The binding of 0-methyl and fluorodeoxy derivatives of methyl P-lactoside to the Ricinus communis toxin (RCA60) and agglutinin (RCA120) was studied in order to determine the donor/acceptor relationships of the hydrogen bonds between the hydroxyl groups of methyl &lactoside and the binding sites of the lectins. Free energy contributions of the hydrogen bonds at each position have been estimated from these data and from those previously reported for the monodeoxy derivatives Analysis of the results indicates that both the C-3' and C-4' hydroxyl groups act as hydrogen-bond donors to charged groups of both RCA60 and RCA120. The C-6' and probably also the C-2' hydroxyl groups participate both as donors and as acceptors of two hydrogen bonds with neutral groups of the lectins. And finally, the C-6 hydroxyl group possibly acts as a donor of a weak hydrogen bond to a neutral group in RCA60, but not in RCA120. The results provide a molecular basis to explain some features of the binding specificity of the lectins. Comparison of RCAGO binding data with the recently refined X-ray crystal structure of the RCA60-lactose complex shows similarities but also some discrepancies that can be attributed to the marked influence of the pH on the carbohydrate -lectin interaction.Carbohydrate-protein interactions have attracted considerable attention in view of their central role in a large number of key events of cell biology. In consequence, there is a growing interest in the understanding of the molecular basis for specificity and affinity in these interactions. X-ray crystallographic studies of several carbohydrate-protein complexes have shown that hydrogen bonds together with van der Waals contacts and stacking interactions are the dominant forces that stabilize the complexes [l -31. Because hydrogen bonds are highly directional, they ensure the correct fit of the ligand and confer stereospecificity to the binding. They also provide a stable solvation shell for the bound sugars. In addition, hydrogen bond strength is high enough to stabilize the complexes but low enough to allow rapid ligand dissociation. All these features make hydrogen bonding the primary interaction in protein-carbohydrate complex formation.Binding studies with selected deoxy, 0-methyl and fluorodeoxy sugar derivatives have been used to probe the involvement of hydrogen bonding in carbohydrate -protein interactions [4-111. The use of fluorodeoxy derivatives is particularly interesting since the electronegative fluorine cannot act as a hydrogen-bond donor but it can be a hydrogenbond acceptor, albeit weakly [6,12]. In addition, the replace- ment of a hydroxyl group by fluorine is more sterically conservative than substitution by hydrogen or a methoxy group. On the other hand, binding studies with mono-0-methyl derivatives provides information on the flexibility of the combining site to steric demands for complexation [lo]. Analysis of the bindmg data of such as derivatives compared with available refined X-ray crystal structure data on the carbohydrate-protein ...
