Highly Effective Recognition of Carbohydrates by Phenanthroline-Based Receptors: alpha- versus beta-Anomer Binding Preference

H-1 NMR spectroscopic titrations in competitive and non-competitive media, as well as binding studies in two-phase systems, such as phase transfer of sugars from aqueous into organic solvents and dissolution of solid carbohydrates in apolar media revealed both highly effective recognition of neutral carbohydrates and interesting binding preferences of an acyclic phenanthroline-based receptor 1. Compared to the previously described acyclic receptors, compound 1. displays significantly higher binding affinities, the rare capability to extract sugars from water into non-polar organic solutions and alpha-versus beta-anomer binding preference in the recognition of glycosides, which differs from those observed for other receptor systems. X-ray crystallographic investigations revealed the presence of water molecules in the binding pocket of 1. that are engaged in the formation of hydrogen-bonding motifs similar to those suggested by molecular modelling for the sugar OH groups in the receptor-sugar complexes. The molecular modelling calculations, synthesis, crystal structure and binding properties of 1 are described and compared with those of the previously described receptors.