Glycosidic Linkage Conformations and Rigidity

A selective 2D J-8 INEPT experiment [129-131] was used for measuring the three-bond, long-range proton-carbon coupling constants, 3JCH [132]. This approach allowed unequivocal detection of doublets for each carbon three-bond, long-range coupling with the selected proton. The coupling constants of JH1"-Cr-O-C3' and JH1-Cl -O-C4 were readily determined through the polarization transfer from the well-isolated an-

omeric protons (Fig. 15). The measuring of the coupling constant /H3 -C3-O-Cl» required selective irradiation of H-3' of £-gal, which was otherwise overlapped with other protons. The 3JCH coupling constants and the corresponding interglycosidic torsional angles were calculated by using the equation [133] 3JCH = 5.7 cos2ft — 0.6 cos ft + 0.5. The results showed the crucial dihedral angles of a(1^3) linkage <f>/ft = —43°/ —40° and dihedral angle for £(1^4) $ + 38°. The literal interpretation would have the trisaccharide 37 assuming a U-shaped conformation in solution state.

In general, the rigidity of the glycosidic linkage can be deduced from the temperature dependence of the long-range 3JCH coupling constants [134]. The interglycosidic coupling constants 3JCH for a(1^3) linkage were measured at different temperatures by performing selective 2D J-8 INEPT experiments. The intraresidue 3JCH coupling constants for a-gal were also obtained in the same 2D experiments at different temperatures. It was found that the NMR-derived average dihedral angles were almost independent of the temperature changes, which suggests that the a(1^3) linkage is relatively rigid. Interresidue nuclear Overhauser effects (NOEs) were also measured to substantiate the structural information obtained from long-range coupling constant measurement (Fig. 16).

Figure 15 Selective 2D J-8 INEPT spectra and cross sections of a(1-3) linkage of trisaccharide 37.
Figure 16 Observed nuclear Overhauser effects (NOEs) in U-shaped trisaccharide 37.

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