The recognition of saccharides by proteins has far reaching implications in biology, technology, and drug design. Within the past two decades, researchers have directed considerable effort toward a detailed understanding of these processes. Early crystallographic studies revealed, not surprisingly, that hydrogen-bonding interactions are usually involved in carbohydrate recognition. But less expectedly, researchers observed that despite the highly hydrophilic character of most sugars, aromatic rings of the receptor often play an important role in carbohydrate recognition. With further research, scientists now accept that noncovalent interactions mediated by aromatic rings are pivotal to sugar binding. For example, aromatic residues often stack against the faces of sugar pyranose rings in complexes between proteins and carbohydrates. Such contacts typically involve two or three CH groups of the pyranoses and the π electron density of the aromatic ring (called CH/π bonds), and these interactions can exhibit a variety of geometries, with either parallel or nonparallel arrangements of the aromatic and sugar units. In this Account, we provide an overview of the structural and thermodynamic features of protein-carbohydrate interactions, theoretical and experimental efforts to understand stacking in these complexes, and the implications of this understanding for chemical biology. The interaction energy between different aromatic rings and simple monosaccharides based on quantum mechanical calculations in the gas phase ranges from 3 to 6 kcal/mol range. Experimental values measured in water are somewhat smaller, approximately 1.5 kcal/mol for each interaction between a monosaccharide and an aromatic ring. This difference illustrates the dependence of these intermolecular interactions on their context and shows that this stacking can be modulated by entropic and solvent effects. Despite their relatively modest influence on the stability of carbohydrate/protein complexes, the aromatic platforms play a major role in determining the specificity of the molecular recognition process. The recognition of carbohydrate/aromatic interactions has prompted further analysis of the properties that influence them. Using a variety of experimental and theoretical methods, researchers have worked to quantify carbohydrate/aromatic stacking and identify the features that stabilize these complexes. Researchers have used site-directed mutagenesis, organic synthesis, or both to incorporate modifications in the receptor or ligand and then quantitatively analyzed the structural and thermodynamic features of these interactions. Researchers have also synthesized and characterized artificial receptors and simple model systems, employing a reductionistic chemistry-based strategy. Finally, using quantum mechanics calculations, researchers have examined the magnitude of each property's contribution to the interaction energy.

Different behavior has been observed for the psi torsion angle of the glycosidic linkages of D-GalNAc-Ser and D-GalNAc-Thr motifs, allowing the carbohydrate moiety to adopt a completely different orientation. In addition, the fact that the water pockets found in alpha-D-GalNAc-Thr differ from those obtained for its serine analogue could be related to the different capability that the two model glycopeptides have to structure the surrounding water. This fact could have important biological inferences (i.e., antifreeze activity).

The first detailed structural model for the hevein-chitin complex is presented on the basis of the analysis of NMR data. The resulting model, in combination with ITC and analytical ultracentrifugation data, conclusively shows that recognition of chitin by hevein domains is a dynamic process, which is not exclusively restricted to the binding of the nonreducing end of the polymer as previously thought. This allows chitin to bind with high affinity to a variable number of protein molecules, depending on the polysaccharide chain length. The biological process is multivalent.