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 (H-1-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 alpha 1-->6 linkage in the sequence of Man alpha 1-->6Man beta 1--> is predominantly confined to a gauche-gauche rotamer (omega = 180 degrees, omega = O6-C6-C5-H5) and not to a gauche-trans rotamer (omega = -60 degrees). We do not know of any previous demonstration that the dihedral angle omega (O6-C6-C5-H5) in Man alpha 1-->6Man beta 1--> is preferentially 180 degrees in complex-type N-linked glycans having no bisecting GlcNAc residue.