Motional properties of E. Coli polysaccharide K5 in aqueous solution analyzed by NMR relaxation measurements

13C NMR relaxation measurements at three different magnetic field strengths have been used to analyse the motional properties of a low molecular weight K5 polysaccharide (ΔUA-[→ 4)-β-d-GlcNAc(1 → 4)-β-d-GlcA(1 → ]n-GlcNAcred) from E. coli. Two-dimensional double INEPT spectra with suppression of cross-correlation effects between dipolar and chemical shift anisotropy relaxation mechanisms were collected in order to determine carbon longitudinal and transverse relaxation times. The values of the overall correlation time and the rate of internal motions were obtained using the model free spectral densities. The data indicate that the overall motion of the molecule is non-isotropic and can be approximated with the symmetric top model with an axial ratio of ≈ 22. The magnitude of the generalized order parameter (S2 ≈ 0.8) and the internal motion correlation time (τe ≈ 30 ps) differ from those found for iduronic acid-containing glycosaminoglycans and suggest that the internal motions in K5 polysaccharide are more limited.