The O-antigen moiety of lipopolysaccharide is important for the function and virulence of many bacteria and has been recognized as a key target for protective immunity against several pathogens. In this study, model oligosaccharides were used to investigate the conformations of glycosidic linkages from structural entities present in the O-antigen repeating units of the well-known pathogens Shigella and Salmonella via a combination of experimental and computational techniques. Model compounds with intramolecular tethering and/or functional group modifications include chlorines that have been reported as high affinity oligosaccharide-based ligands were also included. Ranging from di- to tetrasaccharides, the compounds were subjected to one-microsecond molecular dynamics (MD) simulations with both the CHARMM36 Additive and Drude Polarizable state-of-the-art force fields. The intramolecular tether parameters were developed as part of the present study. In addition to yielding insights into the linkage conformational properties, the results allowed for validation of the force fields for sugars having the l absolute configuration present in most of the compounds. Analysis of the MD simulations included dihedral angle distributions of φ and ψ, potentials of mean force obtained from φ – ψ rotamer distributions, effective distances and ring puckering measurements allowing elucidation of structural information with the guidance of NMR chemical shift perturbation data. Three-bond spin–spin NMR coupling constants (³J) related to φ and ψ dihedral angles were calculated with Karplus-type relationships and showed notable agreement with ³J obtained from 1DLR, J-HMBC and IPAP selHSQMBC NMR experiments. The present structural insights of the studied oligosaccharides give a good understanding of the range of conformations adopted in the O-antigens and how tethering affects conformational properties of glycan macrocycles.

Kevin Dorst

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