Carbohydrate-carbohydrate interactions in water with glycophanes as model systems

The synthesis and conformational properties of glycophanes 2 and 3 (cyclodextrin-cyclophane hybrid receptors containing two maltose units linked by (4-hydroxymethyl) benzoic acid spacer) are described. The binding properties in water of these receptors with a series of 4-nitrophenyl glycosides with alpha- and beta-configurations at the anomeric center have been studied using H-1 NMR spectroscopy and molecular mechanics calculations. A comparison of these properties with those of glycophane 1 (an alpha,alpha-trehalose containing glycophane) and alpha-cyclodextrin (alpha CD) using the same glycosides shows the existence of a stabilizing contribution to the free energy of binding in the case of of glycophanes but not in the case of the alpha CD system. This contribution is due to carbohydrate - carbohydrate interactions between both host and guest lipophilic sugar surfaces. Glycophanes 1, 2, and 3 show similar alpha/beta selectivity on binding the ligands, despite the great flexibility of 3 related to 1 and 2. Parallels are drawn between the thermodynamic behavior of these model systems and that proposed for sugar-protein interactions.