Intermolecular repulsion forces between negatively charged glycosaminoglycan (CS-GAG) macromolecules are a major determinant of cartilage biomechanical properties. It is thought that the electrostatic component of the total intermolecular interaction is responsible for 50-75% of the equilibrium elastic modulus of cartilage in compression, while other forces (e.g., steric, hydration, van der Waals, etc.) may also play a role. To investigate these forces, radiolabeled CS-GAG polymer chains, with a fully extended contour length of 35 nm, were chemically end-grafted to a planar surface to form model biomimetic polyelectrolyte "brush" layers whose environment (e.g., ionic strength, pH) was varied to mimic physiological conditions. The total intersurface force (enN) between the CS-GAG brushes and chemically modified probe tips (SO 3 -and OH) was measured as a function of tip-substrate separation distance in aqueous solution using the technique of high-resolution force spectroscopy (HRFS). These experiments showed long-range, nonlinear, purely repulsive forces that decreased in magnitude and range with increasing ionic strength and decreasing pH. To estimate the contribution of the electrostatic component to the total intersurface force, the data were compared to a theoretical model of electrical double layer repulsion based on the Poisson-Boltzmann formulation. The CS-GAG brush layer was approximated as either a flat surface charge density or a smoothed volume of known fixed charge density and the probe tip was modeled as a smooth hemisphere of constant surface charge density. Modeling the CS-GAG brush as a volume charge yielded theoretical fits much closer to the experimental data and is a good first step toward deconvolution of the force components.

Many synthetic and natural peptides are known to self-assemble to form various nanostructures such as nanofibers, hollow tubes, or ring-like structures. Some of the synthetic peptide molecules are specifically designed to produce well-defined nanostructures by controlling intermolecular interactions. Many environmental conditions such as salt concentration, pH, temperature, and surface characteristics influence intermolecular interactions, hence the process of the self-assembly. Here we studied self-assembly of a genetically engineered protein polymer composed of silk-like and elastin-like repeats on a mica surface. Silk-elastinlike protein polymers (SELPs) consist of tandem repeats of Gly-Ala-Gly-Ala-Gly-Ser from Bombyx mori (silkworm) and Gly-Val-Gly-Val-Pro from mammalian elastin. At a very low polymer concentration of 1 μg/ml, SELPs self-assembled into nanofibrous structures on a mica surface. Examination using atomic force microscopy (AFM) and dynamic light scattering techniques showed that SELPs self-assembled into nanofibers in the presence of the mica surface but not in the bulk state. Ionic strength had a significant influence on nanofiber growth, indicating the importance of electrostatic interactions between the polymer and the mica surface. At low ionic strength, the kinetics of nanofiber growth indicates that the mica surface effectively removed a lag phase by providing nucleating sites, facilitating nanofiber selfassembly of SELPs. Further examination of self-assembly on various surfaces such as silicon, positively charged surface, and hydrophobic surface revealed that negatively charged hydrophilic surface provides optimal surface to facilitate self-assembly of SELPs.

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