Coassembly in Binary Mixtures of Peptide Amphiphiles Containing Oppositely Charged Residues

The self-assembly in water of designed peptide amphiphile (PA) C-16-ETTES containing two anionic residues and its mixtures with C-16-KTTKS containing two cationic residues has been investigated Multiple spectroscopy, microscopy, and scattering techniques are used to examine ordering extending from the beta-sheet structures up to the fibrillar aggregate structure. The peptide amphiphiles both comprise a hexadecyl alkyl chain and a charged pentapeptide headgroup containing two charged residues. For C-16-ETTES, the critical aggregation concentration was determined by fluorescence experiments. FTIR and CD spectroscopy were used to examine beta-sheet formation. TEM revealed highly extended tape nanostructures with some striped regions corresponding to bilayer structures viewed edge-on. Small-angle X-ray scattering showed a main 5.3 nm bilayer spacing along with a 3 nm spacing. These spacings are assigned respectively to predominant hydrated bilayers and a fraction of dehydrated bilayers. Signs of cooperative self-assembly are observed in the mixtures, including reduced bundling of peptide amphiphile aggregates (extended tape structures) and enhanced beta-sheet formation.