Magnetic Resonance in Chemistry

Afonin, Andrei V.; Ushakov, Igor A.; Vashchenko, Alexander V.; Kondrashov, Evgeniy V.; Rulev, Alexander Yu.

doi: 10.1002/mrc.2643pmid: 20661903

In the series of diaminoenones, large high‐frequency shifts of the 1H NMR of the NH group in the cis‐position relative to the carbonyl group suggests strong NH···O intramolecular hydrogen bonding comprising a six‐membered chelate ring. The NH···O hydrogen bond causes an increase of the 1J(N,H) coupling constant by 2–4 Hz and high‐frequency shift of the 15N signal by 9–10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge‐independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3‐bis(isopropylamino)‐1‐(aryl)prop‐2‐en‐1‐one (12) for conformations with the Z‐ and E‐orientations of the carbonyl group relative to the NH group. The effects of the NH···O hydrogen‐bond on the NMR parameters are analyzed with the atoms‐in‐molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NH···O hydrogen bond as compared with that of 1,1‐di(pyrrol‐2‐yl)‐2‐formylethene (13) where NH···O hydrogen bridge establishes a seven‐membered chelate ring, and the corresponding 1J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) →σ*NH hyperconjugative interaction is weakened on going from the six‐membered chelate ring to the seven‐membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the 1J(N,H) value as a consequence of the NH···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd.

Mamedov, Ibrahim Garib; Eichhoff, Uwe; Maharramov, Abel Mammadali; Bayramov, Musa Rza; Mamedova, Yegana Vagif

doi: 10.1002/mrc.2644pmid: 20625986

The formation of hydrogen bonds and molecular dynamics for the molecules cis‐1‐(2‐hydroxy‐5‐methylphenyl)ethanone oxime (I) and N‐(2‐hydroxy‐4‐methylphenyl)acetamide (II) have been investigated in solution using NMR. The results confirm the formation of OH···O, OH···N and O···HN type inter‐ and intramolecular hydrogen bonds. Spin‐lattice relaxation times (T1), activation energy of molecular dynamics and energy of intramolecular hydrogen bonds have been determined. Copyright © 2010 John Wiley & Sons, Ltd.

Kleinmaier, Roland; Gschwind, Ruth M.

doi: 10.1002/mrc.2648pmid: 20641131

Monoalkylated acylguanidines are important functional groups in many biologically active compounds and additionally applied in coordination chemistry. Yet a straightforward assignment of the individual NH chemical shifts and the acylguanidine conformations is still missing. Therefore, in this study, NMR spectroscopic approaches for the chemical and especially the conformational assignment of protonated monoalkylated acylguanidines are presented. While NOESY and 3JH, H scalar couplings cannot be applied successfully for the assignment of acylguanidines, 4JH, H scalar couplings in 1H,1H COSY spectra allow for an unambiguous chemical shift and conformational assignment. It is shown that these 4JH, H long‐range couplings between individual acylguanidinium NH resonances are observed solely across all‐trans (w) pathways. Already one cis orientation in the magnetisation transfer pathway leads to signal intensities below the actual detection limit and significantly lower than cross‐peaks from 2JNH, NH couplings or chemical exchange. However, it should be noted that also in the case of conformational exchange being fast on the NMR time scale, averaged cross‐peaks from all‐trans 4JH, H scalar couplings are detected, which may lead at first glance to an incomplete or even wrong conformational analysis. Copyright © 2010 John Wiley & Sons, Ltd.

Afonin, Andrei V.; Ushakov, Igor A.; Pavlov, Dmitry V.; Ivanov, Andrei V.; Mikhaleva, Al'bina I.

doi: 10.1002/mrc.2650pmid: 20623827

The 1H, 13C and 15N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole‐2‐carbaldehyde oxime is stabilized by the NH···N and NH···O intramolecular hydrogen bonds, respectively. The NH···N hydrogen bond in the E isomer causes the high‐frequency shift of the bridge proton signal by about 1 ppm and increase the 1J(N, H) coupling by ∼3 Hz. The bridge proton shows further deshielding and higher increase of the 1J(N, H) coupling constant due to the strengthening of the NH···O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by ∼3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of 1H shielding and 1J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the NH···N and NH···O hydrogen bondings to be estimated. The NBO analysis suggests that the NH···N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the NH bond through the NH···O hydrogen bond occurs in the Z isomer. Copyright © 2010 John Wiley & Sons, Ltd.

Castejón, David; Villa, Palmira; Calvo, Marta M.; Santa‐María, Guillermo; Herraiz, Marta; Herrera, Antonio

doi: 10.1002/mrc.2652pmid: 20641130

High‐resolution magic angle spinning (HRMAS) NMR spectroscopic data of smoked Atlantic salmon (Salmo salar) were fully assigned by combination of one‐ and two‐dimensional‐HRMAS experiments. Complete representative spectra, obtained after few minutes of analysis time, revealed a large number of minor and major compounds in the sample. The methodology is limited by the low sensitivity of NMR, and therefore HRMAS only enables the determination of the most relevant components. These were fatty acids (FAs), carbohydrates, nucleoside derivatives, osmolytes, amino acids, dipeptides and organic acids. For the first time, spectra were resolved sufficiently to allow semiquantitative determination in intact muscle of the highly polyunsaturated FA 22:6 ω‐3. Additionally, the feasibility of 1H‐HRMAS NMR metabolite profiling was tested to identify some bioactive compounds during storage. This profiling was carried out by the non‐destructive and direct analysis (i.e. without requiring sample preparation and multiple step procedures) of intact salmon muscle. The proposed procedure can be applied to a large number of samples with high throughput due to the short time of analysis and quick evaluation of the data. Copyright © 2010 John Wiley & Sons, Ltd.

deAzevedo, E. R.; Ayrosa, A. M. I. B.; Faria, G. C.; Cervantes, H. J.; Huster, D.; Bonagamba, T. J.; Pitombo, R. N. M.; Rabbani, S. R.

doi: 10.1002/mrc.2653pmid: 20641133

This article describes a solid‐state NMR (SSNMR) investigation of the influence of hydration and chemical cross‐linking on the molecular dynamics of the constituents of the bovine pericardium (BP) tissues and its relation to the mechanical properties of the tissue. Samples of natural phenetylamine‐diepoxide (DE)‐ and glutaraldehyde (GL)‐fixed BP were investigated by 13C cross‐polarization SSNMR to probe the dynamics of the collagen, and the results were correlated to the mechanical properties of the tissues, probed by dynamical mechanical analysis. For samples of natural BP, the NMR results show that the higher the hydration level the more pronounced the molecular dynamics of the collagen backbone and sidechains, decreasing the tissue's elastic modulus. In contrast, in DE‐ and GL‐treated samples, the collagen molecules are more rigid, and the hydration seems to be less effective in increasing the collagen molecular dynamics and reducing the mechanical strength of the samples. This is mostly attributed to the presence of cross‐links between the collagen plates, which renders the collagen mobility less dependent on the water absorption in chemically treated samples. Copyright © 2010 John Wiley & Sons, Ltd.

Migda, Wojciech; Stankiewicz, Anna; Szneler, Edward; Ulman, Lucyna; Rys, Barbara

doi: 10.1002/mrc.2657pmid: 20641136

Temperature‐dependent 1H and 13C‐NMR spectra of the title compounds are presented. Coalescence effects are discussed and assigned to dynamic process—the interconversion of bicyclic system. The free energies of activation covered the range 39–52 kJ/mol. The dioxepane ring adopts twist‐chair (TC) conformation. GIAO/DFT calculation of isotropic shieldings for the set of low‐energy conformations showed that only one conformer is present at 298 K in solution that matched well with experimental data. Copyright © 2010 John Wiley & Sons, Ltd.

Dračínský, Martin; Pohl, Radek; Slavětínská, Lenka; Buděšínský, Miloš

doi: 10.1002/mrc.2658pmid: 20661941

A series of model sulfides was oxidized in the NMR sample tube to sulfoxides and sulfones by the stepwise addition of meta‐chloroperbenzoic acid in deuterochloroform. Various methods of quantum chemical calculations have been tested to reproduce the observed 1H and 13C chemical shifts of the starting sulfides and their oxidation products. It has been shown that the determination of the energy‐minimized conformation is a very important condition for obtaining realistic data in the subsequent calculation of the NMR chemical shifts. The correlation between calculated and observed chemical shifts is very good for carbon atoms (even for the ‘cheap’ DFT B3LYP/6‐31G* method) and somewhat less satisfactory for hydrogen atoms. The calculated chemical shifts induced by oxidation (the Δδ values) agree even better with the experimental values and can also be used to determine the oxidation state of the sulfur atom (S, SO, SO2). Copyright © 2010 John Wiley & Sons, Ltd.

Pontoizeau, Clément; Herrmann, Torsten; Toulhoat, Pierre; Elena‐Herrmann, Bénédicte; Emsley, Lyndon

doi: 10.1002/mrc.2661pmid: 20648569

Unambiguous identification of individual metabolites present in complex mixtures such as biofluids constitutes a crucial prerequisite for quantitative metabolomics, toward better understanding of biochemical processes in living systems. Increasing the dimensionality of a given NMR correlation experiment is the natural solution for resolving spectral overlap. However, in the context of metabolites, natural abundance acquisition of 1H and 13C NMR data virtually excludes the use of higher dimensional NMR experiments (3D, 4D, etc.) that would require unrealistically long acquisition times. Here, we introduce projection NMR techniques for studies of complex mixtures, and we show how discrete sets of projection spectra from higher dimensional NMR experiments are obtained in a reasonable time frame, in order to capture essential information necessary to resolve assignment ambiguities caused by signal overlap in conventional 2D NMR spectra. We determine optimal projection angles where given metabolite resonances will have the least overlap, to obtain distinct metabolite assignment in complex mixtures. The method is demonstrated for a model mixture composition made of ornithine, putrescine and arginine for which acquisition of a single 2D projection of a 3D 1H–13C TOCSY‐HSQC spectrum allows to disentangle the metabolite signals and to access to complete profiling of this model mixture in the targeted 2D projection plane. Copyright © 2010 John Wiley & Sons, Ltd.

Pimenta, Antonia Torres Ávila; Braz‐Filho, Raimundo; Delprete, Piero Giuseppe; de Souza, Elnatan Bezerra; Silveira, Edilberto Rocha; Lima, Mary Anne Sousa

doi: 10.1002/mrc.2656pmid: 20641135

Two unusual monoterpene indole alkaloids, stachyoside (1) and nor‐methyl‐23‐oxo‐correantoside (2), have been isolated from the aerial parts of Psychotria stachyoides. The structural elucidation of both compounds was performed by the aid of HRESIMS, FT‐IR, and 1D‐ and 2D‐NMR techniques including COSY, HSQC, HMBC, and NOESY. Copyright © 2010 John Wiley & Sons, Ltd.