How Electrostatic Interactions Determine the Shape of Chiral Assemblies

Charged, chiral molecules are ubiquitous in biology. Prominent examples include many amino acids that constitute proteins and lipids that constitute cell membranes. Assemblies of such molecules are expected to be dictated by an interplay between electrostatic and chiral interactions. We combine in situ small-/wide-angle X-ray scattering (SAXS/WAXS), cryo-transmission electron microscopy (TEM) and atomic force microscopy (AFM) to analyze the self-assembly of charged chiral amphiphiles consisting of an amino acid (lysine) head group coupled to alkyl tails (C_n=12,14,16). By tuning the solution ionic conditions, we modulate the ionization tendency of the head groups as well as the range of electrostatic interactions. Our principal finding is that molecules assemble into helical ribbons or nanotubes when the electrostatic interactions are weak, but long ranged. By contrast, scroll-like cochleate are formed when the electrostatic interactions are short-ranged. The role of alkyl tail lengths that modulate the intermolecular van der Waals' interactions will also be discussed. Overall, our study shows how electrostatic interactions control the polymorphism in mesoscopic chiral shapes.