Dynamics of the Escherichia coli O91 O-Antigen Polysaccharide in Solution as Studied by Carbon-13 NMR Relaxation

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Abstract
The dynamics of the O-antigen part of the lipopolysaccharide from the enterohemorrhagic Escherichia coli O91 has been determined in solution using 13C NMR relaxation measurements at two magnetic field strengths, 9.4 and 14.1 T, thereby facilitating the testing of several dynamical models. The biological repeating unit, consisting of five sugar residues and substituents, could be determined by spectral analysis of different 1H,13C correlations and corroborated by the relaxation data. The site specifically 13C-labeled material was shown to have ∼10 repeating units with a narrow distribution. A model-free analysis of the relaxation data revealed a complex dynamical behavior where the sugar residues could be described by a global correlation time (τm = 5.4 ns), generalized order parameters (S2 ≈ 0.63), and different correlation times for internal motions related to their position in the repeating unit along the polymer (τe ≈ 360−520 ps). One of the sugar residues showed, in addition, a chemical exchange contribution. Furthermore, a substituent on another sugar residue was described by two order parameters (Sf2 = 0.51 and Ss2 = 0.21). The solution dynamics of the polysaccharide are thus described by highly intricate motions, both in amplitude and time scales. These results are of significance in the general description of polysaccharides surrounding bacterial cell surfaces and in the presentation of antigenic epitopes to the immune system of an invaded host.