Dynamics in the cyclic Enterobacterial common antigen as studied by 13C NMR relaxation.

The motional properties of the cyclic enterobacterial common antigen (cECA), consisting of four trisaccharide repeat units, have been investigated by carbon-13 spin relaxation. R(1), R(2) and NOE relaxation parameters have been determined at three magnetic field strengths. The data were interpreted within the model-free framework to include the possibility of motional anisotropy, and overall as well as local dynamical parameters were fitted separately for each ring carbon. The motional anisotropy was addressed by assuming an axially symmetric diffusion tensor, which was fitted from the overall correlation times for each site in the sugar residues using the previously determined crystal structure. The data were found to be in agreement with an oblate shape of the molecule, and the values for D(iso) and D(||)/D(perpendicular sign) were in good agreement with translational diffusion data and an estimate based on calculation of the moment of inertia tensor, respectively. The local dynamics in cECA were found to be residue-dependent. Somewhat lower values for the order parameters, as well as longer local correlation times, were observed for the beta-linked ManNAcA residue compared to the two alpha-linked residues in the trisaccharide repeat unit.