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 C-13 NMR relaxation data with various express ions for the spectral densities. The model-free, DLM, and diffusion-in -a-cone spectral densities all reproduced the experimental data in a r easonable manner. The validity of the resulting motional models was ev aluated by comparison to data obtained from hydrodynamic theory and mo lecular modeling. The diffusion-in-a-cone spectral densities afforded both the best fit to experimental data and the most precise descriptio n of secondary structure. The optimum motional model could be describe d as anisotropic reorientation of two-fold helical segments containing 29 residues. The corresponding average axial length of the helical se gments of 13 nm is in excellent agreement with the persistence length from small-angle neutron scattering and molecular mechanics. The trans verse or hydrodynamic radius of the helical segments was 0.8 nm as com pared to a covalent radius of 0.4-0.45 nm. The increase in hydrodynami c radius with respect to the covalent radius indicated significant cou nterion condensation in keeping with polyelectrolyte theory. (C) 1997 Elsevier Science Ltd.