Oppositely Charged Homopolypeptides Form Ordered Helix Assemblies With Hierarchical Structure

Oppositely charged polyelectrolytes are known to form polyelectrolyte complexes. Polyelectrolyte complexes are typically structurally disordered. In recent years, complexes of oppositely charged polypeptides have received increasing attention. Polypeptides provide additional interaction motifs such as hydrogen bonds and salt bridges that could stabilize ordered assemblies. Under specific conditions, the formation of secondary structures has been reported. So far, it has been difficult to obtain long-range ordered assemblies of polypeptide complexes. Here we show that liquid-solid phase separation of oppositely charged homopolypeptides spontaneously leads to the formation of extended polypeptide helices that form highly ordered hexagonally or square-packed helix arrays with helix-helix distances of 1.6-2.0 nm. The helices exhibit an additional periodic staggering along their axis with a periodicity of 15 - 30 nm, equal to the length of the extended helices. This hierarchical helix assembly was demonstrated for polyarginine assembled with polyaspartic acid or polyglutamic acid as examples. The materials can be shear-aligned to achieve macroscopic uniaxial orientation of the helices. Crosslinking leads to solid elastic materials. Upon heating to temperatures above 40°C, the structure disorders due to a reversible helix-coil transition. The structural and mechanical properties suggest a new route to synthetic peptide-based materials.