Electrostatic shape control of a charged molecular membrane from ribbon to scroll

Bilayers of amphiphiles can organize into a number of distinct mesoscopic shapes, which interconvert under suitable conditions. The pathway for such transformations is often elusive. We use a charged amphiphile (palmitoyl-lysine, C₁₆-K₁) to elucidate the planar nanoribbon to cochleate transition induced by salt (NaCl) concentration (c). In-situ small- and wide-angle X-ray scattering (SAXS/WAXS), atomic force and cryogenic transmission electron microscopies (AFM and cryo-TEM) tracked transformations over Å to µm length scales. AFM reveals that the large length (L) to width (W) ratio nanoribbons (L/W > 10) convert to sheets (L/W ~ 1) before rolling into cochleates. A theoretical model based on electrostatic and surface energies shows that the ribbon to sheet conversion is a first order transition, occurring at a critical Debye length. SAXS shows that the interbilayer spacing (D) in the cochleates scales linearly with Debye length (or c ^-1/2). Theoretical arguments that include electrostatic, bending and van der Waals energies explain the membrane rolling and the linearity between D and Debye length. These models suggest that the salt-induced ribbon to cochleate transition should be common to all charged bilayers possessing an intrinsic curvature.