Application of 2D and 3D NMR experiments to the conformational study of a diantennary oligosaccharide.

By the application of homonuclear 3D NOE-HOHAHA and heteronuclear 3D HMQC-NOE experiments in studies of complex oligosaccharides, NOEs can be investigated which are hidden in conventional 2D NOE spectra. In the 3D NOE-HOHAHA spectrum omega 3 cross sections were considered to be the most suitable for assignment of NOEs. Alternatively, these cross sections could be measured separately in selective 2D HOHAHA-NOE spectroscopy. The advantages and limitations of the 2D alternative are compared with those of the 3D NOE-HOHAHA approach. In 3D HMQC-NOE spectroscopy the larger chemical shift displacement of the carbon spectrum with respect to the proton spectrum can be used to unmask NOEs hidden in the bulk region. If the extra proton dimension is not needed, 2D HMQC-NOE is a good alternative. The suitability of 2D and 3D NOE-HOHAHA and HMQC-NOE experiments for the estimation of proton-proton distances is demonstrated by comparing the results of these experiments on a diantennary asparagine-linked oligosaccharide with those of a conventional 2D NOE experiment. NOEs identified in the 2D and 3D NOE-HOHAHA as well as HMQC-NOE experiments, so far not identified or not quantified in 2D NOE experiments, are discussed in relation to each glycosidic linkage. The flexibility of the Man alpha (1-3)Man linkage is demonstrated, confirming the existence of an ensemble of conformations for this linkage.