Sugar coupling constants

Fig. 9. — (a) H-N.m.r. Spectrum (270 MHz, Resolution-enhanced) of Beef-lung Heparin in D20 (40%, w/v) at 90° (b) Computer-simulated Spectrum, with Calculated, Interproton Coupling-Constants (/, at 35°) for the Amino Sugar (A) and L-Iduronic acid (I) Residues.84... 

The employment of NMR-active isotopes permits to access experimental parameters which are intrinsically difficult to measure, unless a significant concentration of the sugar is present in the NMR tube. For instance, aqueous solutions of N-acetyIncuraminic acid, labeled with 13C at Cl, C2, and/or C3, were analyzed to detect and quantify the various chemical species present in equilibrium at different pHs. In fact, in addition to the expected a and (3 pyranose forms, acyclic keto, keto hydrate and enol forms were identified on the basis of 13C NMR spectroscopic data. Besides, DFT methods were employed to predict the effect of enol and hydrate structure on the coupling constant values Jc,u and /c c involving C2 and C3, finding that 2/c2,h3 can be safely used to differentiate the cis and tram isomers of the enol forms.9... 

A new NMR method for the determination of the anomeric configuration in mono- and disaccharides has been described.18 The protocol is based on the different cross-correlated relaxation between proton chemical shift anisotropy (CSA) and dipolar relaxation for the a and (3 anomers of sugars. Only the ot-anomers show the presence of CSA (HI or Hl )-proton dipole (H1-H2 or Hl -H2 ) in the longitudinal relaxation of the anomeric protons. The method is of special interest for cases in which vicinal coupling constants between HI and H2 in both anomers a and (3 are similar and small, such as D-mannose, and the non-ambiguous description of the anomeric configuration needs additional measurements. 

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

Pedersen, A.T. et al., Anomeric sugar configuration of anthocyanin 0-pyranosides determined from heteronuclear one-bond coupling constants, Phytochem. Anal, 6, 313, 1995. 

The major uses of 13C-NMR spectra of derivatives of compounds 4 and 5 have been for assignment of site of attachment of sugar residues (N-l, N-2, or N-3). A detailed discussion of the spectra of some [l,2,3]triazolo[4,5-b]pyridines, including principal coupling constants, has been provided by Townsend et al.222 Less detailed reports are available for 2,5-disubstituted [l,2,3]triazolo[4,5-b]pyridin-5-ones,184 for compound 5, for D-ribosyl derivatives (enabling assignment of site of ribosylation),204 for similar derivatives of compound 4,220 and for some [l,2,3]triazolo-[4,5-c]pyridin-4-ones.196... 

The -n.m.r. parameters for these P sugars are summarized in Table IV. In these detailed, spectral data, there are some interesting features with regard to the chemical shifts (S values) and coupling constants (J values) of the ring protons. The general trends of these values are summarized in Table V. 

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