Solution conformation of the branch points of N-linked glycans: synthetic model compounds for tri'-antennary and tetra-antennary glycans

Abstract

The solution conformation of model compounds for the tri''-antennary and tetraantennary (six-arm) branch point of N-linked glycans has been determined through the use of chemical shift, relaxation, and nuclear Overhauser enhancement data. The object was to establish the conformation about the glycosidic linkages in the N-linked substructure GlcNAc(.beta.1,6) [GlcNAc(.beta.1,2)]Man(.alpha.)- by estimation of values for the appropriate glycosidic torsional angles. The GlcNAc(.beta.1,6) linkage in a trisaccharide model compound was found to be constrained to a narrow rotameric subpopulation about the substituted Man C5-C6 bond (.omega.=-60.degree.) and a narrow range of possible .vphi.-.psi. values. Free rotation about the Man C5-C6 bond was obstructed by unfavorable steric interactions between the GlcNAc(.beta.1,6) and GlcNAc(.beta.1,2) residues. A .omega., .psi. value of 55.degree., 190.degree. was found to be consistent with the NMR data for the GlcNAc(.beta.1,6) linkage. However, the value of .psi. appears to be "virtual" in that the molecule is in equilibrium between two different values (90.degree. and 252.degree.). For the GlcNAc(.beta.1,2) linkage, complete agreement between all the observed NMR parameters and all the calculated ensemble average values could only be obtained with a set of potential energy functions which included hydrogen bonding. Other choices of potentials yielded calculated values that disagreed with a least two of the observed quantities. As a result, we infer that an interresidue hydrogen bond is formed, and we find it to be between the GlcNAc(.beta.1,2) ring oxygen and the Man C3 hydroxyl. In this conformation the values of .vphi. and .psi. are restricted to values in the vicinity of 40.degree. and -5.degree., respectively. This conformation differs from that reported for the same linkage in glycopeptides.