Chemoenzymatic synthesis of sialic acid-containing carbohydrates

Sialic acids are a family of negatively charged monosaccharides with nine-carbon backbone. They have been found predominantly in nature as the outermost terminal units on glycans, glycoconjugates, or as components of surface polysaccharides of some pathogenic bacteria. More than 50 structurally distinct naturally occurring sialic acid forms have been identified. Sialic acid-containing carbohydrates (sialosides) are believed to play important roles in many biological processes including cell-cell recognition, communication, adhesion, migration, tumor metastasis, protein structure and function, as well as bacterial and viral infections. Study of the important roles of sialosides is hampered by the limited access to these complex carbohydrates. Therefore, homogenous sialosides are exciting synthetic targets for better understanding their biological significance.;Other than structures containing terminal sialic acids, oligosaccharides with internal sialic acid residue have been found as building blocks of the capsular polysaccharides and cell wall components of some pathogenic bacteria. These ionic polysaccharides are believed to play important roles in the virulence of these pathogenic organisms. Structurally defined polysaccharides containing internal sialic acid residues are extremely difficult to obtain in homogeneous forms either by isolation from natural sources or by chemical synthesis. We have developed a novel chemoenzymatic approach to obtain size-defined polysaccharides with internal sialic acid residues by sialyltransferase-catalyzed block transfer of oligosaccharides. In this approach, we combined the flexibility of chemical synthesis and high regio- and stereoselectivity of sialyltransferases to obtain size-defined polysaccharides containing internal sialic acid residues.;In addition, based on our experience with the synthesis of polysaccharides containing internal sialic acids, we have developed a highly efficient chemoenzymatic method for the preparation of a new class of macrocyclic oligosaccharides. This method involves chemical or chemoenzymatic synthesis of galactosides containing a propargyl group at the reducing end as sialyltransferase acceptors. Introducing an azido-containing sialic acid to the non-reducing end of the galactosides through a sialyltransferase-catalyzed enzymatic reaction followed by copper(I)-catalyzed Huisgen's 1,3-dipolar cycloaddition of alkyne and azide provides size-defined macrocyclic oligosaccharides. The produced negatively charged macrocyclic oligosaccharides have high solubility in water and interact with hydrophobic small molecules.