Unambiguous Assignment Of Signals Research Articles

Identification of isomeric dicaffeoylquinic acids from Eleutherococcus senticosus using HPLC-ESI/TOF/MS and 1H-NMR methods.

Liquid chromatography-electrospray time-of-flight mass spectrometry (HPLC-ESI/TOF/MS) and a novel NMR technique, developed to maximise the sensitivity obtained from the standard NMR spectrometer, have been applied to the identification of the phenolic constituents of Eleutherococcus senticosus. In addition, molecular modelling and dihedral bond angle calculations based on the vicinal 3JHH-coupling constants have been used in the unambiguous assignment of signals in the 1H-NMR spectra. 5'-O-Caffeoylquinic acid and three isomeric compounds, 1',5'-O-dicaffeoylquinic acid, 3',5'-O-dicaffeoylquinic acid and 4',5'-O-dicaffeoylquinic acid, have been isolated and identified from a sample. The isolation and structure determination of the latter two compounds are reported for the first time from this plant.

In vivo 1H NMR spectroscopy of the human brain at 7 T.

In vivo 1H NMR spectra from the human brain were measured at 7 T. Ultrashort echo-time STEAM was used to minimize J-modulation and signal attenuation caused by the shorter T2 of metabolites. Precise adjustment of higher-order shims, which was achieved with FASTMAP, was crucial to benefit from this high magnetic field. Sensitivity improvements were evident from single-shot spectra and from the direct detection of glucose at 5.23 ppm in 8-ml volumes. The linewidth of the creatine methyl resonance was at best 9 Hz. In spite of the increased linewidth of singlet resonances at 7 T, the ability to resolve overlapping multiplets of J-coupled spin systems, such as glutamine and glutamate, was substantially increased. Characteristic spectral patterns of metabolites, e.g., myo-inositol and taurine, were discernible in the in vivo spectra, which facilitated an unambiguous signal assignment.

Unambiguous signal assignments of 1H and 13C NMR spectra for chlorinated benzenes using 37Cl/35Cl isotope effects

AbstractOne‐bond 37Cl/35Cl isotope effects on 13C NMR chemical shifts were used to assign the 1H and 13C NMR spectra of bis(4‐chlorophenyl)sulfone, 4,5‐dichlorophthalic acid and 2,2′‐thiobis(4,6‐dichlorophenol). The chlorine‐bonded carbons showed characteristic peaks separated by about 4 ppb with a 3 : 1 area ratio, corresponding to the natural abundance of 35Cl and 37Cl. The determination of the chlorine‐bonded carbon signals based on the isotope effects was useful when the empirical additivity rules of the chemical shifts and several 2D NMR techniques were insufficient for a complete definitive assignment. Copyright © 2001 John Wiley & Sons, Ltd.

Special reactions of α,β‐unsaturated ketones: Part II. Structure and stereochemistry of perhydro‐5a,11a‐epoxydibenz[b,f]oxepin‐4a‐ol, a bridged Diels–Alder dimer of 2‐methylenecyclohexanone

AbstractThe paper deals with the structure elucidation of (4aRS,5aRS,9aRS,11aRS)‐perhydro‐5a,11a‐epoxydibenz[b,f]oxepin‐4a‐ol (6), which is unexpectedly formed from 2‐dimethylaminomethylcyclohexanone hydrochloride (1·HCl) in the presence of phenylguanidine in boiling ethanol. In detail, two molecules of 2‐methylenecyclohexanone (generated from 1 by elimination of dimethylamine) afforded via Diels–Alder reaction, subsequent addition of water and ring closure a product C14H22O3, which finally proved to be the title compound. The structure of the tetracyclic hemiacetal 6 was deduced from one‐ and two‐dimensional NMR experiments (1D TOCSY, DEPT‐135, 1H,1H‐COSY, gs‐HSQC, gs‐HMBC, gs‐HSQC‐TOCSY). In particular, the one‐dimensional selective TOCSY and the gradient‐selected HSQC‐TOCSY, performed as a pseudo‐3D experiment without the evolution period in the TOCSY part, proved to be very helpful for the unambiguous assignment of signals in overcrowded regions. NOE data confirmed the stereochemistry of epoxydibenzoxepinol 6. Copyright © 2001 John Wiley & Sons, Ltd.

Solution 1H NMR Study of the Electronic Structure and Magnetic Properties of High-Spin Ferrous or Deoxy Myoglobins

Solution 1D and 2D NMR, together with limited isotope labeling, have led to the assignment of the heme, axial His, and numerous heme contact residues in sperm whale, horse, and human deoxy myoglobin. The paramagnetic relaxivity leads to increased line widths and shorter T1s with little compensation in increased dispersion due to dipolar shifts. Hence only limited, but crucial F helix standard backbone sequence specific assignment could be made for heme cavity residues. Numerous other residues with significant dipolar shifts could be assigned from the characteristic scalar connectivities and dipolar contacts to the heme predicted by the crystal structure. It is concluded that the complete and unambiguous assignment of the heme pyrrole substituent signals is not attainable by 2D NMR alone without either partial deuterium labeling of the heme or parallel assignment of key residues in dipolar contact with the heme; hence the present study revises some earlier assignments. The resulting dipolar shifts for nonligated residues, together with the crystal coordinates of deoxy myoglobin, were used to determine the orientation relative to the heme and the anisotropy of the paramagnetic susceptibility tensor. The significant anisotropy, |Δχ| ∼1 × 10-9 m3/mol, however, is shown to result in dipolar shift with reciprocal square, rather than just reciprocal, absolute temperature dependence, which is indicative of large zero field splitting rather than g-tensor anisotropy. The appropriate equation for a 5B2 ground state allows an estimate of the zero-field splitting, D ∼−10 cm-1, which is in good agreement with earlier results. The present NMR data favor a spin-only magnetic moment with S = 2 and D ∼−10 cm-1 over a ground state with S < 2 and significant orbital contribution (Hendrich and Debrunner, 1989).

Chemical Shift Assignment of Geminal Protons in 3,7-Diazabicyclo[3.3.1]nonanes: An Unexpected Deviation from the Axial/Equatorial Chemical Shift Order

The chemical shift order of axial and equatorial methylene protons in 1,5-disubstituted 3,7-diazabicyclo[3.3.1]nonan-9-ones may be altered by substituents in the 1,5-positions, but the corresponding alcohols behave differently. Unambiguous signal assignments for a series of the title compounds are provided, based on 3JCH coupling constants and on {1H} 13C heteronuclear Overhauser effects. Substituent anisotropy effects as a source of the chemical shift changes are discussed. © 1997 by John Wiley & Sons, Ltd.

1H‐NMR study of some new acetyl dimethylbiphenyls: unambiguous signal assignment for the methyl groups

The abundance 1H‐NMR spectra of some new mono‐ and diacetyl compounds derived from 2,2′‐dimethyl‐, 3,3′‐dimethyl‐, and 4,4′‐dimethylbiphenyls were recorded. Unambiguous signal assignment of the methyl groups of the aromatic biphenyl rings were made for the structure elucidation.

Numerical abnormalities of chromosome 7 in human prostate cancer detected by fluorescence in situ hybridization (FISH) on paraffin‐embedded tissue sections with centromere‐specific dna probes

Fluorescence in situ hybridization (FISH) using chromosome-specific alpha-satellite DNA probes for chromosomes 7, 8, and 12 was performed on paraffin-embedded tissue sections and touch imprint preparations of 53 cases of human prostate cancer. Subsequent haematoxylin and eosin (H & E) staining of the hybridized tissue sections allowed unambiguous assignment of hybridization signals either to tumour or to non-tumorous parenchyma. Fifty-three cases of human prostate cancer were evaluated for numerical aberrations of chromosome 7. Scoring 200 cells of tumour and non-tumorous parenchyma in each case revealed abnormalities exclusively in tumour parenchyma in 41 cases (77 per cent). Ten of 41 cases (24 per cent) showed trisomy 7, and 15 cases (37 per cent) monosomy 7 or trisomy 7 in combination with monosomy 7, respectively. Sixteen cases (39 per cent) exhibited polysomy 7 in cells of the tumour parenchyma. In the tumour tissue in one case, different polyploid clones (triploid, tetraploid) and polysomy 7 could be identified by double hybridization with chromosome-specific DNA probes for chromosome 7, plus 8 or 12. The indicated numerical aberrations of chromosome 7 were correlated with 78 per cent of advanced pathological stages or poorly differentiated tumours (pT3/4 or G3) of prostate carcinomas. A statistical analysis of the data revealed significant relationships of particular numerical abnormalities of chromosome 7 to different pathological categories (pT, G, pN) of tumour classification. For the T-classification, the frequency of cells carrying polysomy 7 and polysomy 7/+7 increases significantly from pT1 to pT3/4 (P = 0.022).(ABSTRACT TRUNCATED AT 250 WORDS)

Assignment of 1H signals in complex overlapping spectral regions using proton-detected 2D heteronuclear shift correlation spectroscopy

Abstract The application of a heteronuclear 2D pulse sequence is demonstrated which correlates chemical shifts of carbons with directly bound protons and with remote protons belonging to the same coupled spin system by using the effect of Hartmann—Hahn cross polarization among protons. The enhanced spectral resolution obtained by spreading individual proton subsystems into the carbon domain and the option for multiplicity discrimination makes these kind of experiments most suitable for proton and carbon signal assignments of natural products as alkaloids and steroids with strongly overlapping proton resonances. With the application to a steroid we want to demonstrate that such unambiguous signal assignments are the prerequisite and the basis for detailed structural investigations using additional, more conventional NMR experiments.

13C NMR study ofN-substituted 3-azachalcones

13 C NMR data are reported for new N-benzyl and N-alkyl derivatives of 3-azachalcone. The unambiguous assignment of signals is based on 13 C/ 1 H heteroshift correlated 2D NMR spectra

1H‐ und 13C‐NMR‐Spektren von N‐Methyl‐chinoliniumchalkonen: Bestimmung von σ‐Konstanten für die 2‐, 3‐ und 4‐Position des N‐Methyl‐chinoliniumrings

1H‐ and 13C‐N.M.R. Spectra of N‐Methyl‐quinolinium Chalcones: Determination of σ‐Constants for the 2‐, 3‐ and 4‐Position of the N‐Methyl‐quinolinium RingFor the N‐methyl‐quinolinium chalcones 1a–c the 1H‐ and 13C‐n.m.r. data are reported. The unambiguous assignment of signals is based upon the 1H/1H homoshift and the 13C/1H heteroshift correlated 2D‐n.m.r. spectra. Using a Hammett relationship we have determined σ‐constants for the 2‐, 3‐ and 4‐position of the N‐methyl‐quinolinium ring from the 13C‐α chemical shifts of 1a–c. Their comparison with the σ‐values for the equivalent positions in the pyridine, N‐methyl‐pyridinium and quinoline ring indicates the additivity of anellation and quaternization effect.

Selektive Metallierung von BicycIo[3.2.1]octa-2,6-dien, II Distannylierte Derivate und stannylierte tricyclische Sekundärprodukte / Selective Metalation of Bicyclo[3.2.1]octa-2,6-diene, II Distannylated Derivatives and Stannylated Tricyclic Secondary Products

Abstract Distannylated bicyclo[3.2.1]octa-2,6-diene (1) with (CH3)3 Sn groups in positions exo-4 and 7 (7a), endo-4 and 7 (7b), exo-4 and 6 (7c), and probably exo-4 and endo-4 (7d) are obtained when 1 is treated first with an excess of n-BuLi/t-BuOK in alkane and then with (CH3)3 SnCl. Byproducts are monostannylated derivatives of 1 and l,4-bis(trimethylstannyl)but-2-ene (3). The formation of 7a/b/c is best understood when the two possible bicyclo[3.2.1]octa-2,6-dienyl dianions are generated as intermediates. 3,exo-4-Bis(trimethylstannyl)bicyclo[3.2.1]octa-2,6-diene (7e) is ob-tained from the corresponding dibromo derivative of 1 and (CH3)3SnLi with 3-bromo-exo-4-trimethylstannylbicyclo[3.2.1]octa-2,6-diene (9) as an intermediate. The bicyclic stannyl com-pounds are susceptible to a 1,5-homopentadienyl shift of a (CH3)3Sn group at relatively low temperature when the starting compound has the stannyl group in position 4 of 1 and when positions 6 or 7 are unsubstituted. Tricyclo[3.2.1.02.7]oct-3-enes with (CH3)3Sn groups in positions exo-6 (5a), endo-6 (5b), 3 and exo-6 (10a), and 3 and endo-6 (10b) are thus obtained. On further heating, 10b gives l-trimethylstannyl-3-vinylbenzene (11). All compounds have been studied systematically by NMR spectroscopy including δ(119Sn), δ(13C), δ(1H), 4/5J(119/117Sn-119Sn), nJ(119/117Sn-13C) , nJ(119/117Sn-1H), 1J(13C-1H ) , and nJ(1H-1H ). Among all nuclei, 119Sn gives the most simple spectra. From the 13C NMR spectra a series of useful increments of (CH3)3Sn groups emerges. The most interesting coupling constants are 5J(119/117Sn-119Sn), for which a considerable change with the dihedral angle is established, and nJ(119/117Sn-13C). For n = 3 and 4 the latter are especially useful because their angular dependence leads to unambiguous signal assignments.

Alkyl phosphotriester modified oligodeoxyribonucleotides. VI. NMR and UV spectroscopic studies of ethyl phosphotriester (Et) modified Rp-Rp and Sp-Sp duplexes, (d[GGAA(Et)TTCC])2.

1H NMR chemical shift assignments for the title compounds were made for all but a few H5' and H5" signals using two-dimensional nuclear Overhauser effect (2D-NOE) data, which was also used for the first time to assign absolute configuration at phosphorus. The chemical shifts were, in general, similar to those reported [Broido, M.S., et al. (1985) Eur. J. Biochem. 150, 117-128] for the B-like conformation of the unmodified, parent duplex, [d(GGAATTCC)]2. Differences in chemical shifts for corresponding protons were mostly localized to the AA(Et)TT region, and showed some stereochemical dependence. Unambiguous assignment of the phosphotriester 31P signals was achieved in a novel way using selective insensitive nucleus enhancement by polarization transfer (selective INEPT) NMR. The Rp-Rp duplex melted ca. 11 degrees C lower than either the Sp-Sp or parent duplexes, as evidenced by Tm and variable temperature 1H/31P NMR measurements. The 2D-NOE data for the Rp-Rp duplex suggested possible steric interactions between the ethyl group and the H3' of the flanking A residue. At low ionic strength, the Sp-Sp and parent duplexes had similar stability but at high ionic strength the Sp-Sp duplex was less stable.

Analysis of the NMR spectra of trimethylsilyl‐substituted sugars by two‐dimensional heteronuclear 29Si–1H shift correlation spectroscopy

AbstractTwo‐dimensional NMR experiments on 29Si‐1H shift correlations in trimethylsilyl‐substituted sugars are described. This technique allows the unambiguous assignment of 29Si and 1H signals of all trimethylsilyl groups.

Natural abundance 13C 13C coupling constants observed via double quantum coherence. The two-dimensional ‘inadequate’ experiment for 13C NMR spectral analysis of erythronolide - B

A one- and a two-dimensional INADEQUATE NMR experiment afforded one-bond 13C 13C coupling constants at natural abundance and total unambiguous signal assignment for erythronolide-B (1). 13C 13C coupling constants at natural abundance and total unambiguous signal assignment for erythronolide-B (1).

13C chemical shifts of symmetrically substituted biphenyls: Unambiguous signal assignment for the carbons ortho and para to an aryl group

AbstractThe natural abundance 13C NMR spectra of 2,2′‐dimethyl‐, 2,2′‐dimethoxy‐ and 2,2′‐dihydroxybiphenyls, and a series of 2,2′‐dimethoxy‐5,5′‐disubstituted biphenyls were recorded. Unambiguous signal assignments of the carbons ortho and para to an aryl ring in biphenyls were made by selective deuteration and/or the graphical method for 1H single frequency off‐resonance decoupled spectra. Contrary to the reported assignments, it was shown that the signal for C‐6 in 2,2′‐dimethylbiphenyl clearly appears at lower field than that for C‐4. The signals for the ortho carbons (C‐6) of 2,2′‐dimethoxy‐5,5′‐disubstituted biphenyls generally appeared at lower fields than those for the para carbons (C‐4). The validity of applying deuterium isotope shifts to the assignments of 13C chemical shifts of di‐ and tetra‐substituted biphenyls is also discussed.

13C{ 1H} off-resonance decoupling with variable decoupler field strength

A 13C{ 1H} off-resonance decoupling technique by varying the decoupler field strength at a constant frequency offset is discussed. The usefulness of the method for spectrometers equipped with only a stepwise variation of the decoupler frequency is demonstrated by the carbon spectrum of fluorene. This leads to an unambiguous assignment of all tertiary carbon signals.

UNEQUIVOCAL SIGNAL ASSIGNMENT OF THE 13C NMR SPECTRUM OF 4-HOMOISOTWISTANE AS PROBED BY DEUTERIUM SUBSTITUTIONS

Abstract Isotope shifts as well as geminal and vicinal couplings with the deuterium atom were clearly observed in the 13C NMR spectrum of tricyclo[5.3.1.03,8]undecane-2-d1 (2 and 3) and -3-d1 (4). The phenomena made possible an unambiguous signal assignment in the spectrum of the undeuterated hydrocarbon (1) that had been impracticable so far.