Direct measurement of forces between self-assembled proteins: temperature-dependent exponential forces between collagen triple helices.

We report direct measurements of force vs. separation between self-assembled proteins. These forces are observed between collagen triple helices in native and reconstituted fibers. They are a combination of a short-range repulsion, which varies exponentially over at least five decay lengths, and an inferred, longer-ranged attraction responsible for spontaneous assembly. From 5 degrees C to 35 degrees C the relative contribution of the attraction to the net force increases with temperature. These forces are strikingly similar to the "hydration" forces measured between several other linear macromolecules (DNA, polysaccharides) and between lipid bilayer membranes. The decay length of the repulsive force agrees well with a theoretical estimate based on axial periodicity of the triple helix, suggesting another connection between molecular architecture and protein-protein interaction.