Proton NMR Study of the Trimannosyl Unit in a Pentaantennary N-Linked Decasaccharide Structure. Complete Assignment of the Proton Resonances and Conformational Characterization

The chemical shifts of all the ring protons of the three Man residues in a pentaantennary glycan chain have been unambiguously assigned by two-dimensional proton nuclear magnetic resonance (1H-NMR) spectroscopic methods. The study, using chemical shift and J values on the conformation of the trimannosyl unit, revealed that the rotamer about the C5-C6 bond of the α16 linkage in the sequence of Manα16Manβ1 is predominantly confined to a gauche-gauche rotamer (ω= 180°, ω= O6-C6-C5-H5) and not to a gauche-trans rotamer (ω=–60°). We do not know of any previous demonstration that the dihedral angle ω (O6-C6-C5-H5) in Manα16Manβ1 is preferentially 180° in complex-type N-linked glycans having no bisecting GlcNAc residue.