Characterization of polyion counterion interactions in cartilage by 23Na NMR relaxation

Nuclear quadrupole relaxation is a sensitive measure of electrolyte environments. We used the relaxation of 23Na to probe mobile ion-matrix interactions and the electrostatic structure of the polyelectrolyte extracellular matrix of cartilage. Specifically, we measured spin-lattice and spin-spin relaxation times of 23Na in bovine nasal cartilage at 132 MHz under several conditions. Matrix fixed charge density was reduced by protonating anionic sites and by matrix digestion with trypsin and the relaxation times compared to controls. Under all conditions studied, measured longitudinal relaxation was monoexponential with values ranging from 16-32 msec. Transverse relaxation exhibited biexponential behavior in all cartilage samples with a fast component in the range of 2 to 5 ms and a slow component between 16 and 53 ms. Reduction in matrix fixed charge density in all cases led to a decrease in the relaxation rates. The results suggest a two-site model for Na+ ions in cartilage and a relaxation mechanism involving both polyion segmental motion and counterion diffusion. In the context of ion condensation theory, the implication of a two-site model is that the mean polyion-polyion spacing may be less than 0.7 nm. The mean polyion-counterion spacings were estimated by calculating correlation times and quadrupole coupling constants. These spacings were found to be 0.5-0.7 nm.