Stabilization of Branched Oligosaccharides: Lewis^x Benefits from a Nonconventional C–H···O Hydrogen Bond

Although animal lectins usually show a high degree of specificity for glycan structures, their single-site binding affinities are typically weak, a drawback which is often compensated in biological systems by an oligovalent presentation of carbohydrate epitopes. For the design of monovalent glycomimetics, structural information regarding solution and bound conformation of the carbohydrate lead represents a valuable starting point. In this paper, we focus on the conformation of the trisaccharide Le^x (Gal­[Fucα(1–3)]­β(1–4)­Glc<i>N</i>Ac). Mainly because of the unfavorable tumbling regime, the elucidation of the solution conformation of Le^x by NMR has only been partially successful so far. Le^x was therefore attached to a ¹³C,¹⁵N-labeled protein. ¹³C,¹⁵N-filtered NOESY NMR techniques at ultrahigh field allowed increasing the maximal NOE enhancement, resulting in a high number of distance restraints per glycosidic bond and, consequently, a well-defined structure. In addition to the known contributors to the conformational restriction of the Le^x structure (exoanomeric effect, steric compression induced by the <i>N</i>HAc group adjacent to the linking position of l-fucose, and the hydrophobic interaction of l-fucose with the β-face of d-galactose), a nonconventional C–H···O hydrogen bond between H–C(5) of l-fucose and O(5) of d-galactose was identified. According to quantum mechanical calculations, this C–H···O hydrogen bond is the most prominent factor in stabilization, contributing 40% of the total stabilization energy. We therefore propose that the nonconventional hydrogen bond contributing to a reduction of the conformational flexibility of the Le^x core represents a novel element of the glycocode. Its relevance to the stabilization of related branched oligosaccharides is currently being studied