Electrostatic and Van der Waal Control of Charged Chiral Assemblies

Charged, chiral molecules assemble into a variety of chiral mesoshapes. However, the interplay between electrostatics and van der waals interactions in chiral shape selection is not well understood. To tackle this, we analyzed self-assembly in a homologous series of amphiphiles (C_nK) consisting of an ionizable amino acid (lysine (K)) coupled to alkyl tails with n = 12, 14 or 16 carbons. Solution small/wide angle X-ray scattering (SAXS/WAXS) reveal that at low pH, where the headgroups are ionized (+1), the assembly is dictated by tail length resulting in disordered aggregates (n = 12), spherical micelles (n = 14) and high aspect ratio (L/W > 10:1) planar bilayers (n = 16). However, regardless of the tail length, assemblies transform to helical bilayers and then to bilayer stacks as the average headgroup charge is reduced by increasing pH. The helix radius (60-100 nm) increases both with decreasing molecular charge (fixed tail length) and decreasing tail length (fixed pH).  Overall, our study represents a step towards a rational electrostatics-based approach for meso-shape selection and for controlling nm-scale structure in a continuous manner for chiral assemblies.