摘要:

AbstractTo test the feasibility of using77Se NMR to distinguish between different proposed mechanisms of action of the selenoenzyme glutathione peroxidase, model compounds containing the proposed selenium moieties have been synthesized and their77Se NMR spectra recorded. It was found that the difference in chemical shift between the two proposed resting states, selenol or selenenic acid, is approximately 1300 ppm. For the next step, oxidation by hydrogen peroxide, a much smaller difference of about 100 ppm was found between selenenic and seleninic acid species. The products of the second reaction step, selenide sulfide vs selenoxide sulfide, differed in chemical shift by about 470 ppm. It was concluded that, for two of the three states of the enzyme,77Se NMR could, in principle, be used unambiguously to distinguish between the two proposed mechanisms.

ISSN:0749-1581    

DOI:10.1002/mrc.1260260102

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe hypermodified nucleotidyl purine queuine [7‐(3,4‐trans‐4,5‐cis‐dihydroxycyclopent‐1‐en‐3‐ylaminomethyl)‐7‐deazaguanine] is a substrate for transfer ribonucleic acid (tRNA) enzymes replacing guanine at nucleotide position 34 of tRNAs for the amino acids aspartic acid, asparagine, bistidine and tyrosine. The structure and conformation of native queuine was studied by one‐dimensional13C and1H NMR spectroscopy and by two‐dimensional NMR spectroscopy (COSY and NOESY). The structure in solution was found to be the 3,4‐trans‐4,5‐cis‐dihydroxycyclopent‐1‐ene form. The plane of the cyclopentene ring is probably perpendicular

ISSN:0749-1581    

DOI:10.1002/mrc.1260260103

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

Abstract13C and15N chemical shift assignments of a number of 1,2‐benzisoxazole 2‐oxides and their deoxygenated analogues are reported and discussed. Similarly, assignments for some of the precursors to theN‐oxides,o‐hydroxyaryl ketoximes, are reported. In theN‐oxides the “push‐pull” effect of the NO dipole is largely localized on the isoxazole unit. A large (ca39 ppm) upfield shift of the C‐3 resonance is caused by the exocyclic oxygen whereas C‐7a and, to a lesser extent, C‐6 are the most heavily deshielded nuclei. The substituents do not have a significant effect on the C‐3 resonance, whereas their effect on the carbocyclic carbons is more pronounced, the most markedly affected being the C‐4 and C‐6 nuclei.N‐Oxidation of 1,2‐benzisoxazole causes a large (50.5

ISSN:0749-1581    

DOI:10.1002/mrc.1260260104

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe1H and13C NMR spectra of a sulphamoyl derivative of DL‐camphor have been examined. The unequivocal assignment of the position of substitution has been achieved by the attached proton test, 2D correlation spectroscopy and NOE difference spectroscop

ISSN:0749-1581    

DOI:10.1002/mrc.1260260105

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

Abstract17O NMR spectroscopic data at natural abundance in acetonitrile at 75°C were obtained for sixteenN‐arylacetamides, threeN‐methyl‐N‐arylacetamides and seven aryl acetates. The17O chemical shifts of theN‐arylacetamides are significantly influenced by both electronic and steric factors. A Hammett plot of the data for 4‐substitutedN‐arylacetamides gives a slope of 22 ppm.Orthosubstituents result in significant shielding effects, in contrast to previously reported results for conjugated ketnoes. A quantitative relationship between17O chemical shift data and torsion angle was found for the sterically hindered arylacetamides. Electronic effects on the aryl acetate17O chemical shifts are, in comparison, greatly reduced. Steric factors for aryl acetates show even smaller effects on the17O c

ISSN:0749-1581    

DOI:10.1002/mrc.1260260106

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractSeveral recent papers have dealt with the modulation of protonated carbon response intensities in long‐range heteronuclear 2D NMR chemical shift correlation spectra. Expressions describing the one‐bond modulation of the response intensity of the C‐3 methylene resonance of 6,7‐dimethoxy‐3,4‐dihydroisoquinoline, and the decoupling of this modulation afforded by a BIRD pulse located midway through the Δ2refocusing delay, are presented with experimental confirmation. Long‐range response intensities are related to the conventional heteronuclear chemical shift correlation spectrum optimized for transfer from the directly a

ISSN:0749-1581    

DOI:10.1002/mrc.1260260107

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractModified long‐range heteronuclear chemical shift correlation pulse sequences which incorporate both BIRD pulses midway through the Δ2magnetization transfer delay and low‐passJfilters are described. The BIRD pulse provides the means of decoupling one‐bond modulations of response intensity when magnetization is transferred to protonated carbons. The one‐ and three‐step low‐passJfilters utilized eliminate responses due to directly attached protons over a range of one‐bond coupling constants.Results obtained by long‐range optimization of the conventional heteronuclear chemical shift correlation pulse sequence, the previously reported pulse sequence containing only the BIRD pulse midway through Δ2and the two new pulse sequences described in this work are compared using 2,3‐dihydrofuran as a model compound.The low‐passJfilters have no adverse effects on the ability of the BIRD pulse to decouple one‐bond modulations. Modulation decoupling in the modified pulse sequences was confirmed using the3J(C‐3, H‐1) coupling pathway of the simple alkaloid norharmane. The response intensities, however, were up to 10% lower with a one‐step low‐passJfilter when compared with the simpler pulse sequence containing only t

ISSN:0749-1581    

DOI:10.1002/mrc.1260260108

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of eleven hydroxy and oxo derivatives of methyl esters of 1‐ and 4‐diamantane carboxylic acids, three hydroxy derivatives of diamantanone and six dibromo derivatives of diamantane have been measured in CDCl3or C6D6solutions. The signals were assiged by the additivity rule. The13C NMR spectra of all compounds were described by two parameters, e.g. the sum of the13C chemical shifts of all carbon atoms in the measured compounds (SCCS) and the ratio of the number of carbon atoms (Nc) and theoretical number of signals (

ISSN:0749-1581    

DOI:10.1002/mrc.1260260109

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of optically active primary and secondary 2‐substituted cyclohexanamines were measured and the chemical shifts were unequivocally assigned. Similar studies were made with four diastereomeric 2‐methyl‐N‐(1‐phenylethyl)cyclohexanamines with three chiralit

ISSN:0749-1581    

DOI:10.1002/mrc.1260260110

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe proton‐coupled13C NMR and1H NMR spectra of azidobenzene, tetrazolo[1,5‐a]pyridine, 2‐azidopyrimidine and tetrazolo[1,5‐a]pyrimidine in different solvents have been analysed, and criteria for using the NMR spectral parameters (especially the13C‐1H coupling constants) for tautomeric structure determination have been elaborated. The efficiency of these criteria has been demonstrated in studies of azide‐‐tetrazole equilibria in 2,4‐ and 4,6‐diazidopyrimidines. The ditetrazolo tautomeric form of 2,4‐diazidopyrimidine has been foun

ISSN:0749-1581    

DOI:10.1002/mrc.1260260111

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY