Peptide amphiphiles are composed of hydrophobic alkyl tails and peptide regions designed to self-assemble into cylindrical supramolecular nanofibers in solution. While some β-sheets are formed by hydrogen bonds between short β-strands (2 or 3 residues) others are formed by extended β-strands.
The strongly-hydrated ions (F- and Cl-) are more attracted to the positively charged lysine residues on the surface of the peptide nanofiber. When peptide residues form β-sheets, an F- or Cl- ion forms a salt bridge between the side chains of lysine residues from two neighboring peptide amphiphile chains. The salt bridge stabilizes the peptide by bringing the backbones closer, which in turn results in a transition from random coil to extended β-sheets structures. The smaller ions (F- and Cl-, 50mM NaF and NaCl solutions) tend to stabilize β-sheets slightly better compared to the larger ions (I-, Br-).
So self-assembly of peptide amphiphiles into supramolecular nanofibers can be regulated by modifying the salt solution.