Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260260101pmid: N/A

Dowd, Diane; Gettins, Peter

doi: 10.1002/mrc.1260260102pmid: N/A

To test the feasibility of using 77Se 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 their 77Se 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.

Sierzputowska‐Gracz, Hanna; Agris, Paul F.; Katze, Jon R.

doi: 10.1002/mrc.1260260103pmid: N/A

The 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‐dimensional 13C and 1H 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 to that of the purine ring structure.

Tsoungas, Petros G.; De Costa, Bruce F.

doi: 10.1002/mrc.1260260104pmid: N/A

13C and 15N 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 the N‐oxides, o‐hydroxyaryl ketoximes, are reported. In the N‐oxides the “push‐pull” effect of the NO dipole is largely localized on the isoxazole unit. A large (ca 39 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 ppm) shielding of the 15N nucleus.

Stanley, P. D.; Cremlyn, R. J.; Nangle, J.; Swinbourne, F. J.

doi: 10.1002/mrc.1260260105pmid: N/A

The 1H and 13C 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 spectroscopy.

Boykin, David W.; Deadwyler, Gina H.; Baumstark, A. L.

doi: 10.1002/mrc.1260260106pmid: N/A

17O NMR spectroscopic data at natural abundance in acetonitrile at 75°C were obtained for sixteen N‐arylacetamides, three N‐methyl‐N‐arylacetamides and seven aryl acetates. The 17O chemical shifts of the N‐arylacetamides are significantly influenced by both electronic and steric factors. A Hammett plot of the data for 4‐substituted N‐arylacetamides gives a slope of 22 ppm. Ortho substituents result in significant shielding effects, in contrast to previously reported results for conjugated ketnoes. A quantitative relationship between 17O chemical shift data and torsion angle was found for the sterically hindered arylacetamides. Electronic effects on the aryl acetate 17O chemical shifts are, in comparison, greatly reduced. Steric factors for aryl acetates show even smaller effects on the 17O chemical shifts.

Salazar, Miguel; Zektzer, Andrew S.; Martin, Gary E.

doi: 10.1002/mrc.1260260107pmid: N/A

Several 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 Δ2 refocusing 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 attached proton.

Salazar, Miguel; Zektzer, Andrew S.; Martin, Gary E.

doi: 10.1002/mrc.1260260108pmid: N/A

Modified long‐range heteronuclear chemical shift correlation pulse sequences which incorporate both BIRD pulses midway through the Δ2 magnetization transfer delay and low‐pass J filters 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‐pass J filters 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 Δ2 and the two new pulse sequences described in this work are compared using 2,3‐dihydrofuran as a model compound. The low‐pass J filters 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 the 3J(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‐pass J filter when compared with the simpler pulse sequence containing only the BIRD pulse midway through Δ2.

Horská, Alena; Podehradská, Jaroslava; Vodička, Luděk; Hájek, Milan; Trška, Petr

doi: 10.1002/mrc.1260260109pmid: N/A

The 13C 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 CDCl3 or C6D6 solutions. The signals were assiged by the additivity rule. The 13C NMR spectra of all compounds were described by two parameters, e.g. the sum of the 13C 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 (Ns).

Knupp, Gerd; Frahm, August W.

doi: 10.1002/mrc.1260260110pmid: N/A

The 13C 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 chirality centres.