled compound indicate that the main perturbation on the central part of the polyene chain is not homogeneous. In addition, changes in the 1250-1400 cm(-1) spectral range indicate that the geometry of the astaxanthin polyene chain is moderately changed upon binding to the protein. Semiempirical quantum chemical modeling studies (Austin method 1) show that the geometry change cannot be solely responsible for the bathochromic shift from 480 to 632 nm of protein-bound astaxanthin. The calculations are consistent with a polarization mechanism that involves the protonation or another interaction with a positive ionic species of comparable magnitude with both ketofunctionalities of the astaxanthin-chromophore and support the changes observed in the resonance Raman and visible absorption spectra. The results are in good agreement with the conclusions that were drawn on the basis of a study of the charge densities in the chromophore in alpha-crustacyanin by solid-state NMR spectroscopy. From the results the dramatic bathochromic shift can be explained not only from a change in the ground electronic state conformation but also from an interaction in the excited electronic state that significantly decreases the energy of the pi-antibonding C==O orbitals and the HOMO-LUMO gap.
Biospectroscopy 02/1999; 5(6):358-70.
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ABSTRACT: Semiempirical AM1 calculations have been carried out for beta-carotene and the three xanthophylls (zeaxanthin, canthaxanthin, and astaxanthin) containing oxygen functions (hydroxy/keto groups) found in the majority of natural pigment. The fully optimized geometries correspond well with the X-ray structures of beta-carotene and canthaxanthin and indicate that substitutions on the terminal rings have a minimal effect on the conformation of the chromophore. Twisting along the polyenic chain results from steric interaction involving methyl substituents, and a Ci point group can be proposed for the four investigated carotenoids. AM1 calculated excitation energies for the strongly allowed excited states can be compared to with the experimental absorption band in the visible region, considering solvent effect. Resonance Raman (RR) and Fourier transform (FT) Raman spectra of natural astaxanthin as well as astaxanthins specifically 13C labeled at the positions 12,12'; 13,13'; 14,14'; 15,15'; 15, and 20,20' were recorded. Furthermore the RR and FT Raman spectra of the asymmetric carotenoid 20-norastaxanthin are presented. The data reveal a substantial amount of information about the coupling between the different vibrations, and enabled an extensive experimental verification of the theoretical normal-coordinate analysis previously performed on polyenic molecules [J Raman Spectrosc 1983, 14, 310-321; Advances in Infrared and Raman Spectroscopy, Vol. 12, 1985, pp. 115-178; Spectrochim Acta 1996, 53, 381-392; Biochim Biophys Acta 1994, 1185, 188-196]. The results make up a very interesting dataset which allowed the interpretation and/or observation of several, hitherto never observed or not well understood, effects in the Raman spectra of the differently labeled astaxanthins.
Biospectroscopy 02/1999; 5(1):19-33.
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ABSTRACT: As a follow-up to structural studies of monohydroxylated flavones, the structural and spectroscopic properties of a tetrahydroxylated flavone, the quercetin molecule, have been investigated. The molecular conformation of quercetin has been obtained from semiempirical treatment with the AM1 Hamiltonian. Some structural modifications have been observed between the molecule in the solid state and in an isolated state, notably in the rotation of the phenyl ring with respect to chromone part of the compound. The theoretical model has been validated by both vibrational and electronic spectroscopies. The calculated vibrational and UV-vis spectra are in good accordance with the experiments. The Raman spectra have been assigned, and the main electronic transitions involved in the absorption spectrum have been characterized. © 1997 John Wiley & Sons, Inc. Biospect 3: 183–193, 1997
Biospectroscopy 12/1998; 3(3):183 - 193.
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ABSTRACT: Selective isotope enrichment, 13C magic angle spinning (MAS) NMR, and semiempirical quantum chemical modeling, have been used to analyze ligand-protein interactions associated with the bathochromic shift of astaxanthin in alpha-crustacyanin, the blue carotenoprotein complex from the carapace of the lobster Homarus gammarus. Spectra of alpha-crustacyanin were obtained after reconstitution with astaxanthins labeled with 13C at positions 4,4', 12,12', 13,13', or 20,20'. The data reveal substantial downfield shifts of 4.9 and 7.0 ppm at positions 12 and 12' in the complex, respectively. In contrast, at the 13 and 13' positions, small upfield shifts of 1.9 ppm were observed upon binding to the protein. These data are in line with previously obtained results for positions 14,14' (3.9 and 6.8 ppm downfield) and 15,15' (0.6 ppm upfield) and confirm the unequal perturbation of both halves after binding of the chromophore. However, these results also show that the main perturbation is of symmetrical origin, since the chemical shift differences exhibit a similar pattern in both halves of the astaxanthin molecule. A small downfield shift of 2.4 ppm was detected for the 4 and 4' positions. Finally, the 20,20' methyl groups are shifted 0.4 ppm upfield by the protein. The full data set provides convincing evidence that charge polarization is of importance for the bathochromic shift. The NMR shifts are compared with calculated charge densities for astaxanthin subjected to variations in protonation states of the ring-functional groups, as models of ligand-protein interactions. Taking into account the color shift and other available optical data, the current model for the mechanisms of interaction with the protein was refined. The results point toward a mechanism in which the astaxanthin is charged and subject to strong electrostatic polarizations originating from both keto groups, most likely a double protonation.
Biochemistry 07/1997; 36(24):7288-96. · 3.42 Impact Factor
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ABSTRACT: Two reconstituted carotenoproteins have been studied by resonance Raman spectroscopy. They were prepared from the apoprotein of the Asterias rubens carotenoprotein, asteriarubin and either astaxanthin or 15,15'-didehydroastaxanthin. Spectral properties of dehydrocarotenoids are first discussed. The spectral properties of the complexes are compared to those of the free carotenoids and of other carotenoproteins containing astaxanthin, and possible protein-carotenoid interactions are discussed. Greater delocalisation of the pi-electron system in the central part of the polyene chain, and the role of lateral methyl groups in binding is emphasised.
Biochimica et Biophysica Acta 07/1987; 913(2):111-6. · 4.66 Impact Factor
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ABSTRACT: Yersinia enterocolitica 195A14J, a milk-isolated strain of biovar 1, produces a non-diffusible yellow pigment and forms star-shaped colonies when grown on egg-yolk agar at 28 degrees C. Solubility properties and in situ Raman spectrum of the pigment support evidence that it is not a carotenoid, although it contains a 9 (+/- 1) double-bond polyenic chain. Pigmentless variants, also star-shaped, appeared with a frequency of ca.10(-3) when bacteria were grown at 38 degrees C. Agarose gel electrophoresis of DNA extracted from the pigmented strain revealed the presence of a 42-kb plasmid which was lost in pigmentless variants.
Annales de l Institut Pasteur Microbiologie 138(6):609-15.
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ABSTRACT: Complex formation between aluminium chloride and 3′4′-dihydroxyflavone (3′4′diOHF) in methanol has been studied by UV–visible and Raman spectroscopies combined with quantum chemical calculations. Job's method of continuous variation and the molar ratio method were applied to ascertain the stoichiometry composition of the chelate in pure methanol. A 1:1 complex was indicated by both the methods. Geometry optimizations of free and complexed molecules by AM1 and DFT methods show that structural modifications of the ligand, induced by complexation, are minor, and are localized on the chelating site. The good agreement between experimental and theoretical electronic spectra of both 3′4′diOHF and complex confirm the structural models. The great similarities between Raman spectra of the free and complexed form constitute an another proof of the absence of pronounced electronic and geometric changes, and notably demonstrate that the quinoidal form induced by the deprotonation of the two hydroxyl groups does not participate in the 3′4′diOHF complex structure. Whereas no complexation occurs in acidic medium, complexes of high stoichiometry are formed in alkaline medium. (Al(3′4′diOHF)2)− and (Al(3′4′diOHF)3)3− species are observed in methanol in the presence of sodium acetate or sodium methanoate.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.
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ABSTRACT: A series of hydroxyflavones, widely occurring in nature, have been used as model compounds for the study of the behaviours of humic substances towards aluminium ion complexation. The chelating power of 3-hydroxy-4-keto, 5-hydroxy-4-keto and catechol functional groups have been compared in methanol solution, first when these binding sites are isolated, and then when they are in competition on the a same ligand. Electronic and vibrational spectroscopy investigations were performed to characterize the Al(III) complexes: stoichiometry, stability constants and involved groups in multi-site ligands. Quantum chemical calculations have been carried out in order to know the structural modifications generated to the ligand by complexation. The influence of the medium, acidic, neutral and alkaline, has been studied. The 3-hydroxy-4-keto group has the strongest chelating power in acidic and neutral media whereas catechol presents the greatest ability to chelate Al(III) in alkaline solution. This investigation has shown that capacity of different studied functions to chelate Al(III) is the same when they are in competition and when they are individually taken.
Journal of Molecular Structure.
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ABSTRACT: Combined molecular mechanics (MMX), semi-empirical (AM1) and ab initio calculations were applied to an investigation of the conformational and electronic properties of the isolated flavone molecule in the ground state. The calculated molecular structure shows that the angle between the benzoypyrone moiety and the phenyl ring is about 30°. The barrier to rotation in this molecule is about 1.8 kcal mol−1. The frequency shifts observed in the vibrational spectra of flavone in solution and in crystalline form are tentatively connected with a conformational change of the molecule with the physical state. The energy gap between a planar and a twisted configuration is very low and the intermolecular interactions in the crystal, which are expected to be larger than the modest intramolecular potential, must strongly reduce the dihedral angle. As a flat molecule should give more favorable packing, it is reasonable that flavone should exist in this form in the crystal. Electronic energy levels, charge distributions and bond orders were calculated using semi-empirical methods. Electronic interactions of the rings are discussed.
Journal of Molecular Structure.
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ABSTRACT: Resonance Raman spectroscopic studies unequivocally indicate that the absorption band at about 500 nm, observed when 2-(4′hydroxyphenylazo)-benzoic acid (HABA) interacts with either avidin or bovine serum albumin in neutral aqueous solution, originates from the protein-bound hydrazone form of the dye. Spectroscopic comparison of the protein-bound and unbound hydrazone forms enables some of the ligand—protein interactions to be defined. Studies with HABA and some selected analogs in aqueous solution, in buffer-doped dimethyl sulfoxide (d6), and in the presence of α-cyclodextrin indicate that two important factors responsible for stabilising the hydrazone forms of HABA analogs are the ring substitution pattern, and intramolecular hydrogen bonding. A possible model for the HABA—protein interaction is proposed.
Spectrochimica Acta Part A: Molecular Spectroscopy.
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ABSTRACT: The infrared solid state, Raman solid state and tetrachloride solution spectra of flavone have been obtained. Assignments of most of the vibrational data have been performed by comparison between the spectra of flavone and three isotopic species, deuterated on the A, B and C rings, respectively. The vibrational frequencies for all the investigated compounds have been calculated from the conformational analysis of flavone using the semi-empirical AM1 method and compared with experimental values. The correlation is more or less satisfactory; however, for some vibrational modes, the calculated isotopic shifts agree better with experiment than do the frequencies themselves. Specific vibrational modes which retain a benzene ring mono-substituted and ortho-distributed character have been recognized in the spectra, according to literature data, isotopic frequency shifts and graphic representation of the atomic displacements.
Spectrochimica Acta Part A: Molecular Spectroscopy.
