Oblique self-assemblies and order-order transitions in polypeptide complexes with PEGylated triple-tail lipids

We report on highly ordered oblique self-assemblies in ionic complexes of PEGylated triple-tail lipids and cationic polypeptides, as directed by side-chain crystallization, demonstrating also reversible oblique-to-hexagonal order-order transitions upon melting of the side chains. This is achieved in bulk by complexing cationic homopolypeptides, poly-l-lysine (PLys), poly-l-arginine (PArg), and poly-l-histidine (PHis), in stoichiometric amounts with anionic lipids incorporating two hydrophobic alkyl tails and one hydrophilic polyethylene glycol (PEG) tail in a star-shaped A(2)B geometry. Based on Fourier transform infrared spectroscopy (FTIR), the PLys and PArg complexes fold into α-helical conformation. Aiming to periodicities at different length scales, that is, hierarchies, the PEG tails were selected to control the separation of the polypeptide helices in one direction while the alkyl tails determine the distance between the hydrophilic polypeptide/PEG layers, resulting in an oblique arrangement of the helices. We expect that the high overall order, where the self-assembled domains are in 2D registry, is an outcome of a favorable interplay of plasticization due to the hydrophobic and hydrophilic lipid tails combined with the shape persistency of the peptide helices and the crystallization of the lipid alkyl chains. Upon heating the complexes over the melting temperature of the alkyl tails, an order-order transition from oblique to hexagonal columnar morphology was observed. This transition is reversible, that is, the oblique structure with 2D correlation of the helices is fully returned upon cooling, implying that the alkyl tail crystallization guides the structure formation. Also PHis complex forms an oblique self-assembly. However, instead of α-helices, FTIR suggests formation of helical structures lacking intramolecular hydrogen bonds, stabilized by steric crowding of the lipid. The current study exploits competition between the soft and harder domains, which teaches on concepts toward well-defined polypeptide-based materials.