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 α1→6 linkage in the sequence of Manα1→6Manβ1→ 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 Manα1→6Manβ1→ is preferentially 180° in complex‐type N‐linked glycans having no bisecting GlcNAc residue.