Abstract
Stimuli-induced structural transformations of molecular assemblies in aqueous solutions are integral to nanotechnological applications and biological processes. In particular, pH responsive amphiphiles as well as proteins with various degrees of ionization can reconfigure in response to pH variations. Here, we use in situ small and wide-angle X-ray scattering (SAXS/WAXS), transmission electron microscopy (TEM), and Monte Carlo simulations to show how charge regulation via pH induces morphological changes in the assembly of a positively charged peptide amphiphile (PA). Monte Carlo simulations and pH titration measurements reveal that ionic correlations in the PA assemblies shift the ionizable amine pK ∼ 8 from pK ∼ 10 in the lysine headgroup. SAXS and TEM show that with increasing pH, the assembly undergoes spherical micelle to cylindrical nanofiber to planar bilayer transitions. SAXS/WAXS reveal that the bilayer leaflets are interdigitated with the tilted PA lipid tails crystallized on a rectangular lattice. The details of the molecular packing in the membrane result from interplay between steric and van der Waals interactions. We speculate that this packing motif is a general feature of bilayers comprised of amphiphilic lipids with large ionic headgroups. Overall, our studies correlate the molecular charge and the morphology for a pH-responsive PA system and provide insights into the Å-scale molecular packing in such assemblies.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 1623-1628 |
| Number of pages | 6 |
| Journal | Journal of Physical Chemistry B |
| Volume | 121 |
| Issue number | 7 |
| DOIs | |
| State | Published - Feb 23 2017 |
Funding
This research was primarily supported by the Department of Energy (DOE), Office of Basic Energy Sciences under Contract DE-FG02-08ER46539. Additional support to L.C.P and S.I.S. for peptide synthesis was provided by the National Science Foundation (Grant DMR-1006713). Peptide synthesis was performed in the Peptide Synthesis Core Facility of the Simpson Querrey Institute at Northwestern University. The U.S. Army Research Office, the U.S. Army Medical Research and Materiel Command, and Northwestern University provided funding to develop this facility.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry