Control of self-assemblies and secondary structures in polypeptide-surfactant complexes

Polyelectrolyte-surfactant complexes are known to form self-assembled structures at nanometer length scale. When polypeptides are used instead of traditional synthetic polymers, another level of structural control is introduced due to the ability of polypeptides to fold into secondary structures. In this thesis, self-assembly and secondary structure formation of selected cationic homopolypeptides and block copolypeptides are studied in ionic complexes with anionic surfactants. The surfactant architecture as well as the amount of surfactant were varied and their effect on the self-assembly and polypeptide secondary structures was investigated. Articles I, II and III describe the self-assembly of stoichiometric complexes with single and double alkyl tail surfactants. The complexes were found to respond to a simple external trigger, i.e. temperature, by showing structural changes. In addition, when the amount of surfactant was increased from the stoichiometric value, a plasticization effect from solid to soft liquid crystalline material was observed. Plasticization could be important for solid state applications requiring processing. In Articles IV and V, hierarchical self-assemblies, i.e. materials with structures at different length scales, are described. Two approaches were used. In Article IV, an asymmetric triple-tail lipid was found to induce a helical conformation in the polypeptides and packing of the helices into a layered structure with 2D correlation between the helices, resulting in an oblique lattice. Crystallization of the lipid alkyl tails was found crucial for such structure formation. Heating past the melting temperature of the side-chain crystallites caused a reversible order-order transition from oblique to hexagonal self-assembly. In Article V, a diblock copolypeptide with one cationic block was complexed with surfactants and hierarchical structure-and-structure-within-structure self-assemblies were observed. The morphology was found to depend on the surfactant architecture, i.e. the amount of branching in the alkyl tail. In conclusion, this thesis encourages pursuing novel rationally designed self-assemblies based on polypeptides to enable new schemes for biomimetic materials.