"Setting paint" analogy for the hydrophobic self-association of tropoelastin into elastin-like hydrogel

Alkaline tropoelastin solutions (pH 11) were optically clear at low temperatures, but a firm gel formed when the temperature was raised to 37 degrees C. Reversion to a clear solution took place if the temperature was lowered to below 20 degrees C within less than 2 h, but not if 37 degrees C was maintained for several hours. The precipitated elastin-like hydrogel thus formed did not visually redissolve at low temperatures. Tropoelastin hydrogel was stable to subsequent washings with alkaline solution at 37 degrees C, but at 4 degrees C some hydrogel redissolved showing that association is at least partly reversible. Washing the hydrogel with neutral 8M urea solution at 4 degrees C dissolved less than 10% of tropoelastin in 24 h. We characterized this phenomenon by combining temperature-controlled light microscopy analysis, 1H NMR spectroscopy (temperature, diffusion, and relaxation time studies), and UV-absorption-based concentration measurements. The self-association of tropoelastin at pH 11 is due to hydrophobic interactions in an emulsion-like system in which the spherules coalesce in a manner like a water-based latex paint that forms a durable hydrophobic sheet as water and the organic solvent evaporate. In the present case, the sedimentation and entanglement of the tropoelastin porous sheets means that reverse dissolution is a kinetically slow process.