Comparison of collagen dynamics in articular cartilage and isolated fibrils by solid-state NMR spectroscopy

Native pig articular cartilage was investigated by (13)C cross polarization (CP) magic angle spinning (MAS) NMR at a magnetic field strength of 17.6 T. CP MAS spectra of cartilage are dominated by resonances from rigid collagen, while only low-intensity signals from the glycosaminoglycans are observed. The spectral resolution of collagen fibrils in native cartilage is somewhat higher than for isolated collagen fibrils from bovine achilles tendon investigated for comparison. This is confirmed qualitatively by (1)H-(1)H wideline separation spectra that show much lower line widths for cartilage collagen compared to isolated collagen. The strength of (1)H-(13)C dipolar couplings was measured in a 2D LG CP experiment providing a motionally averaged dipolar coupling value for each resolved signal. These scaled couplings were converted to molecular order parameters for the CH bond vector. Typical order parameters for isolated collagen were 0.91-0.96 for sidechains and 0.98-1.00 for the backbone. Somewhat lower order parameters were determined for cartilage collagen; 0.79-0.90 for the sidechain and 0.92-0.97 for the backbone. The only glycosaminoglycan signals that could be detected by CP MAS show order parameters of 0.48-0.92 and are assigned to relatively rigid hyaluronan and keratan sulfate. The higher mobility of collagen in cartilage is due to the high water content and collisions with the isotropically mobile glycosaminoglycans, such as chondroitin sulfate. Therefore, the mobility of cartilage macromolecules is broadly distributed from almost completely rigid to highly mobile, which lends cartilage its mechanical strength and shock-absorbing properties.