Magnetic Resonance in Chemistry

Navarro, Pilar; Reviriego, Felipe; Alkorta, Ibon; Elguero, José; López, Concepción; Claramunt, Rosa M.; García‐España, Enrique

doi: 10.1002/mrc.1897pmid: 16991111

The crystals obtained by mixing equimolar amounts of diethyl 1H‐pyrazole‐3,5‐dicarboxylate and the primary amines phenethylamine and homoveratrylamine are ammonium pyrazolate salts as determined by 13C and 15N CPMAS NMR. Copyright © 2006 John Wiley & Sons, Ltd.

Žáček, Petr; Dransfeld, Alk; Exner, Otto; Schraml, Jan

doi: 10.1002/mrc.1900pmid: 16991110

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, XC6H4CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6–31 + G*//B3LYP/6–31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent‐induced chemical shifts of the CN carbon. Copyright © 2006 John Wiley & Sons, Ltd.

Alvarez‐Salgado, Francisco; Desvaux, Hervé; Boulard, Yves

doi: 10.1002/mrc.1902pmid: 16972306

We have carried out a solution study of two non‐labelled self‐complementary DNA dodecamers d(GACTGTACAGTC)2 and d(GACTGTGCAGTC)2 by NMR, the second sequence composed of two GT mismatches. Structures were determined using distances extracted from NOE effects alone or using both NOE and RDC constraints, measured in three different liquid crystalline media. We ensured that our data on the influence of the mesogen on the DNA structures, and the way in which the RDCs were incorporated as constraints in the protocol refinement, were consistent. We also tested the influence of different sets of RDCs and the best means of optimizing the calculation of Da and R. Resolution and accuracy of the ten best energy final structures were compared. The addition of a small set of RDC constraints significantly improves the final determined structures. We took advantage of the specificity of the RDC, i.e. it contains orientational information, and explored the global shape of the DNA duplexes; it was found that the duplexes do not have a large curvature. For the GT base pair, we observed, in this particular sequence (tandem of GT mismatches), a new pattern of base pairing, which involved the formation of a bifurcated hydrogen bond. Copyright © 2006 John Wiley & Sons, Ltd.

Zou, Peng; Fu, Jing; Yu, He‐shui; Zhang, Jie; Kang, Li‐ping; Ma, Bai‐ping; Yan, Xian‐zhong

doi: 10.1002/mrc.1904pmid: 16991112

The detailed NMR studies and full assignments of the 1H and 13C spectral data for two new furostanol saponins isolated from Agave sisalana leaves are described. Their structures were established using a combination of 1D and 2D NMR techniques including 1H, 13C, 1H1H COSY, TOCSY, HSQC, HMBC and HSQC‐TOCSY, and also FAB‐MS spectrometry and chemical methods. The structures were established as (25S)‐26‐(β‐D‐glucopyranosyl)‐22 ξ‐hydroxyfurost‐12‐one‐3β‐yl‐O‐α‐L‐rhamnopyranosyl‐(1→4)‐β‐D‐glucopyranosyl‐(1→3)‐O‐[O‐β‐D‐glucopyranosyl‐(1→2)]‐O‐β‐D‐glucopyranosyl‐(1→4)‐β‐D‐galacto‐ pyranoside (1) and (25S)‐26‐(β‐D‐glucopyranosyl)‐22ξ‐hydroxyfurost‐5‐en‐12‐one‐3β‐yl‐O‐α‐L‐rhamno‐ pyranosyl‐(1→4)‐β‐D‐glucopyranosyl‐(1→3)‐O‐[O‐β‐D‐glucopyranosyl‐(1→2)]‐O‐β‐D‐glucopyranosyl‐ (1→4)‐β‐D‐galactopyranoside (2). Copyright © 2006 John Wiley & Sons, Ltd.

Beguin, Laetitia; Courtieu, Jacques; Ziani, Latifa; Merlet, Denis

doi: 10.1002/mrc.1905pmid: 16991108

This work presents a technique to simplify overcrowded proton spectra in chiral liquid crystal solvents using rotation of the sample near the magic angle, VASS, combined with homonuclear selective refocusing 2D NMR experiments, SERF. This methodology provides a powerful tool to visualise enantiomers out of unresolved proton spectra. A modified SERF sequence is presented where the resulting 2D spectrum can be phased to increase the resolution. Accurate enantiomeric excesses are determined that are not possible to measure on static samples. Two examples are presented. Copyright © 2006 John Wiley & Sons, Ltd.

Bernstein, Michael A.; Stoddart, Simon

doi: 10.1002/mrc.1906pmid: 17001735

We describe a program for the viewing, analysis, and reporting of 1D NMR spectra. The program provides an intuitive environment to display 1D NMR spectra, and assists with data reduction and reporting. It is particularly well suited for multiple NMR spectra derived from the recording of spectra of samples in microtitre plates, where it adopts a unique, effective, and user‐friendly approach. This also allows for quick overviews of the underlying NMR data and offers an effective tool for quality control in parallel chemistry. Copyright © 2006 John Wiley & Sons, Ltd.

Seba, Hadj B.; Thureau, Pierre; Ancian, Bernard; Thévand, André

doi: 10.1002/mrc.1909pmid: 17016868

Complementary results from 13C intermolecular nuclear Overhauser effects (NOE), 1H13C heteronuclear Overhauser spectroscopy (HOSEY) and 1H‐NMR diffusion measurements were used for probing the structure of the first solvation shell of uridine in water. It is demonstrated that a cyclic dihydrate is formed. The two water molecules produce two hydrogen bonds with the two oxygen atoms from the pyrimidine ring and accept only one hydrogen bond from the amide proton. The dihydrate has only a short lifetime as compared with the rotational correlation time of the free nucleoside. The chemical exchange constant of the amide proton with water is then estimated by diffusion experiments. The results are consistent with previous data obtained for uracil in water and provide interesting information about water accessibility in nucleic acid bases. Copyright © 2006 John Wiley & Sons, Ltd.

Subramaniam, Gopal; Karimi, Sasan; Phillips, David

doi: 10.1002/mrc.1908pmid: 17001750

We describe the conformation and stereospecific 1H and 13C chemical shift assignments of longifolene 1 and its penultimate precursor 2 through the combined use of ab initio calculations and experimental NMR techniques. The predicted stable conformation for both compounds was similar and adopts a twisted chair conformation at the seven‐membered ring where C4 lies on top of the exocyclic double bond. The calculated chemical shifts for the stable conformation agree well with the experimental values. Copyright © 2006 John Wiley & Sons, Ltd.

Barros, Ana I. R. N. A.; Silva, Artur M. S.

doi: 10.1002/mrc.1895pmid: 17001752

Twenty‐six new aminoflavones have been synthesised by two different methods and the structure elucidation was accomplished using extensive 1D (1H, 13C) and 2D NMR spectroscopic studies (COSY, HSQC and HMBC experiments). Copyright © 2006 John Wiley & Sons, Ltd.

Liu, Yi; Zhang, Qingying; Chen, Hubiao; Wang, Bin; An, Dongge; Zhao, Yuying

doi: 10.1002/mrc.1903pmid: 17001751

Four saponins (1–4) were isolated from the roots of Hedysarum polybotrys Hand.‐Mazz. and obtained from Hedysarum (Tourn.) L for the first time. Their structures were identified on the basis of spectroscopic data. MS, 1D and 2D NMR including HMQC, HMBC, 1H–1H COSY and NOESY permitted correct assignments of all 1H and 13C signals of 1 and 2, and the 13C NMR data of 2 were reported for the first time. Copyright © 2006 John Wiley & Sons, Ltd.