Electrostatic Forces between Charged Macromolecules Measured by Equilibrium Sedimentation: Relevance to Cartilage Mechanics

Introduction: Molecular interactions between glycosaminoglycans (GAGs) affect the behavior of extracellular matrix in health and disease. However, their heterogeneous composition, distribution, and physical-chemical properties complicate their characterization in situ. Here we present a new approach to measuring intermolecular electrostatic repulsive forces between GAGs (and between other matrix macro-molecules), using equilibrium sedimentation (centrifugation) and a new model of macromolecular electrostatic interactions in a centrifugal field. We then demonstrate this method using a well-characterized molecular analog of GA Gs, polymethacrylic acid (PMAA). Theory, Materials and Methods: With a centrifuge, we apply a known, spatially varying mechanical force to a macromolecular solute (e.g., see (!)). From the resulting measured macroscopic solute concentration profile, Ci(r) within the centrifuge cell (Fig!), we calculate a profile of intermolecular spacing . At thermodynamic and mechanical equilibrium, the electrostatic, centrifugal, and diffusive forces, respectively, balance, leading to: Mj(l -vjp)w2r2 RT l () (I) (r) o = 2ziF ziF n Ci r .