Switchable peptide surfactants with designed metal binding capacity

By modifying a well-studied peptide sequence, we have designed two biosurfactants with the ability to reversibly and precisely control the stability of foams. Foam stabilization occurs when the peptide forms a cohesive interfacial film cross-linked by metal ions, while foam destabilization occurs when peptide-metal binding is disrupted. The parent sequence is an amphipathic peptide that adsorbs at fluid interfaces, but forms neither cohesive interfacial films nor stable foams at the concentrations tested. Two modified peptide sequences were designed in which internal sites were substituted with metal-binding histidine residues. The first derivative, AM1, contains two histidines and can undergo intermolecular cross-linking by metal at the air-water interface. AM1 forms cohesive interfacial films and stable foams in the presence of Zn(II), Co(II), or Ni(II), but not in the absence of metal ions. The second derivative, AFD4, has four histidine substitutions, and can undergo both intra- and intermolecular cross-linking by metal ions. AFD4 forms stronger interfacial films and more stable foams than AM1 in the presence of the same metal ions, and also undergoes helical structuring in solution in the presence of added metal ions. For both peptides, film formation and foam stabilization can be reversed by acidification of the bulk solution, or addition of a metal chelator.