Conformational Preferences of the O‑Antigen Polysaccharides of Escherichia coli O5ac and O5ab Using NMR Spectroscopy and Molecular Modeling

Escherichia coli serogroup O5 comprises
two different subgroups (O5ab and O5ac), which are indiscernible from
the point of view of standard immunological serotyping. The structural
similarities between the O-antigen polysaccharides (PSs) of these
two strains are remarkable, with the only difference being the glycosidic
linkage connecting the biological tetrasaccharide repeating units.
In the present study, a combination of NMR spectroscopy and molecular
modeling methods were used to elucidate the conformational preferences
of these two PSs. The NMR study was based on the analysis of intra-
and inter-residue proton–proton distances using NOE build-up
curves. Molecular models of the repeating units and their extension
to polysaccharides were obtained, taking into account the conformational
flexibility as assessed by the force field applied and a genetic algorithm.
The agreements between experimentally measured and calculated distances
could only be obtained by considering an averaging of several low
energy conformations observed in the molecular models.