Structural and Conformational Analysis of Bacterial Polysaccharides using NMR Spectroscopy

Sammanfattning: Carbohydrates is one of the three classes of biomolecules found in nature. It is the most common one in comparison to the other two classes, lipids and proteins. However, this simple categorization does not reflect the reality since carbohydrates often are covalently linked to e.g., proteins, so-called glycoproteins where, for example, N-glycans are used as markers of quality control during the process of protein folding. Another example is lipopolysaccharides, which cover the cell surfaces of gram-negative bacteria and which contain both a lipid moiety (Lipid A) and a carbohydrate chain. The outer part of the carbohydrate chain is a polysaccharide, also called O-antigen, as it interacts with the immune system of the host. The polysaccharide has, like a polymer, a repeating unit consisting of 2-7 monosaccharides. The repeating unit varies between different bacteria. Determining the structure of these polysaccharides is important in order to be able to categorize the various strains that exist, but also to be able to develop future glycoconjugate vaccines. This is important as the WHO estimates that antibiotic resistance is expected to be more lethal than cancer by 2050, and therefore a vaccine is needed to slow down this development.Nuclear Magnetic Resonance Spectroscopy (NMR) is a useful analytical tool to analyze these carbohydrates at the atomic level in order to determine their structures.The first part (Paper I-III) of this thesis will summarize the structural determination of three Escherichia coli serogroups with hitherto unknown lipopolysaccharides.The second part (Paper IV) will discuss the structure determination, using NMR spectroscopy, for various mono-, di-, and tri-saccharides that have recently been implemented in the structure-determination program, CASPER. The chapter will also present examples of predictions of complex carbohydrates that CASPER can perform.The third part (Paper V) of the thesis will investigate conformational aspects of the polysaccharides from Shigella flexneri serotypes 7a and 7b using NMR spectroscopy and molecular dynamics simulations.