An NMR study of the dynamic single-stranded conformation of sodium pectate

The conformation of aqueous sodium pectate has been probed by fitting multi-field experimental 13C NMR relaxation data with various expressions for the spectral densities. The model-free, DLM, and diffusion-in-a-cone spectral densities all reproduced the experimental data in a reasonable manner. The validity of the resulting motional models was evaluated by comparison to data obtained from hydrodynamic theory and molecular modeling. The diffusion-in-a-cone spectral densities afforded both the best fit to experimental data and the most precise description of secondary structure. The optimum motional model could be described as anisotropic reorientation of two-fold helical segments containing 29 residues. The corresponding average axial length of the helical segments of 13 nm is in excellent agreement with the persistence length from small-angle neutron scattering and molecular mechanics. The transverse or hydrodynamic radius of the helical segments was 0.8 nm as compared to a covalent radius of 0.4 – 0.45 nm. The increase in hydrodynamic radius with respect to the covalent radius indicated significant counterion condensation in keeping with polyelectrolyte theory.