RESUMEN

Aggrecan, the major cartilage proteoglycan, is the macromolecular species primarily involved in the resiliency of cartilage tissue. Most aggrecan molecules occur in cartilage extracellular matrix as aggregates. Each aggregate has a supramolecular structure, with many highly anionic, brushlike aggrecan subunits noncovalently bound to a hyaluronan chain. To better examine the viscoelastic properties of aggrecan aggregate, contaminating subunits were removed by exclusion chromatography. At physiologic ionic strength, concentrated solutions of purified aggrecan aggregate exhibit predominantly elastic behavior at small shear strains. However, above a critical strain, gamma c, the shear moduli show a pronounced strain-softening transition, where the storage modulus decreases suddenly, and the loss modulus exhibits a maximum. At small stresses, the creep function is also highly elastic, exhibiting an equilibrium compliance and large recoverable compliance. When the stress is amplified, a discrete transition to viscous flow occurs at a yield stress sigma y. These nonlinear responses are similar to previous observations for close-packed colloidal suspensions of soft spheres, such as microgel or emulsion particles, for which a yield transition occurs when the stress and deformation are sufficient to move a particle past its neighbors.

Asunto(s)

Proteínas de la Matriz Extracelular , Proteoglicanos/química , Agrecanos , Animales , Cartílago/química , Bovinos , Elasticidad , Técnicas In Vitro , Lectinas Tipo C , Sustancias Macromoleculares , Tamaño de la Partícula , Proteoglicanos/aislamiento & purificación , Reología , Soluciones , Viscosidad

RESUMEN

We report static and dynamic light scattering measurements on bovine serum albumin (BSA) solutions at high ionic strength (I) where potential and hydrodynamic interactions between BSA molecules are of comparable strengths. Measurements of the concentration dependence of the osmotic compressibility, (dpi/dc), and the translational diffusion coefficient, D(m), are presented for several solvent systems: (a) at the isoelectric pH = 4.7 and I = 0.1, where long-range electrostatic repulsions are absent; (b) at pH = 7.4 and I = 0.15, 1.5, and 3.3, where a well-screened electrostatic repulsion is present. The results are compared with theoretical predictions which involve a microscopic hard-sphere treatment of the potential and hydrodynamic interactions. At pH = 7.4 and I = 1.5, our experimental results for dpi/dc are in good agreement with the hard-sphere prediction, and our values for D(m) are, likewise, consistent with a hard-sphere hydrodynamic analysis in which contributions from the divergence terms in the velocity field are neglected. At the isoelectric pH, similar agreement with theory is obtained, provided the contribution of an attractive potential is included; at pH 7.4 and I = 0.15, the contribution from a long-range repulsion must be included; at pH 7.4 and I = 3.3, onset of protein aggregation is observed. Copyright 1999 Academic Press.