A Zoo of Chiral Structures: Electrostatic Directed Assembly of Charged, Chiral Amphiphiles

In self-assembled systems, molecular chirality is manifested in fascinating membrane shapes at nano- to meso-scale. The most common shapes are helical membranes, helical scrolls (cochleates) and twisted membranes. These shapes have been observed in natural and synthetic systems. Despite the prevalence, an understanding of the interconversion mechanisms between different chiral morphologies is lacking. In this study, we were able to generate all the common chiral structures in perhaps the simplest chiral molecular series Cn-K, where one amino acid [lysine (K)] is coupled to alkyl tails of n (= 8-18) carbons. This was accomplished by tuning the solution ionic conditions. Scroll-like assemblies were observed when the molecular degree of ionization was high, and the electrostatic interactions were short-ranged. By contrast, helical and twisted membranes are observed for long (n = 10-18) and short (n = 8-10) molecular tails, respectively, when the molecular degree of ionization was low and the electrostatic interactions were long-ranged. These results were derived by combining X-ray scattering and electron/atomic force microscopy. Overall, our study reveals that electrostatic interactions can be used to guide chiral shape selection and nano-scale structure in molecular self-assemblies.