Native β-lactoglobulin self-assembles into a hexagonal columnar phase on a solid surface

Using Electron Scanning Microscopy we have studied the protein deposit left on silicon and mica substrates by dried droplets of aqueous solutions of bovine β-lactoglobulin at low concentration and pH=2–7. We have observed different self-assembled structures: homogeneous layers, hexagonal platelets and flower-shaped patterns laying flat on the surface, and rods formed by columns. Homogeneous layers covered the largest area of the droplet deposit. The other structures were found in small isolated regions, where the protein solution dried in the form of microdroplets. The presence of hexagonal platelets, flower-shaped patterns and columnar rods shows that β-lactoglobulin self-assembles at the surface in a hexagonal columnar phase, which has never been observed in solution. A comparison with proteins showing similar aggregates suggests that β-lactoglobulin structures grow from hexagonal germs composed of discotic nanometric building blocks, possibly possessing an octameric structure. We propose that discotic building blocks of β-lactoglobulin may be produced by the anisotropic interaction with the solid surface.