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.

大肠杆菌O5血清群包含两个亚型（O5ab与O5ac），二者无法通过标准免疫血清分型加以区分。这两类菌株的O抗原多糖（O-antigen polysaccharides，PS）结构相似性极高，二者唯一的差异在于连接生物学四糖重复单元的糖苷键。本研究结合核磁共振波谱法（NMR spectroscopy）与分子建模方法，阐明了这两种多糖的构象偏好。本次核磁共振波谱分析基于NOE增长曲线，对残基内与残基间的质子-质子距离进行解析。研究人员结合所采用的力场与遗传算法评估的构象柔性，构建了重复单元及其延伸形成多糖的分子模型。仅当考虑分子模型中观测到的多种低能构象的平均效应时，实验测得的距离与计算得到的距离方可实现良好吻合。