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J C Merlin

Publications and source records attributed to J C Merlin.

14 recordsLinked to original sources
PubMed2002

Spectroscopic and structural study of complexes of quercetin with Al(III).

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.

Aluminum↗
PubMed2002

Self-modelling analysis applied to nanosecond transient absorption spectroscopy of flavone: an aid to elucidate and characterise reaction intermediates.

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.

Acetonitriles↗
PubMed2001

Complexes of Al(III) with 3'4'-dihydroxy-flavone: characterization, theoretical and spectroscopic study.

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.

Aluminum Chloride↗
PubMed2000

Vibrational and theoretical study of the 2',6'-dimethoxyflavone cis-formic acid inclusion compound.

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.

Flavonoids↗
PubMed2000

Conformational and spectroscopic investigation of 3-hydroxyflavone-aluminium chelates.

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).

Acids↗
PubMed1999

Semiempirical and Raman spectroscopic studies of carotenoids.

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.

Absorption↗
PubMed1999

Resonance raman spectroscopy and quantum chemical modeling studies of protein-astaxanthin interactions in alpha-crustacyanin (major blue carotenoprotein complex in carapace of lobster, Homarus gammarus).

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.

Animals↗
PubMed1997

13C Magic angle spinning NMR analysis and quantum chemical modeling of the bathochromic shift of astaxanthin in alpha-crustacyanin, the blue carotenoprotein complex in the carapace of the lobster Homarus gammarus.

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.

Animals↗
PubMed1987

Resonance Raman study of reconstituted carotenoproteins incorporating astaxanthin and 15,15'-didehydroastaxanthin.

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.

Animals↗
PubMed1975

[Introduction to Raman spectrometry].

The frequency shift observed when light is scattered by molecules is called Raman effect. Raman spectroscopy like infrared spectroscopy is a method of studying molecular vibrations. The two methods are complementary, they both give much informations about the structure of molecules and crystals, the nature of chemical bonds and intermolecular interactions. Infrared absorption is allowed if the vibration is accompanied by a variation of electric dipole moment, however Raman scattering will only be observed if a variation of molecular polarizability appears during the vibration. Symetry properties of molecules of crystals lead to the determination of the number of normal vibrational modes and their Raman or infrared activity. The discovery of Laser light source has permitted a great development of Raman instrumentation. Raman spectrometers can easily record the whole spectrum of molecular vibrations (0-4000 cm-1) of samples in solid, liquid or gazeous state. Very small quantities of material are required (several milligrams). Aqueous solutions are easily investigated. Owing to the easy exploration of the low frequency range by modern spectrometers, new areas are opened in the study of the solid state and polymeric chains. Resonance Raman effect allows the spectra of very dilute solutions to be obtained. With the development of rapid scanning systems and electro-optical spectrometers, study of transients species is now possible. Among the physical analysis methods, Raman spectroscopy is now more and more used, and this technic has already been successfully used in numerous biological and biochemical problems.

Crystallography↗
PubMed1975

Resonance Raman spectroscopic studies of the interactions between trypsin and a competitive inhibitor.

Raman spectroscopy was used to study the interactions between bovine trypsin and a competitive inhibitor. For this purpose, a chromophoric substrate analogue, 4-amidino-4'-dimethylamine azobenzene, was synthesized. This compound competitively inhibits the enzyme with a 1:1 stoichiometry and an inhibition constant Ki of 2.3 muM at pH 6.08 and 15 degrees. Resonance Raman spectra in aqueous solution of free or enzyme-bound inhibitor were analyzed. The main spectral changes observed upon enzyme-inhibitor complex formation were changes in the relative intensities of four bands (1171, 1206, 1315, 1608 cm-1) while no large frequency shifts occurred. The binding of the inhibitor molecule to the enzyme did not induce a twisting of the phenyl groups around the N=N bond. Some modifications of the band widths are interpreted in terms of a restriction of rotational motions in the inhibitor molecule. The possible involvement of specific interactions between trypsin and the benzamidinium ion part of the inhibitor molecule is discussed.

Azo Compounds↗
PubMedDate not supplied

Characteristics of the yellow pigment from a strain of Yersinia enterocolitica biovar 1.

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.

DNA, Bacterial↗