Synthetic study of bacterial glycan containing pseudaminic acid

Abstract

Pseudaminic acid is a nonulosonic acid unique to bacterial species, found as a component of important cell surface glycans and glycoproteins in various pathogenic species, such as the critical hospital threat Pseudomonas aeruginosa. Psuedaminic acid is deemed to be closely related to the pathogenicity of the bacteria due to the structural similarity of these bacterial nonulosonic acids to that of eukaryote sialic acids, while the real role of pseudaminic acid and the pathogenic mechanism is still mysterious. It was proposed that the similarity in structure may contribute to bacterial virulence by dampening the immune response to invading bacterium that utilize these important carbohydrates. Furthermore, the relative structural similarities these sugars have with the sialic acids may also enable a potential interaction with host sialic acid-specific lectins, which could explain their possible influence in adhesion and infection. Studies discovering the biological functions of pseudaminic acids are seriously hampered because of the difficulty in acquiring glycan samples in homogeneous form and sufficient amount. The lack of commercially available monosaccharide sources and structural complexity increase the difficulty in synthesizing glycans containing pseudaminic acid. Thus, a de novo synthetic strategy towards pseudaminic acid was developed. The synthesis starts with commercially available L-allo-threonine, contains two carbon-chain elongation steps constructing three chiral centers and gives stable pseudaminyl donor in gram-scale. The absolute configuration of chiral centers generated were confirmed via derivatization of the intermediates to the corresponding cyclic hexosamines. Also, conditions controlling stereoselectivity of new chiral centers were developed. Thus, the application of the synthesis could be expanded for constructing nonulosonic acids and the corresponding six-carbon precursor with different configurations. Based on the synthesis of pseudaminic acid, azido-modified hexosamine precursor of pseudaminic acid was synthesized and used in metabolic labeling of pseudaminic acid. Moreover, a novel nonulosonic acid found in Fusobacterium nucleutum with similar structure was also successfully synthesized based on the synthetic strategy of pseudaminic acid. Glycosylation of pseudaminic acid was also investigated. Pseudaminyl donors with different reactivity and various substitutions were successfully synthesized and tested in glycosylation. Luckily, a robust methodology for the stereocontrolled chemical glycosylation of pseudaminic acid to afford both α- and β- glycosides with complete stereoselectivity, using a common glycosyl donor under different conditions was established. This method now enables efficient access to pseudaminylated molecules which should underpin chemical glycobiology research and carbohydrate-based antibacterial vaccine development on the pseudaminic acid motif.