Controlled structure in artificial protein hydrogels

Small-angle X-ray scattering (SAXS) and circular dichroism (CD) were used to study the structure of artificial, multidomain protein hydrogels. Preliminary sedimentation equilibrium results indicate that the flanking leucine zipper domains used in the design of the artificial multidomain protein form tetrameric helical bundles. The leucine zipper domain alone precipitates at high concentrations (7% w/v) and near-neutral pH, but the multidomain protein remains soluble owing to a hydrophilic central domain. The resulting solution displays characteristic properties of physical gels. SAXS data from gels fit well to a cylindrical model with the following dimensions: length 63 Å, radius 13.6 Å, and a 1 Å axial pore. These results match the dimensions of a tetrameric helical bundle and indicate that the low concentration equilibrium structure of the leucine zipper domain is maintained within the multidomain protein, even at high concentrations. Altogether, these results confirm a static picture of the gel structure where tetrameric self-associations of leucine zipper domains act as physical cross-links in the protein hydrogel.