Contribution of C-H…π Interactions to the Affinity and Specificity of Carbohydrate Binding Sites

X-ray crystallographic studies have shown that C-H groups of saccharides interact with aromatic amino acid residues in binding sites in proteins. Such C-H…pi interactions have been shown to be dominated by London's dispersion interactions. The strength of interaction depends on a number of factors: (i) the nature of aromatic residue and saccharide, (ii) the form (acyclic, pyranose or furanose) and conformation of the saccharide, (iii) the functional groups present in modified saccharides (e.g., N-acetyl group), and (iv) the mutual position-orientation of the interacting moieties; this, in turn, determines the number of interacting C-H groups and the geometry of interaction. The strength of interaction also depends on the surrounding medium and on the microenvironment in a protein's binding sites. A variety of experimental techniques such as isothermal titration calorimetry, turbidity measurements, NMR spectroscopy, infrared ion depletion spectroscopy and fluorescence spectroscopy have been used to investigate C-H…pi interactions. Quantum chemical calculations of saccharide - aromatic systems have shown that their interaction is stabilizing. The interaction energy ranges between 1 and 12 kcal/mol and is thus comparable in strength to a conventional hydrogen bond. However, experiments that mimic double mutant cycles are needed to be designed, and performed, to determine the contribution of C-H…pi interactions to the affinity of glycans to proteins and the glycan specificity of proteins.