Magnetic Resonance in Chemistry

Muri, Dieter; Corminboeuf, Clémence; Carreira, Erick M.; Jeannerat, Damien

doi: 10.1002/mrc.2476pmid: 19591195

We discuss and demonstrate the potential of HSQC‐TOCSY and HSQC‐NOESY experiments to offer solutions for overlap problems in COSY and NOESY spectra, leading to improved signals that can be unambiguously assigned to individual carbons. Direct comparison of experimental 1H and 13C chemical shielding with density functional theory (DFT)‐calculated values are uninformative; in contrast, the relative differences in experimental shielding between pairs of molecules correlates well with the relative differences in DFT‐GIAO shielding for the computed lowest energy conformers. A detailed application of both experimental and theoretical techniques is illustrated for slowly exchanging conformers of an erythronolide A derivative, which demonstrates that structure determination can strongly benefit from the interplay between experiment and theory. Copyright © 2009 John Wiley & Sons, Ltd.

López, Concepción; Claramunt, Rosa M.; Pinilla, Elena; Torres, M. Rosario; Alkorta, Ibon; Elguero, José

doi: 10.1002/mrc.2483pmid: 19650063

Five charge‐transfer complexes 1–5 derived from indoles (including a carbazole) and halogenopolynitrobenzenes (ClDNB, FDNB, ClTNB) as well as their individual components have been studied in the solid state by 13C CPMAS NMR. The stacking effects on the 13C chemical shifts have been rationalized by means of M05‐2X functional and GIAO/B3LYP/6‐311 ++G(d,p) calculations. The results, although only semiquantitative, are very promising for studying such structures. Copyright © 2009 John Wiley & Sons, Ltd.

Neuman, Robert C.; Gerig, John T.

doi: 10.1002/mrc.2489pmid: 19634131

Melittin dissolved in 42% trifluoroethanol‐water at pH 2 has been shown to be α‐helical between residues 6 and 12 and between residues 13 and 25, with the two helical regions separated by a bend at the Leu13 residue. The inter‐helix angle was found to be 154 ± 3° at 0 °C and 135 ± 3° at 25 °C. The dominant conformation of the peptide is thus similar to those observed by previous workers for the peptide in a variety of media. At 25 °C, intermolecular nuclear Overhauser effects arising from nuclear spin dipole‐dipole interactions between melittin hydrogens and fluorines of the solvent are essentially those expected for a system that is homogeneous as regards concentration and translational diffusion of the peptide and fluoroalcohol components. However, at 0 °C, peptide‐trifluoroethanol cross‐relaxation terms are negative, a result consistent with the conclusion that fluoroalcohol molecules associate with the peptide for times (∼1 ns) that are long compared to the time of a typical peptide‐fluoroalcohol diffusive encounter (∼0.2 ns). Such interactions may be responsible for the reduction of the translational diffusion coefficient of trifluoroethanol produced by dissolved peptides. Copyright © 2009 John Wiley & Sons, Ltd.

Pazderski, Leszek; Pawlak, Tomasz; Sitkowski, Jerzy; Kozerski, Lech; Szłyk, Edward

doi: 10.1002/mrc.2491pmid: 19691018

1H, 13C, 15N and 195Pt NMR studies of gold(III) and platinum(II) chloride organometallics with N(1),C(2′)‐chelated, deprotonated 2‐phenylpyridine (2ppy*) of the formulae (Au(2ppy*)Cl2), trans(N,N)‐(Pt(2ppy*)(2ppy)Cl) and trans(S,N)‐(Pt(2ppy*)(DMSO‐d6)Cl) (formed in situ upon dissolving (Pt(2ppy*)(µ‐Cl))2 in DMSO‐d6) were performed. All signals were unambiguously assigned by HMBC/HSQC methods and the respective 1H, 13C and 15N coordination shifts (i.e. differences between chemical shifts of the same atom in the complex and ligand molecules: Δ1Hcoord = δ1Hcomplex − δ1Hligand, Δ13Ccoord = δ13Ccomplex − δ13Cligand, Δ15Ncoord = δ15Ncomplex − δ15Nligand), as well as 195Pt chemical shifts and 1H‐195Pt coupling constants discussed in relation to the known molecular structures. Characteristic deshielding of nitrogen‐adjacent H(6) protons and metallated C(2′) atoms as well as significant shielding of coordinated N(1) nitrogens is discussed in respect to a large set of literature NMR data available for related cyclometallated compounds. Copyright © 2009 John Wiley & Sons, Ltd.

Morais, Cláudia M.; Montouillout, Valérie; Deschamps, Michael; Iuga, Dinu; Fayon, Franck; Paz, Filipe A. A.; Rocha, João; Fernandez, Christian; Massiot, Dominique

doi: 10.1002/mrc.2492pmid: 19685487

From one‐ to two‐ and three‐dimensional MAS NMR solid‐state experiments involving 31P and 27Al, we show that the structure of microporous alumino‐phosphate AlPO4‐40 contains at least four times more sites than expected, and we attribute two types of AlIV sites. The newly described 27Al‐31P MQ‐HMQC opens new possibilities of describing details of three‐dimensional bounded networks. Copyright © 2009 John Wiley & Sons, Ltd.

Kato, Takayuki; Tsubono, Koji; Kamo, Osamu; Kato, Toshiyo; Furihata, Kazuo; Fujimoto, Takashi; Machinami, Tomoya; Tashiro, Mitsuru

doi: 10.1002/mrc.2495pmid: 19655368

The 1H and 39K longitudinal relaxation times (T1) and 1H diffusion coefficients were measured to investigate the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. Although the 1H‐T1 values of H3′, H5′, H1″ and H4″ decreased in the presence of potassium ions, 1H chemical shifts and 1H diffusion coefficients did not show significant changes. The long‐range coupling constants of 3JC−H around the glycosyl bonds did not show significant changes either. In the measurements of 39K spectra, the 39K signal obviously broadened and the 39K‐T1 values decreased in the presence of 1,6‐anhydro‐β‐maltotriose, indicating the complex formation of 1,6‐anhydro‐β‐maltotriose and potassium ions. These results indicate that the conformation and molecular volume were unaffected in the complex formation. Copyright © 2009 John Wiley & Sons, Ltd.

Pesyan, Nader Noroozi

doi: 10.1002/mrc.2498pmid: 19655369

NMR spectra of the synthesized azo dyes, 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (5a–g), 1,3‐dimethyl‐5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (6a–g), and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones (7a–g) were studied in (CD3)2SO (three drops of CD3OD were added into solutions of the dyes in two different concentrations). All dyes showed intramolecular hydrogen bonding. Dyes 5a–7a showed bifurcated intramolecular hydrogen bonds. Tautomeric behaviours of some of N‐methylated azo dyes (6a‐g) were studied in two different concentrations. The solvent–substrate proton exchange of dyes 5a–d, 6a and 7a–e was examined in presence of three drops of CD3OD. The dyes which were soluble in (CD3)2SO containing CD3OD showed isotopic splitting (β‐isotope effect) in the 13C NMR spectra. Copyright © 2009 John Wiley & Sons, Ltd.

Kupka, Teobald

doi: 10.1002/mrc.2500pmid: 19681101

Good performance of segmented contracted basis sets XZP, where X = D, T, Q and 5, for obtaining H2O, H2, HF, F2 and F2O nuclear isotropic shielding constants in the BHandH Kohn–Sham basis set limit was shown. The results of two‐ and three‐parameter complete basis set limit extrapolation schemes were compared with experimental results, earlier literature data and benchmark ab initio results. Similar convergence patterns of shieldings obtained from calculations using general purpose XZP basis sets and from polarization‐consistent basis sets pcS‐n and pcJ‐n, where n = 0, 1, 2, 3 and 4, designed to accurately predict magnetic properties were observed. On the contrary, the SSCCs were more sensitive to the XZP basis set size and generally less accurate than those estimated using pcJ‐n basis set family. The BHandH density functional markedly outperforms B3LYP method in predicting heavy atom shieldings and SSCCs values in the studied systems. Copyright © 2009 John Wiley & Sons, Ltd.

Furihata, Kazuo; Shimotakahara, Sakurako; Shibusawa, Yoichi; Tashiro, Mitsuru

doi: 10.1002/mrc.2493pmid: 19637209

An efficient pulse sequence for observing the ligand signals resonating close to the water signal has been developed by incorporating the WET technique into the saturation transfer difference pulse sequence. Although several pulse sequences have been developed for observing a ligand binding with a protein receptor, the ligand signals resonating close to the water were undetectable owing to the interference of the huge water signal in the samples containing 95% 1H2O. On the point of sample preparation, it is preferable to avoid the solvent exchange in the protein samples. In the proposed pulse sequence, a WET sequence is incorporated for the selective suppression of the water resonance. The efficient water suppression and the clear observation of the bound ligand signals close to the water have been demonstrated using the lysozyme‐glucose complex. Copyright © 2009 John Wiley & Sons, Ltd.

Grycová, Lenka; Dommisse, Roger; Pieters, Luc; Marek, Radek

doi: 10.1002/mrc.2494pmid: 19653253

Malaria is one of the most serious global health problems. Isolating new therapeutic agents with potential antimalarial activity from natural sources or preparing such agents either semisynthetically or synthetically is one strategy for solving the problem of resistance constantly evolving to the drugs currently in use. For alkaloids, the acid–base dissociation constant, pKa, is an important characteristic, thought to be associated with biological activity. In this contribution, pKa values for several indoloquinoline alkaloids were determined by using 1H NMR spectroscopy in a mixture of solvents. The data were recalculated for water solutions using the correction factors reported previously. The structural dependence of the pKa values for cryptolepine and its isomers neocryptolepine, isocryptolepine and isoneocryptolepine as well as some substituted neocryptolepine derivatives is discussed. Copyright © 2009 John Wiley & Sons, Ltd.