THE IMPORTANCE OF POLYSACCHARIDE CONFIGURATIONAL ENTROPY IN DETERMINING THE OSMOTIC SWELLING PRESSURE OF CONCENTRATED PROTEOGLYCAN SOLUTION AND THE BULK COMPRESSIVE MODULUS OF ARTICULAR-CARTILAGE

One important contribution to the osmotic swelling pressure of concentrated proteoglycan and hence the elasticity of articular cartilage arises from the configurational entropy of the polysaccharide chains in the extracellular matrix. The work presented here provides a theoretical determination of this entropy and an analysis of its effect on the equilibrium osmotic swelling pressure of concentrated proteoglycan solutions. This effect is calculated in a manner similar to the Flory-Huggins technique where the solution is treated as a lattice (P. Flory, Principles of polymer chemistry (Cornell Univ. Press, Ithaca, 1953); J. Chem. Phys. 12 (1944) 425). In addition, the charge-related contribution to the elasticity of these materials is reviewed in the form of a Donnan equilibrium model (T. Hill, Faraday Soc. Discussions 21 (1956) 31; A.G. Ogston and J.D. Wells, Biochem. J. 119 (1970) 67; C. Tanford, Physical Chemistry of Macromolecules (Wiley, New York, 1961)). It is found that the configurational entropy of the glycosaminoglycan (GAG) chain polysaccharides together with the charge effects reproduce the equilibrium swelling pressure of concentrated proteoglycan solutions as experimentally determined by J.P.G. Urban et al., Biorheol. 16 (1979) 447. In addition this theoretical model is manifestly independent of the proteoglycan molecular weight, consistent with prior experimental findings (J.P.G. Urban et al., Biorheol. 16 (1979) 447). The model is also extended to include polydispersity of proteoglycan size and to predict the equilibrium bulk compressive modulus of articular cartilage. This work represents the first comprehensive theoretical description of the equilibrium elastic properties of proteoglycan solutions. It establishes the important role for polysaccharide configurational entropy in determining the equilibrium elastic properties of proteoglycan solutions, and serves to conceptually link their molecular structure with their important mechanical properties.