~(13)C-~1H and ~(13)C-~(13)C scalar couplings in isotopically-labeled mono-, di- and trisaccharides: NMR and DFT studies of O-glycoside and hydroxymethyl group conformation

摘要

In contrast to proteins and nucleic acids, determinations of the three-dimensional structures of complex oligosaccharides by NMR are complicated by limited signal dispersion, the presence of strong coupling (second-order effects), and the lack of a single mode of monomer coupling. Oligosaccharides can be assembled in vivo in various ways (i.e., different linkage types) using a range of monomers (e.g., Glc, Gal, Xyl, GlcNAc, GalNAc, Fuc, NANA). Their constituent O-glycoside linkages are composed of two or three torsion angles (φ, ψ and ω) that display different degrees of flexibility which are likely to be context dependent. In addition, the number of available NMR constraints is often less than required to define their conformational and dynamic properties well (i.e., the problem is highly under-determined).