Anomeric resonances

A second example [IJ) is that of the anomeric spectral region of dextran B-742 fraction S, a polysaccharide for which per-methylation data indicate Structure 2, when n=0. This is an unusual polymer, as every backbone residue is 3-0-substituted. It is fortunate that this polymer exists, as the dextrans branching through 3,6-di-O-substituted residues present a problem in the anomeric spectral region, displaying only a single branching anomeric resonance in addition to the linear dextran resonance. 

It was therefore postulated that one of the two expected minor resonances was unresolved from the linear dextran anomeric resonance. For dextran B-742 fraction S there are essentially no normal linear dextran 6-0-substituted a-D-glucopyranosyl residues, only branch-point and terminal residues -- and one of the two approximately equal resonances appears at the same chemical shift as the linear dextran anomeric resonance. 

Figure 5.1.4 shows examples of what kind of structural information can be obtained from such an LC-NMR-MS run. This figure shows the NMR chromatogram of a total asterosaponin fraction obtained from MSPD extraction. For clarity, the regions of the methyl and olefinic/anomeric resonances are shown. 

Finally, the anomeric resonance at 5.068 ppm showed a direct correlation to the C resonance at 108.66 ppm in the HMQC spectrum, indicating that galactose residue D is the /3-furanoside form (Beier et al. 1980). 

One-dimensional (ID) NMR investigations usually revealed genuine characteristics present in the glycan molecule, for example, methyl groups and carbonyl carbons from A-acetylghreosamine (GIcNAc) and fucose (Fuc) units. The detection of the downfield placed (typically between 5h 4.3 and 5.7 Sc 95 and 105) anomeric resonances and the measurement of their respective vicinal coupling... 

The DEPT spectrum (Figure 4.15) immediately confirms the seven-saccharide repeat, since there are seven anomeric resonances such a confirmation of the results of methylation analysis by NMR is important, because incomplete methylation can give the appearance of additional branch points. Moreover, the hydroxymethylene carbons at high field show inverted peaks, which identifies them as the C6 carbons of the various residues. 

The final step in the assignment of all resonances is correlating the C peaks with the proton peaks. This can often be done from chemical shift alone for the anomeric resonances, but not for other ring atoms. The most important pulse sequence here is HSQC (Heteronuclear Single Quantum Correlation), which involves six pulses to protons and four to C. The spectrum is now a non-symmetrical map with a peak at each carbon attached to a proton the projection on the C axis is the C DEPT spectrum and on the proton axis the ordinary proton spectrum (Figure 4.18). 

The C anomeric resonances in alginate are sufficiently dispersed that the frequency of the eight possible mannuronate and guluronate triads (MMM, MMG, MGM, GMM, GGM, GMG, MGG, GGG) can be analysed from the intensity of the signals from the central saccharide of the triad. A study of the... 

F. HMQC experiment performed on the derivatized saponin (256 experiments of 2K, reverse mode with decoupling in the carbon dimension experiment is not phased). The part of the map corresponding to the high field resonances (methyls) is shown on top. At the bottom is the part of the map corresponding to anomeric resonances (plus H,C-3 and -12 of the triterpene). 

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