Collagen dynamics in articular cartilage under osmotic pressure

Cartilage is a complex biological tissue consisting of collagen, proteoglycans and water. The structure and molecular mobility of the collagen component of cartilage were studied by (13)C solid-state NMR spectroscopy as a function of hydration. The hydration level of cartilage was adjusted between fully hydrated ( approximately 80 wt% H(2)O) and highly dehydrated ( approximately 30 wt% H(2)O) using the osmotic stress technique. Thus, the conditions of mechanical load could be simulated and the response of the tissue macromolecules to mechanical stress is reported. From the NMR measurements, the following results were obtained. (i) Measurements of motionally averaged dipolar (1)H-(13)C couplings were carried out to study the segmental mobility in cartilage collagen at full hydration. Backbone segments undergo fast motions with amplitudes of approximately 35 degrees whereas the collagen side-chains are somewhat more mobile with amplitudes between 40 and 50 degrees . In spite of the high water content of cartilage, collagen remains essentially rigid. (ii) No chemical shift changes were observed in (13)C cross-polarization magic angle spinning spectra of cartilage tissue at varying hydration indicating that the collagen structure was not altered by application of high osmotic stress. (iii) The (1)H-(13)C dipolar coupling values detected for collagen signals respond to dehydration. The dipolar coupling values gradually increase upon cartilage dehydration, reaching rigid limit values at approximately 30 wt% H(2)O. This indicates that collagen is essentially dehydrated in cartilage tissue under very high mechanical load, which provides insights into the elastic properties of cartilage collagen, although the mechanical pressures applied here exceed the physiological limit.