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pH changes trigger transformations in peptide amphiphile assemblies, shifting from micelles to nanofibers and bilayers. Ionic interactions alter lysine headgroup charge, influencing molecular packing and membrane structure.

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Area of Science:

  • Supramolecular Chemistry
  • Materials Science
  • Biophysics

Background:

  • Stimuli-responsive molecular assemblies are crucial for nanotechnology and biological systems.
  • pH variations influence the ionization and conformation of amphiphiles and proteins.
  • Peptide amphiphiles (PAs) offer tunable properties for self-assembly.

Purpose of the Study:

  • To investigate pH-induced morphological transitions in a positively charged peptide amphiphile (PA) system.
  • To elucidate the role of charge regulation and ionic correlations in PA self-assembly.
  • To determine the molecular packing details within the assembled structures.

Main Methods:

  • In situ small and wide-angle X-ray scattering (SAXS/WAXS) for structural analysis.
  • Transmission electron microscopy (TEM) for visualizing morphology.
  • Monte Carlo simulations and pH titration for understanding charge effects.

Main Results:

  • Ionic correlations shift the pKa of the lysine headgroup from ~10 to ~8.
  • Increasing pH induces transitions from spherical micelles to cylindrical nanofibers to planar bilayers.
  • Bilayer structures exhibit interdigitated, tilted PA lipid tails crystallized on a rectangular lattice.

Conclusions:

  • Molecular charge and assembly morphology are directly correlated in pH-responsive PAs.
  • Steric and van der Waals interactions govern molecular packing in the observed bilayer membranes.
  • The identified packing motif may be common in bilayers with large ionic headgroups.