Showing papers in "Magnetic Resonance in Chemistry in 2001"
TL;DR: In this paper, the bulk magnetic susceptibility (BMS) shifts of 1H NMR resonance signals for inert reference compounds caused by the addition of paramagnetic lanthanide(III) compounds were exploited in order to determine the concentration of the compound in solution.
Abstract: The bulk magnetic susceptibility (BMS) shifts of 1H NMR resonance signals for inert reference compounds caused by the addition of paramagnetic lanthanide(III) compounds were exploited in order to determine lanthanide(III) concentrations in solution. The BMS shifts of an inert reference compound in the samples were measured by comparing the chemical shifts with those of an external reference. The concentration values obtained were in good agreement with results acquired from inductively coupled plasma analysis. Copyright © 2001 John Wiley & Sons, Ltd.
182 citations
TL;DR: In this paper, the GIAO/DFT approach was used to calculate 3.1H, 13C and 15N NMR chemical shifts for 28 substituted 2-phenacylpyridines (ketimine forms) and their enolimine tautomers.
Abstract: 1H, 13C and 15N NMR chemical shifts for 28 substituted 2-phenacylpyridines (ketimine forms) and their enolimine tautomers, (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines, were calculated via the GIAO/DFT approach. Among four tested methods at the B3LYP level of theory, the 6–311G, 6–311++G and 6–311G** basis sets gave acceptable result for 13C NMR chemical shifts whereas the 6–311++G** basis set was the minimum needed for reproduction of 15N NMR chemical shifts. Satisfactory reproduction of 13C and 15N NMR chemical shifts for different tautomers revealed that intramolecular hydrogen bonding could be modeled reliably by these calculations when the geometry optimizations were done with the HF/3–21G method. Agreements between theoretical and experimental 13C and 15N NMR chemical shifts and also HF/3–21G and HF/6–31G** optimized structural parameters with those obtained by x-ray crystallographic measurements suggests that it is not necessary to select too sophisticated and CPU time-intensive methods for geometry optimizations. Copyright © 2001 John Wiley & Sons, Ltd.
166 citations
TL;DR: A survey of heteronuclear correlation experiments for the measurement of coupling constants is presented in this article, where the authors evaluate several recent experiments for measuring nJ(C,H) (n = 2, 3) couplings in one comprehensive review.
Abstract: A survey of heteronuclear correlation experiments for the measurement of heteronuclear coupling constants is presented. The purpose of this survey is to evaluate several recent experiments for measuring nJ(C,H) (n = 2, 3) couplings in one comprehensive review. Ten experiments are presented and evaluated relative to one another in terms of experimental usability, ease of data interpretation and number of usable correlations. The experiments compared in this survey are the sensitivity-improved hetero-(ω1)-half-filtered TOCSY (HETLOC), carbon-sorted HETLOC (HSQC-HECADE), coupled/decoupled HSQC-TOCSY, GSQMBC, HSQMBC, G-BIRDR,X-HSQMBC, J-resolved HMBC-2, J-IMPEACH-MBC and methods for extracting heteronuclear coupling constants from gradient-selected variants of the magnitude-mode and phase-sensitive HMBC. The plant alkaloid strychnine was used as a model compound for this study. Copyright © 2001 John Wiley & Sons, Ltd.
156 citations
TL;DR: In this article, the use of tetramethylsilane (TMS) as an internal diffusion reference for DOSY measurements related to molecular association forced by H-bond in solution is proposed.
Abstract: The use of tetramethylsilane (TMS) as an internal diffusion reference for DOSY measurements related to molecular association forced by H-bond in solution is proposed. By using such a reference, it is possible to determine the extent to which the diffusion of the solutes is affected by changes in viscosity or H-bonding, after modifications in the composition of the solution under study. Therefore, changes in hydrodynamic radii due to H-bonding can be calculated. Three examples of the use of this reference in diffusion studies related to H-bonding are presented: the addition of an H-bond acceptor to a mixture of two compounds with very similar hydrodynamic radii and molecular weights, but where only one can be involved in H-bonding, the addition of an H-bond acceptor to a mixture of alcohols with different H-bond acidities and the addition of an H-bond acceptor to a mixture of two isomeric alcohols with different steric hindrances. Copyright © 2001 John Wiley & Sons, Ltd.
155 citations
TL;DR: In this article, a low-pass J filter was employed to eliminate one-bond J(C,H) responses for the measurement of heteronuclear and homonuclear coupling constants.
Abstract: New methods for the measurement of homonuclear J(H,H) and heteronuclear long-range J(C,H) coupling constants are presented. The heteronuclear and homonuclear coupling constants are measured by schemes based on J-HMBC and XLOC, respectively, and they all employ a low-pass J filter to eliminate one-bond J(C,H) responses. The J-HMBC experiments are automatically broadband in their excitation over a range of J(C,H) coupling constants whereas the broadband feature for XLOC is achieved by employing the approach used in broadband HMBC. Copyright © 2001 John Wiley & Sons, Ltd.
147 citations
TL;DR: In this article, a series of hydrogen-bonded solid 1:1 acid-base complexes of 2,4,6-trimethylpyridine (collidine) with carboxylic acids and their hydrogen bond deuterated analogs were synthesized and studied by H-1 magic angle spinning (MAS) and N-15 cross-polarization NMR with and without MAS.
Abstract: A novel series of hydrogen-bonded solid 1:1 acid-base complexes of N-15-labeled 2,4,6-trimethylpyridine (collidine) with carboxylic acids and their hydrogen bond deuterated analogs were synthesized and studied by H-1 magic angle spinning (MAS) and N-15 cross-polarization NMR with and without MAS. Not only zwitterionic complexes with the H-bond proton closer to nitrogen than to oxygen but also molecular complexes have been observed, where the proton is located closer to oxygen. For these complexes, the isotropic H-1 and N-15 chemical shifts and the N-15 chemical shielding tensor elements were measured (the latter by lineshape simulation of the static powder spectra) as a function of the hydrogen bond geometry. For the deuterated analogs H-1/H-2 isotope effects on the isotropic N-15 chemical shifts were obtained under MAS conditions. Lineshape simulations of the static N-15 powder spectra revealed the dipolar H-2, N-15 couplings and hence the corresponding distances. The results reveal several hydrogen bond geometry-NMR parameter correlations which are analyzed in terms of the valence bond order model. (1) The collidine and apparently other pyridines isotropic N-15 chemical shifts depend in a characteristic way on the nitrogen-hydrogen distance. This correlation can be used in the future to evaluate hydrogen bond geometries and solid-state acidities in more complicated systems. (2) A correlation of the H-1 with the N-15 isotropic chemical shifts is observed which corresponds to the well-known hydrogen bond geometry correlation indicating a strong decrease of the A . . .B distance in an AHB hydrogen bond when the proton is shifted to the hydrogen bond center. This contraction is associated with a low-field H-1 NMR chemical shift. (3) The N-15 chemical shift anisotropy principal tensor elements delta (t), delta (r) and delta (tangential, radial and perpendicular with respect to the pyridine ring) exhibit a linear relation with the isotropic N-15 chemical shifts. A crossing point of delta (t) = delta (r) is observed. Further correlations of the hydrogen bond geometry with the geometric H/D isotope effects on the N-15 chemical shifts and with the pK(a) values of the associated acids are reported. Copyright (C) 2001 John Wiley & Sons, Ltd.
134 citations
TL;DR: In this article, the influence of solvent polarity on the properties of hydrogen-bonded 1 : 1 complexes of 2,4,6-trimethylpyridine-15N with HF and DF, labeled below as FHN and FDN, has been studied by multinuclear magnetic resonance spectroscopy in the slow hydrogen bond exchange regime reached below 190 K.
Abstract: The influence of solvent polarity on the properties of hydrogen-bonded 1 : 1 complexes of 2,4,6-trimethylpyridine-15N with HF and DF, labeled below as FHN and FDN, has been studied by multinuclear magnetic resonance spectroscopy in the slow hydrogen bond exchange regime reached below 190 K. Mixtures of CDF3/CDClF2 were employed as solvent, which is liquid down to 90 K. In order to evaluate their polarity, the static dielectric constants eo of the CHF3, CHClF2 and of the binary 1 : 1 mixture were measured from 160 K down to 90 K. A strong increase of eo from 14 at 190 K to 38 at 103 K is observed for the mixtures used in the NMR measurements. Upon cooling, i.e. increase of the dielectric constant, the NMR spectra indicate a gradual transformation of an asymmetric molecular complex FH···N to a quasi-symmetric complex Fδ−···H···Nδ+ and eventually to a more or less zwitterionic species F−···H N+. These changes are not only manifested in the scalar couplings J(1H,19F) and J(1H,15N) but also lead to characteristic primary and secondary H/D isotope effects on the chemical shifts of the hydrogen bonded nuclei. Whereas the primary isotope chemical shift effect pΔ(D/H) ≡ δ(F2HN) − δ(F1HN) = −0.2 ppm is negative at 190 K and in agreement with an asymmetric hydrogen bond in the molecular complex, it changes its sign when the temperature is lowered, goes through a maximum of +0.27 ppm at eo ≈ 22 and finally decreases again. The positive value of pΔ(D/H) is in agreement with D more confined to the hydrogen bond center compared with H, which constitutes a fingerprint of a quasi-symmetric hydrogen bond involving a single well potential for the proton motion. The quasi-symmetric complex is further characterized by the following NMR parameters, J(1H,19F) = 30 Hz, J(1H,15N) = −50 Hz, J(19F,15N) = −96 Hz, δ(F1HN) = 20.0 ppm, δ(19FHN) = −114.2 ppm, δ(FH15N) = −63.5 ppm, and the one-bond H/D-isotope effects δ(F2HN) − δ(F1HN) = +0.27 ppm, δ(19FDN) − δ(19FHN) = 1.4 ppm and δ(FD15N) − δ(FH15N) = −3.4 ppm. Copyright © 2001 John Wiley & Sons, Ltd.
127 citations
TL;DR: In this article, complete assignments of 1H and 13C NMR chemical shifts for stigma-5-en-3-O-β-glucoside, isolated from Brassica rapa sb.sp. campestris and Aloe barbadensis, and its acetyl derivative are presented, based on DEPT, HETCOR, COLOC, HMBC, COSY-45°, NOESY and J-resolved experiments.
Abstract: Unambiguous and complete assignments of 1H and 13C NMR chemical shifts for stigma-5-en-3-O-β-glucoside, isolated from Brassica rapa sb.sp. campestris and Aloe barbadensis, and its acetyl derivative are presented, based on DEPT, HETCOR, COLOC, HMBC, COSY-45°, NOESY and J-resolved experiments. Copyright © 2001 John Wiley & Sons, Ltd.
116 citations
TL;DR: In this paper, the authors reported that the locked conformation of OH and OCH3 groups in solids results in an increased shielding of carbon adjacent to C-OH or C-OCH3 hydrogens, enabling conclusions as to the orientations of these groups to be drawn.
Abstract: 13C CP/MAS NMR spectra were recorded to characterize the solid-state conformation of flavonoids: galangin, kaempferol, quercetin, myricetin, chrysin, apigenin, luteolin, acacetin, hesperetin and naringenin. Correct assignment of the signals in solution-state spectra (using APT, HSQC and HMBC) and solid-state spectra (using dipolar dephased and short contact sequences) for the above flavonoids is reported. The locked conformation of OH and OCH3 groups in solids results in an increased shielding of carbon adjacent to C—OH or C—OCH3 hydrogens, enabling conclusions as to the orientations of these groups to be drawn. The C-5—OH hydroxyl (and C-3—OH) points towards the carbonyl group oxygen, forming intramolecular hydrogen bonds. Considerations of C2′ and C6′ carbon shielding suggest that ring B is not coplanar with the benzopyran fragment in kaempferol, acacetin and myricetin. Copyright © 2001 John Wiley & Sons, Ltd.
103 citations
TL;DR: A chronology of NMR studies over the past three decades which have led to the present understanding of active site H-bonding in serine proteases can be found in this article.
Abstract: This article is a chronology of NMR studies over the past three decades which have led to our present understanding of active site H-bonding in serine proteases. The story of H-bonding in this highly scrutinized family of enzymes, as revealed by NMR and x-ray crystallography, is to a large extent the foundation upon which our understanding of the catalytic mechanism of this archetypical enzyme system rests. Various theories have come into and out of favor as each bit of hard-earned evidence has been established, particularly with regard to the active-site imidazole, its state of protonation, its acid–base and tautomeric equilibrium constants and its H-bonding. Specific bioincorporation of 15N and 13C isotopic labels at the active site has proven especially useful in revealing the locations of protons and H-bond interactions through chemical shift and spin-coupling information, the latter useful also for spectral editing that reveals only the resonances of interest. However, the final chapter in the serine protease mechanism is yet to unfold. Currently competing proposals for the catalytic mechanism, one involving a low-barrier H-bond and the other a reaction driven histidine ring flip, are presented and discussed. Copyright © 2001 John Wiley & Sons, Ltd.
88 citations
TL;DR: The physicochemical properties of low barrier hydrogen bonds (LBHBs) and single well hydrogen bonds are contrasted with the properties of conventional hydrogen bonds as discussed by the authors, which are characterized by shortened hydrogen bond lengths and correspondingly lengthened covalent bonding.
Abstract: The physicochemical properties of low barrier hydrogen bonds (LBHBs) and single well hydrogen bonds (SWHBs) are contrasted with the properties of conventional hydrogen bonds. The LBHBs are much more common than the rare, symmetrical SWHBs. LBHBs are characterized by shortened hydrogen bond lengths and correspondingly lengthened covalent bonding, shortened distances between participating heteroatoms, far downfield proton NMR signals, low deuterium fractionation factors and high values of the enthalpy of activation for exchange of the proton with solvent protons. For many years, it was thought that LBHBs could exist only in aprotic media or crystalline or vapor states. Recently, however, the downfield protons associated with hydrogen maleate, hydrogen cis-cyclohexane-1,2-dicarboxylate and hydrogen 2,2-dimethylmalonate have been observed in 0.32 mol fraction water in acetone-d6 at −50 °C. The activation enthalpy for exchange of the downfield proton in hydrogen maleate with solvent water has been found to be 7.2 kcal mol−1, consistent with strong hydrogen bonding. Two examples of LBHBs in enzymes are the downfield proton bridging His 57 and Asp 102 in transition state analog complexes of chymotrypsin, and the proton bridging Tyr 14 and the equilenin-OH group in its complex with 3-ketosteroid isomerase. These protons display the structural, spectroscopic and chemical properties of LBHBs. Their presence in transition state analogue complexes suggests their importance in the transition states of enzymatic reactions. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: The metabonomics approach to drug toxicity evaluation is exemplified using 1H NMR spectroscopy to investigate the biochemical perturbations in urine after dosing Fischer rats with the cationic, amphiphilic drugs chloroquine, amiodarone and DMP 777, a neutrophil elastase inhibitor.
Abstract: The application of high-field 1H NMR spectroscopy to the analysis of biological fluids has proved to be a valuable approach to toxicological screening. Coupled with data reduction and chemometric analyses, this technology has a high capacity for elucidating relevant metabolic information pertaining to the nature and mechanism of toxic processes in an organism. This approach is encapsulated in the concept of metabonomics, which is defined as ‘the quantitative measurement of the dynamic multiparametric metabolic response of living systems to pathophysiological stimuli or genetic modification’, and can be regarded as complementary to studies of the genome (genomics) and the proteins in an organism (proteomics). Here the metabonomics approach to drug toxicity evaluation is exemplified using 1H NMR spectroscopy to investigate the biochemical perturbations in urine after dosing Fischer rats with the cationic, amphiphilic drugs chloroquine, amiodarone and DMP 777, a neutrophil elastase inhibitor. These drugs are known to induce phospholipidosis, characterized by lysosomal lamellar body and drug accumulation. Using a metabonomic approach, combinations of specific urinary biomarkers were identified for each compound, together with a putative general marker of phospholipidosis, phenylacetylglycine. Furthermore, in the case of chloroquine, clear evidence of hepatotoxicity was determined. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the proton transfer equilibrium in series of Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxyl-2-acetonaphthone was measured by means of 1H, 13C and 15N NMR spectra and deuterium isotope effects on 13C/15N chemical shifts at variable temperature and in different solvents.
Abstract: The proton transfer equilibrium in series of Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone were measured by means of 1H, 13C and 15N NMR spectra and deuterium isotope effects on 13C and 15N chemical shifts at variable temperature and in different solvents. The latter made it possible to investigate a broad range of equilibrium positions. All compounds exist mainly as the NH tautomer over the full range of temperatures in CDCl3 solution. The position of the equilibrium was estimated using 1J(15N,1H) and 3J(15NH,1H) coupling constants. The correlation between deuterium isotope effects nΔ13C-2(XD) or nΔ15N(XD), where X = O or N, and the mole fraction of the NH form show the characteristic, non-monotonic function as for other intramolecular hydrogen bonded systems. The position of the minimum depends on substituents and the maximal negative values of nΔ13C-2(XD) varies with substituents and solvents. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, a short model tetrapeptide, NAc-Ser-Phe-Val-Gly-OMe, was dissolved in water, in trifluoroethanol (TFE) and in mixtures of both.
Abstract: Homonuclear and heteronuclear intermolecular NOE measurements were applied to study the solvation of a short model tetrapeptide, NAc-Ser-Phe-Val-Gly-OMe, dissolved in water, in trifluoroethanol (TFE) and in mixtures of both. Site specificity of homonuclear NOEs between water and different protons of the peptide was proved. Preferential solvation of the peptide by TFE in water–TFE mixtures was demonstrated by a significant decrease in water–peptide interaction and a concomitant buildup of heteronuclear NOE between the fluorine atoms of TFE and the peptide. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the design and use of a system consisting of a disposable solid-phase extraction cartridge coupled to an NMR flow-probe (SPE-NMR) is demonstrated.
Abstract: The design and use of a system consisting of a disposable solid-phase extraction cartridge coupled to an NMR flow-probe (SPE–NMR) is demonstrated. Fractions obtained from high-performance liquid chromatography of an extract of Habanero pepper (Capsicum chinense) were analysed. The fractions were loaded on SPE cartridges and the compounds were eluted with deuterated methanol directly into the NMR flow-probe. Good-quality NMR spectra were acquired and the structure of the compounds could be determined. The main advantages with SPE–NMR are the simple handling of small amounts of samples, the removal of polar additives and solvents from the preceding separation step allowing optimal conditions for both the chromatography and the NMR spectroscopy and the possibility of automating the procedure. Hence the method can facilitate the structural characterization of small amounts of natural products in complex mixtures. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the known lone pair orientation effect of an α-nitrogen atom on 1J(C,C) couplings was studied using the natural J coupling (NJC) dissection method for the Fermi contact (FC) term.
Abstract: The known lone pair orientation effect of an α-nitrogen atom on 1J(C,C) couplings was studied using the natural J coupling (NJC) dissection method for the Fermi contact (FC) term. This dissection was performed at the DFT-B3LYP level using three different basis sets. It was verified that non-contact contributions, which are much smaller than the FC term, do not depend on the orientation of the N lone pair. Acetone oxime (1), protonated acetone oxime (2) and ethylamine (3), were taken as model compounds to perform this study. In 1 the difference between 1J(Z) and 1J(E) is mainly determined by three NJC terms, i.e the nitrogen lone pair, the carbon–carbon bond containing the coupled carbon atoms and the carbon inner core orbitals contributions. In 3 the angular dependence of 1J(C,C) vs the amine group conformation is dominated by the same three contributions that define the 1J(Z) and 1J(E) difference in 1. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the N···H distance within the unusual hydrogen-bonding arrangement adopted by a pair of methyl-substituted benzoxazine dimers (C6H3(OH)2CH2)2N(CH3) was determined by solid-state NMR to be 194 ± 5 pm.
Abstract: The N···H distance within the unusual hydrogen-bonding arrangement adopted by a pair of methyl-substituted benzoxazine dimers (C6H3(OH)2CH2)2N(CH3) has been determined by solid-state NMR to be 194 ± 5 pm. This indicates that the proton is shared between the nitrogen and oxygen atoms, with a preference for an O H rather than an N H bond character. It is to be noted that a previous X-ray single crystal study was unable to localize the position of this hydrogen-bonded proton. The advanced solid-state NMR methods employed utilize REDOR-type recoupling under fast magic-angle spinning to recouple the heteronuclear 1H–15N dipole–dipole interaction, such that rotor-encoded spinning-sideband patterns are obtained, the analysis of which yields the 1H 15N dipole–dipole coupling and hence the N···H distance. Different designs of recoupling pulse sequences are discussed, which allow the experiment to be adapted to the system under investigation in terms of the required 15N or 1H chemical shift resolution, conventional (15N) or inverse (1H ) detection as well as the importance of the perturbing influences of further spins. The chosen recoupling scheme employs inverse, i.e. 1H , detection, because it provides a dramatic increase in signal sensitivity, resulting in savings in measurement time by a factor of at least 20, as well as 1H chemical-shift resolution in the directly detected spectral dimension. This is the method of choice for cases such as this, where chemical shift resolution is not required in the 15N dimension. In addition, the perturbing effect of further protons on the N···H coupling of interest is minimized, such that a relatively long N···H distance can be determined despite the presence of several other couplings of comparable strength. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the EOM-CCSD coupling constants for a series of cationic complexes stabilized by either traditional or proton-shared N−H+N hydrogen bonds were derived.
Abstract: Predictive quantum chemical methods based upon coupled cluster theory of spin–spin coupling constants offer a direct tool to explore a variety of questions concerning the relationship between coupling constants and intermolecular distances, molecular orientation, changes in hybridization and related issues. Of particular interest are 2hJ(15N,15N) couplings across hydrogen bonds. In this work we present EOM–CCSD coupling constants [2hJ(15N,15N)] for a series of cationic complexes stabilized by either traditional or proton-shared N—H+—N hydrogen bonds, and relate these to 15N,15N coupling constants in neutral complexes with N—H—N hydrogen bonds. The computed 15N,15N coupling constants in these complexes vary smoothly with N—N distance, regardless of the charge or the particular binding at the N atoms. We propose that the curves which show this dependence have sufficient generality that they should be useful for determining N—N distances from experimentally measured coupling constants. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: Results suggest that micelle–induced changes in peptide conformation and peptide hydrophobicity both play important roles in governing peptide–micelle binding.
Abstract: Pulsed-field gradient (PFG) NMR spectroscopy was used to characterize the interactions between a series of small peptides and sodium dodecyl sulfate (SDS) micelles. The study was undertaken because peptide–micelle interactions are often exploited in the chromatographic analysis of complex mixtures, and therefore an accurate knowledge of peptide–micelle association constants is essential for a thorough understanding of the separation mechanism in these techniques. In the PFG-NMR experiment, diffusion coefficients for each peptide were measured in free solution and in the presence of SDS micelles. These diffusion coefficients allowed the mole fraction of peptide molecules associated with SDS micelles and the peptide–micelle association constants, Keq, to be calculated. In a study of dipeptides derived from the leucine enkephalin (YGGFL) and D-(Ala)2-leucine enkephalin (YAGFL) amino acid sequences, only Phe–Leu was found to associate strongly with the micellar aggregates. The equilibrium constant for this dipeptide was found to be 69 ± 6 M−1. The dipeptides Tyr–Gly, Tyr–Ala, Gly–Gly and Gly–Phe exhibited little to no micelle binding, whereas Tyr–Leu had an association constant that was a factor of four less than the Phe–Leu value. Furthermore, the association constants for the pentapeptides leucine enkephalin and D-(Ala)2-leucine enkephalin (130 ± 8 and 584 ± 38, respectively) were both higher than the Keq values for all dipeptides investigated. These results suggest that micelle–induced changes in peptide conformation and peptide hydrophobicity both play important roles in governing peptide–micelle binding. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, a nuclear magnetic resonance (NMR) was applied to measure the high-frequency (500 MHz) 13C spectra at natural abundance in olive oil samples in order to ascertain the suitability of NMR for discriminating oils from different Italian geographical areas of production and, in particular, from areas labelled with the denomination of protected origin.
Abstract: Nuclear magnetic resonance (NMR) spectroscopy is applied to measure the high-field (500 MHz) 13C spectra at natural abundance in olive oil samples in order to ascertain the suitability of NMR for discriminating oils from different Italian geographical areas of production and, in particular, from areas labelled with the ‘Denomination of Protected Origin’ (DOP). The NMR methodology, which uses the distortionless enhancement by polarization transfer (DEPT) pulse sequence for improving the signal-to-noise ratio of 13C spectra, is optimized for the acquisition, processing and integration parameters to validate the quantitative measurements of the intensities of 13C resonances of the whole olive oil spectrum. Principal components analysis, which is applied to the 13C intensity data, correlates the oil samples from the same geographical area provided the composition of the olive oil is monovarietal. The correlation is obtained from properties related to the fatty acid composition of the triglyceride fraction of olive oils. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the coupling constants of N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (1) and its asymmetric analog N-phenyl-N′-(1,3,4-triazol)- 6-amino-enzyme-1,aldimines type were studied.
Abstract: Multinuclear liquid state magnetic resonance experiments have been performed on two seven-membered 15N-labeled H-chelates of the 6-aminofulvene-1-aldimines type in order to characterize the strong intramolecular NHN hydrogen bonds as a function of the molecular symmetry. In particular, the symmetrically substituted N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (1) and its asymmetric analog N-phenyl-N′-(1,3,4-triazol)- 6-aminopentafulvene-1-aldimine-15N5 (2) have been studied. For 1, an NN coupling constant across the hydrogen bridge of 2hJ(15N,15N) = 10.6 Hz was determined indirectly by 13C NMR at two different Larmor frequencies, 125.76 and 67.93 MHz; this coupling constant is characteristically enhanced compared with the value of 8.6 Hz obtained previously for 2. Because of a fast degenerate proton tautomerism the hydrogen bond proton in 1 is coupled with both nitrogen atoms with a coupling constant of −40.8 Hz. {15N} tickling experiments were performed on 2 in order to determine the relative signs of the coupling constants of the NHN hydrogen bridge. We find that 2hJ(15N,15N) and 1hJ(1H,15N) = +4.4 Hz exhibit the same sign, i.e. the opposite sign compared with 1J(15N,1H) = −88.6 Hz. This finding proves that 1hJ(1H,15N) corresponds to an intrinsic coupling, which is not induced by a tautomerism absent in 2 because of the large difference in basicities of the aniline and the amino-1,3,4-triazole substituents. Therefore, these observations indicate a sign change of J(15N,1H) when the proton is transferred successively from one nitrogen to the other, as observed previously for FHF hydrogen bonds. The relation between the values of the coupling constants and the hydrogen bond geometries is discussed in terms of the valence bond order model, as are the implications for obtaining equilibrium constants of tautomerism from coupling constant data. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In particular, the experimental data indicate the unexpected effect of a partial multiple bond between phosphorus and fluorine in 7JP and 3JP-H as mentioned in this paper, and the long-range coupling constant either disappears or at least reduces.
Abstract: The examination of spectra of a selected series of phosphoramidates showed that, in addition to a double bond, whenever there are one or two partial multiple bonds in the molecule the long-range coupling constant, such as 6JP–H, 7JP–H and 3JP–H, either disappears or at least reduces. In particular, the experimental data indicate the unexpected effect of a partial multiple bond between phosphorus and fluorine in 7JP–H and 3JP–H. The preparation of the compounds p-CH3C6H4NHP(O)Cl2 (II), p-CH3C6H4NHP(O)(Cl)OC6H4CH3-p (XIV), m-CH3C6H4NHP(O)(Cl)OC6H4CH3-p (XV), m-CH3C6H4NHP(O)(Cl)OC6H4CH3-m (XVI) and the rigid molecule CH3C6H3(NH)2P(O)Cl (XVII) are described and they are characterized by 1H, 13C, 31P NMR, IR spectroscopy and elemental analysis. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the conformational changes of free, monomeric glucagon-like peptide-1-(7-36)-amide (GLP-1) in aqueous solution with increasing concentrations of 2,2,2-trifluoroethanol (TFE) were monitored by NMR spectroscopy.
Abstract: The conformational changes of free, monomeric glucagon-like peptide-1-(7–36)-amide (GLP-1) in aqueous solution with increasing concentrations of 2,2,2-trifluoroethanol (TFE) were monitored by NMR spectroscopy. It was found that GLP-1 gradually assumes a stable, single-stranded helical structure in water solution when the TFE concentration is increased from 0 to 35% (v/v). No further structural changes were observed at higher TFE concentrations. The structure of GLP-1 in 35% TFE was determined from 292 distance restraints and 44 angle restraints by distance geometry, simulating annealing and restrained energy minimization. The helical structure extends from T7 to K28, with a less well-defined region around G16 and a disordered six-residue N-terminal domain. The folding process of GLP-1 from random coil (in water) to helix (in 35% TFE) is initiated by the formation of the C-terminal segment of the helix that is extended gradually towards the N-terminus of the peptide with increasing concentration of TFE. The exchange rates of the slow exchanging amide protons indicate that the C-terminal part of the helix is more stable than the N-terminal part. Copyright 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the shielding of the double-bonded alkenes was analyzed in terms of magnetic anisotropy and steric effects of double bond together with a model (CHARGE7) for the calculation of the two-and three-branch electronic effects.
Abstract: The 1H NMR spectra of a number of alkenes of known geometry were recorded in CDCl3 solution and assigned, namely ethylene, propene, 4-methylcyclohexene, 1,4-dimethylcyclohexene, methylene cyclohexane (in CFCl3–CD2Cl2 at 153 K), 5-methylene-2-norbornene, camphene, bicyclopentadiene, styrene and 9-vinylanthracene. These results together with literature data for other alkenes, i.e. 1,3- and 1,4-cyclohexadiene, norbornene, norbornadiene, bicyclo[2.2.2]oct-2-ene and α- and β-pinene, and other data allowed the determination of the olefinic shielding in these molecules. The shielding was analysed in terms of the magnetic anisotropy and steric effects of the double bond together with a model (CHARGE7) for the calculation of the two- and three-bond electronic effects. For the aromatic alkenes ring current and π-electron effects were included. This analysis showed that the double bond shielding arises from both anisotropic and steric effects. The anisotropy is due to the perpendicular term only with a value of Δχ(CC) of −12.1 × 10−6cm3mol−1. There is also a steric deshielding term of 82.5/r6 (r in A). The shielding along the π-axis changes sign from shielding at long range (>2.5 A) to deshielding at short range (<2 A). The model gives the first comprehensive calculation of the shielding of the alkene group. For the data set considered (172 proton chemical shifts) ranging from δ=0.48 to 8.39, the r.m.s. error of observed vs calculated shifts was 0.11 ppm. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the diffusion of pyridine confined in mesoporous silica MCM-41 was studied with stray field gradient (SFG) NMR diffusometry as a function of the filling factor of the mesopores at room temperature, employing a laboratory-built SFG setup.
Abstract: The diffusion of pyridine confined in mesoporous silica MCM-41 (d(pore) = 3.3 nm) was studied with stray field gradient (SFG) NMR diffusometry as a function of the filling factor of the mesopores at room temperature, employing a laboratory-built SFG setup. The translational diffusion of pyridine in MCM-41 is found to be anisotropic and the diffusion parallel to the pores' cylinder axes is much faster than that perpendicular to them. The parallel diffusion coefficient depends strongly on the filling level of the guest liquid inside the pores. For a filling level of 25, which corresponds approximately to a monomolecular layer of pyridine molecules hydrogen bonded to surface-SiOH groups, a parallel diffusion coefficient of D-parallel to = 1.0 X 10(-9) m(2) s(-1) is found, which is slower than the diffusion coefficient of the bulk liquid (D = 1.6 X 10(-9) m(2) s(-1)). For higher filling factors the parallel diffusion coefficient increases and at a filling factor of 85 a diffusion coefficient of D-parallel to = 6.8 X 10(-9) m(2) s(-1) is reached. The perpendicular diffusion coefficient of D-perpendicular to = 3.7 +/- 2.0 X 10(-11) m(2) s(-1) is independent of the filling factor. Employing additional N-15 MAS data for the pyridine inside the mesopores, a microscopic model of the diffusion is proposed, which depends on the exchange of the slowly diffusing hydrogen-bonded surface pyridine molecules with fast-diffusing free pyridine molecules inside the pores. Copyright (C) 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, a recent reformulation of the theory of the lineshapes, which describes the spectrum as a sum of transitions, simplifies setting up the iterative process.
Abstract: The lineshapes in an NMR spectrum, due to dynamic processes within the sample, provide an excellent way of measuring the rates of these processes. As the process speeds up, lines broaden, coalesce and finally sharpen into an average peak. Mathematical techniques for simulating these lineshapes have long been available, and rates can be extracted by fitting the calculated lineshape to the experiment. This is often done by manual iteration and visual comparison. The principal iterative program, DNMR5, is somewhat clumsy by modern standards. A recent reformulation of the theory of the lineshapes, which describes the spectrum as a sum of transitions, simplifies setting up the iterative process. This approach is described in general, and illustrated with the two‐site equally populated case. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this article, the authors compared the performance of a 5 mm Varian 1H-15N Cryo-Q NMR probe with those from a 3 mm gradient inverse triple resonance probe using a 2 mg sample of the oxazolidinone antibiotic eperezolid.
Abstract: The development of new NMR probe technologies has been an active area of research effort for the past decade. Recently, cryogenically cooled NMR probes have been the subject of considerable interest in the light of the large gains in sensitivity and hence savings in spectrometer time that can be realized by utilizing this technology. With low gamma nuclides, such as 15N, time savings may be less of an issue than the ability to work with available samples rather than having to isolate additional material for analysis. Results of 5–10 Hz optimized CIGAR-HMBC 1H–15N experiments at natural abundance obtained using a 5 mm Varian 1H–15N Cryo-Q NMR probe are compared with those from a 3 mm gradient inverse triple resonance probe using a 2 mg sample of the oxazolidinone antibiotic eperezolid in a 3 mm NMR tube. Using the cryogenic NMR probe, a 1H–15N CIGAR-HMBC spectrum in which most previously reported long-range couplings are observed was recorded in ∼10 min; almost all of the expected long-range responses were observed within 26 min. Acquiring the same data set with identical parameters using a conventional 3 mm gradient inverse triple resonance probe gave data that were of substantially lower quality. Acquiring data comparable to the 26 min cryogenic probe data with the conventional 3 mm probe required ∼4 h to observe all of the responses and ∼18 h to obtain a spectrum with a signal-to-noise ratio comparable to the data set acquired with the cryogenic NMR probe. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the 2D 2f-DipShift method was used to measure the N,H dipolar coupling in imidazolium-carboxylate pairs.
Abstract: N — H bond lengths in imidazolium–carboxylate pairs were studied as models for enzyme active site motifs. The bond lengths were measured using ‘2D 2f-DipShift,’ a solid-state NMR method wherein the N,H dipolar coupling is determined using heteronuclear dipolar spinning sideband patterns. Like the situation for compressed O — H ··· O systems, some complexes exhibit very short N ··· O distances and weaker N,H dipolar coupling. The weaker time-average N,H dipolar coupling for systems with short N ··· O hydrogen bonds can be most likely associated with an altered potential well for the proton, or a ‘stretched’ covalent bond, although other interpretations involving a double well potential are discussed. The range or variation in average bond lengths seen in this study is much narrower than that previously reported for O — H ··· O systems (1.01–1.07 u A for N — H vs 1.0–1.3 u A for O — H bonds). Copyright 2001
TL;DR: In this paper, a new and improved NMR experiment for F1 scalable, J-Resolved HMBC, was presented, which utilizes a minimum number of 180° pulses to preserve maximum signal-to-noise ratio and provides a variable scaling factor (N) that can be used to scale the heteronuclear couplings in F1.
Abstract: A new and improved NMR experiment for F1 scalable, J-Resolved HMBC is presented. This experiment, called J-IMPEACH-MBC, utilizes a minimum number of 180° pulses to preserve maximum signal-to-noise ratio and provides a variable scaling factor (N) that can be used to scale the heteronuclear couplings in F1. It should prove to be useful for the determination of relative stereochemistry and other structural problems. In addition, a description of the advantages of the J-IMPEACH-MBC compared with the J-Resolved HMBC-2 is presented. The method is exemplified with strychnine. Copyright © 2001 John Wiley & Sons, Ltd.
TL;DR: In this paper, the structure and signal assignments of (Z)-3-arylidene-1-thioflavan-4-ones were elucidated by extensive application of one-and two-dimensional 1H and 13C NMR spectroscopy.
Abstract: Epoxidation of (Z)-3-arylidene-1-thioflavan-4-ones (1) yielded trans,cis (2) and trans,trans (3) isomers. The structure and signal assignments were elucidated by extensive application of one- and two-dimensional 1H and 13C NMR spectroscopy. The conformational analysis was achieved by the application of 3J(C,H) couplings and ab initio MO calculations. Both the preferred ground-state conformers (envelope-A conformations) obtained as global minima of the HF ab initio structures and the 13C chemical shifts calculated by the GIAO method from the global minima structures of the trans,cis and trans,trans isomers are in agreement with the experimentally obtained NMR results. Copyright © 2001 John Wiley & Sons, Ltd.