Selective Recognition of beta-Mannosides by Synthetic Tripodal Receptors: A 3D View of the Recognition Mode by NMR

Unravelling the structural features of carbohydrate recognition by receptors is a topic of major interest. In recent years, several synthetic receptors capable of binding different sugars with moderate to good affinities and selectivities have been developed. Here we report on the analysis of the three-dimensional structures of the complexes of two recently derived synthetic tripodal receptors with octyl beta-D-mannopyranoside, a monosaccharidic glycoside selectively recognized in a polar solvent, by a combination of NMR methods and assisting molecular mechanics calculations. The variations in the chemical shifts upon complexation and the observed intermolecular NOEs were employed to validate the molecular-mechanics-derived structures. The structures of the obtained complexes explain the observed mannose selectivity in chemical terms, suggesting that a combination of van der Waals, CH-pi and hydrogen-bonding forces are involved in the formation of the complexes, together with stabilizing conformational effects of the substituents.