The viscoelasticity of self-assembled proteoglycan combs.

Particle tracking microrheology with a fast digital camera allowed the slow and intermediate time regimes (10(-4)-10(1) s) of the linear viscoelasticity of giant aggrecan proteoglycans to be mapped. Combined with diffusing wave spectroscopy experiments this enabled us to probe the linear viscoelasticity of aggrecan over seven orders of magnitude in time (10(-6)-10(1) s) [Palmer et al., Biophys. J., 1999, 76, 1063; Papagiannopoulos et al., Biomacromolecules, 2007, 7, 2162]. When the comb side-groups self-assemble on the hyaluronic acid backbones they cause a dramatic increase in the relaxation time of the solutions and consequently the viscosity of the sample, but leave the elasticity of the solutions relatively unchanged. The experiments illustrate the modular nature of aggrecan's viscosity and clearly demonstrate the role of this molecule in vivo in cartilaginous composites, where it dissipates energy. Both one- and two-particle tracking microrheology were used to investigate the length-scale dependent viscoelasticity of the comb superstructures [Lui et al., Phys. Rev. Lett., 2006, 96, 118104] and the errors inherent in the two techniques were quantified [Savin and Doyle, Biophys. J., 2005, 88, 623; Waigh, Rep. Prog. Phys., 2005, 68, 685]. The behaviour of the viscoelasticity is compared with the predictions of dynamic scaling theory, indicating a significant contribution of the side-chain dynamics to the reptative motion of both the aggrecan aggregate and the monomers. The results have important implications for a molecular understanding of tissue function and pathology in osteoarthritis.