Application of NMR based binding assays to identify key hydroxy groups for intermolecular recognition

In general, few functional groups are sufficient for the binding of ligands to receptor proteins. For carbohydrates, characteristically spaced hydroxy groups often determine the binding affinity. Key functional groups may be identified by testing derivatives that have the potential sites blocked, which is most effectively achieved by simultaneous testing of all these derivatives. We employed such a parallel approach to identify the binding specificity of Sambucus nigra agglutinin (SNA), a lectin from elderberry. Compound libraries of randomly methylated O-methyl glycopyranosides were screened for binding activity toward SNA using trNOESY experiments, as well as 1D- and 2D-STD NMR experiments. The individual compounds have identical molecular weights and display rather similar physicochemical properties. Nevertheless, fast and reliable identification of the active compounds directly from the mixture was possible. The results are consistent with earlier biochemical work, showing that SNA has a binding specificity for D-galactose with the hydroxy functions at carbon atoms C3 and C4 of methyl beta-D-galactopyranoside being critically important for binding. Finally, we show that the screening protocol can be extended to heteronuclear STD pulse sequences if C-13-labeled derivatives are present in the library.