Electrostatic Control of Polymorphism in Charged Amphiphile Assemblies.
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.
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Changrui
C
Gao
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Honghao
H
Li
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Yue
Y
Li
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Sumit
S
Kewalramani
Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
Liam C
LC
Palmer
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Vinayak P
VP
Dravid
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.|Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.|Department of Biomedical Engineering, Northwestern University , Evanston, Illinois 60208, United States.|Department of Medicine and Simpson Querrey Institute for BioNanotechnology, Northwestern University , Chicago, Illinois 60611, United States.
Samuel I
0000-0002-5491-7442
SI
Stupp
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.|Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.|Chemical and Biological Engineering Department, Northwestern University , Evanston, Illinois 60208, United States.|Department of Physics and Astronomy, Northwestern University , Evanston, Illinois 60208, United States.
Monica
M
Olvera de la Cruz
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.|Department of Physics and Astronomy, Northwestern University , Evanston, Illinois 60208, United States.
Michael J
0000-0002-1026-4558
MJ
Bedzyk