Polyelectrolyte-like Behavior, pH-Dependent Self-Assembly, and Emulsion Stabilizing Properties of a Model Surfactant-like Peptide Bearing Pyridine Groups.

A surfactant-like peptide (SLP) bearing six non-native 3-(4-pyridyl)-l-alanine (Pal) residues and a C-terminal arginine residue, Pal6R, is shown to exhibit pH-dependent self-assembly which arises from the acid-base properties of the Pal residue (pKa ∼ 5). At a native pH of 2.4, "polyelectrolyte" correlation hole scattering is observed due to the electrostatic repulsion of highly charged molecules. The scaling of the domain size with concentration agrees with theoretical predictions for weakly charged flexible polyelectrolytes in a semidilute solution. In contrast, twisted nanotapes are observed at pH 7. The nanotapes are shown to comprise β-sheet structures packed in interdigitated bilayers. Atomistic molecular dynamics (MD) simulations confirmed the bilayer structure of the nanotapes, with extensive hydrogen bonding, and a twisting tendency. The novel SLP can stabilize water-in-oil emulsions at pH 7, forming β-sheet bilayer structures at the water droplet interface. Pal6R represents a model polyelectrolyte system with additional self-assembly and emulsion stabilization properties at neutral pH.