Research Projects

Bacterial cell-surface is decorated with unusual sugars which are virtually absent in human hosts. Due to this structural difference, the rare sugars-containing bacterial oligosaccharides are looked upon as valuable epitopes to construct carbohydrate-based vaccines. However, due to inherently low antigenicity of carbohydrates, they need to be conjugated to a carrier protein for T-cell recognition in order to elicit a long-lasting immune response. Although this approach has been successfully used over the years to construct glycoconjugate vaccines, one of the limitations is that antibodies are also produced in response to the carrier proteins, which is not desired. On the other hand, zwitterionic polysaccharides (ZPS) have been shown to directly bind to T-cells without the need to be tagged to carrier proteins. This unique property of ZPS opens up a possibility of constructing entirely carbohydrate-based vaccines. Although discovered three decades ago, not much progress has been achieved in this direction, mainly due to lack of access to pure and well defined zwitterionic oligosaccharide fragments. This is attributed to the difficulties in their isolation from natural sources as well as their challenging synthesis. This project focuses on the total synthesis of ZPS repeating units from Fusobacterium nucleatum, Bacteroides fragilis, Clostridium symbiosum, and Staphylococcus aureus. Native ZPSs, such as those found in gut microbiota Bacteroides fragilis are produced as complex, high-molecular-weight polysaccharides with heterogeneous lengths, structural microheterogeneity, and variable chain modifications, making it virtually impossible to isolate well-defined, functional oligosaccharide fragments suitable for detailed structure–function studies. To address this limitation, the project aims to synthesize structurally well-defined ZPS repeating units (RUs) 1-4. The fully protected counterparts with orthogonal protecting groups would enable chain elongation to access higher oligosaccharides. Total synthesis of all the zwitterionic RUs featuring rare sugars such as bacillosamine, fucosamine, including AAT (4-amino-2-acetamido-2,4,6-trideoxygalactose) and uronic acid derivatives, involve synthetic challenges such as, procurement of rare sugar building blocks and their stereoselective 1,2-cis glycosylations, late-stage amino acid conjugation and oxidation, as well as installation of phosphate ester and non-carbohydrate linkers. The chemically synthesized glycans will be used for collaborative studies to probe the interactions of bacterial glycans with host receptors through 2D and STD NMR studies with lectins and their molecular dynamics. These synthetic targets will enable systematic exploration of how specific molecular features, particularly zwitterionic charge motifs, contribute to the secondary structure, and immune receptor recognition. The studies would bridge the gaps and bring about a paradigm shift from conventional glycoconjugate vaccine to purely carbohydrate vaccines.