Structural and Conformational Transformations in Self-Assembled Polypeptide−Surfactant Complexes

Abstract

Self-assembled polypeptide−surfactant complexes are usually infusible solids in the absence of solvent and do not allow fluidlike liquid crystallinity even when heated, which seriously limits their polymer-like applications in the solid state due to processing problems. This work is partly inspired by nature's liquid crystalline processing of silk and subsequent structural interlocking due to β-sheet formation. We demonstrate here polypeptide−surfactant complexes that are fluidlike liquid crystalline at room temperature with hexagonal cylindrical self-assembly. The hexagonal structure with α-helical polypeptide chains is then partially converted to lamellar self-assembly with β-sheet conformation through thermal treatment. We use poly(l-lysine)−dodecylbenzenesulfonic acid complexes, PLL(DBSA)x (x = 1.0−3.0), where the branched dodecyl tails suppress the side-chain crystallization. In the stoichiometric composition, x = 1.0, there is one anionic DBSA molecule ionically complexed to each cationic lysine residue. Such a PLL(DBSA)1.0 is an infusible solid material at all temperatures until degradation. Introduction of additional DBSA, i.e., x = 1.5 or 2.0, plasticizes the material to be shear-deformable and birefringent. In organic solution, as witnessed by small-angle neutron scattering (SANS), the PLL(DBSA)x complexes form bottle-brush-like cylinders, which upon evaporation of the solvent self-assemble into hexagonal cylindrical morphology with α-helical PLL secondary structure. Heating of PLL(DBSA)x with x = 1.0−2.0 up to the range 120−160 °C leads to the formation of lamellar self-assembled domains with β-sheet conformation of PLL, which coexist with the hexagonal self-assembled structures with α-helical conformation, as shown by Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS). Higher complexation ratio, i.e., x = 3.0, results in soft and shear-deformable hexagonally packed cylinders at room temperature, but heating irreversibly converts the PLL to a random coil conformation, which leads to a disordered structure. The present model studies show that in polypeptide−surfactant self-assemblies it is possible to change the properties of the material by thermal treatment due to irreversible structural and conformational transformations.