Ionic Magic: Small Ions Transform Peptides into Stable β-Sheets

Peptide amphiphiles are composed of hydrophobic alkyl tails and peptide regions designed to self-assemble into cylindrical supramolecular nanofibers in solution. While hydrogen bonds form some β-sheets between short β-strands (2 or 3 residues), others are formed by extended-strands.

Smaller Ions Stabilize β-sheets

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, resulting 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, the self-assembly of peptide amphiphiles into supramolecular nanofibers can be regulated by modifying the salt solution.