Insensitivity to Salt of Assembly of a Rigid Biopolymer Aggrecan

Many polyelectrolytes, ranging from sulfonated polystyrene to DNA, exhibit a strong sensitivity of their phase behavior to salt concentration, especially to higher valence salts, which often lead to phase separation. We show that the stiff polyelectrolyte aggrecan exhibits a qualitatively different behavior. Specifically, the scattering properties of aggrecan solutions are exceptionally insensitive to the addition of calcium salt, conferring on aggrecan the role of an ion reservoir mediating calcium metabolism in cartilage and bone, and also providing osmotic resilience to compressive load.

Figure 1

Structure of aggrecan molecule consisting of a protein core (pc) with GAG side chains. The negative charges are spaced between 10 to 15 Å apart along the CS and KS side chains. Gl, G2, and G3: globular protein domains, N: amine terminal of the protein core. Inset A: Structure of aqueous aggrecan solution. Inset B: Entangled linear polyelectrolyte chains in solution.

Figure 2

Variation of the osmotic pressure $\Pi$ as a function of aggrecan concentration in 100 mM NaCl solution, with different amounts of ${\mathrm{CaCl}}_2$. Inset shows $\Pi (c)$ for DNA gels containing different amounts of ${\mathrm{CaCl}}_2$ in 100 mM NaCl.

Figure 3

Combined SLS, SANS, and SAXS spectra of aggrecan solutions ($c=0.001\mathrm{g}/{\mathrm{cm}}^3$) in 100 mM NaCl with 3 different ${\mathrm{CaCl}}_2$ concentrations: 0 (○), 100 (+), and 200 mM (×). Inset shows the SANS spectra of two DNA gels in 40 mM NaCl solutions with 0.05 mM ${\mathrm{CaCl}}_2$ (lower curve) and with 0.20 mM ${\mathrm{CaCl}}_2$ (upper curve), respectively.

Figure 4

Intensity correlation function $G(\tau )-1$ for aggrecan solutions ($c=0.0007\mathrm{g}/{\mathrm{cm}}^3$) in 100 mM NaCl without ${\mathrm{CaCl}}_2$ (●) and with 25 mM ${\mathrm{CaCl}}_2$ (×) at scattering angle 150°. Inset A: Intensity correlation function for a CS solution ($c=0.02\mathrm{g}/{\mathrm{cm}}^3$, scattering angle 150°) without and with ${\mathrm{CaCl}}_2$ showing the fast (continuous line) and slow (dashed line) components. Inset B: Dependence of the characteristic relaxation rate $\Gamma \eta$ on the wave vector $q$, where $\eta$ is the solvent viscosity. Data sets show measurements by SADLS, DLS, and NSE for solutions of aggrecan in 100 mM NaCl (×) and in 100 mM NaCl with 100 mM ${\mathrm{CaCl}}_2$ (○).