Effect of the counterion behavior on the frictional–compressive properties of chondroitin sulfate solutions

Chondroitin sulfate is a major constituent of articular cartilage, which is known to affect in a decisive way the mobility and flexibility of our joints. A deviation from the physiological conditions, like e.g. a deficiency of water and salt content, in the cartilage tissue has long been suspected to be a possible trigger for rheumatoid diseases. Progresses in understanding the frictional–compressive behavior on the molecular level have been hindered due to the lack of reliable experimental data and the multitude of controlling parameters, influencing the structure and properties of cartilage tissue in its natural environment. In this paper we study the thermodynamic response of aqueous chondroitin sulfate solutions to changes in the monomer and added salt concentrations, using a recently developed field-theoretic approach beyond the mean field (MF) level of approximation. Our approach relies on the method of Gaussian equivalent representation, which has recently been shown to provide reliable thermodynamic information for polyelectrolyte solutions without and with added salt over the whole range of monomer concentrations. We compare our calculation results to experimental as well as molecular modeling data, and demonstrate that it provides useful estimates for important thermodynamic properties. Moreover, we obtain conclusive insights about the hydration effects and counterion behavior under various conditions, which show that, at the physiological salt concentration, CS solutions have optimal compressive and tribological properties. Finally, our work provides support for the possibility that a long-term deviation from the physiological conditions may trigger rheumatoid diseases.