A novel metal-assisted assembly of mulitivalent carbohydrate ligands is described. A bipyridine-modified N-acetylgalactosamine (bipy-GalNAc) undergoes Fe(II)-induced self-association to form a trimeric GalNAc ligand (FeII(bipy-GalNAc)3). The synthetic GalNAc cluster strongly binds to Vicia villosa B4 lectin and Glycin Max lectin, which recognizes multiple GalNAc residues. The trimeric GalNAc ligand is formed as a mixture of four diastereomeric isomers: Δ-fac, Λ-fac, Δ-mer, and Λ-mer. These stereoisomers are in a dynamic equilibrium at room temperature. The equilibrium allows the spatial arrangement of the three GalNAc residues to change in order to fit into a multivalent carbohydrate binding site of the lectins. Detailed analysis of the kinetic and thermodynamic data for the isomerization can provide structural information of the carbohydrate binding site of the lectins.
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Sakai et al. (1999) studied this question.