Parallel β-sheet assemblies at interfaces

Polypeptide assemblies may exhibit various topologies[1-9] that are of interest for nanometer-scale surface patterning and its potential applications. The success in designing such ordered molecular architectures entails control over peptide conformations and intermolecular interactions. At the air-water interface peptides composed of alternating hydrophilic and hydrophobic amino acids tend to adopt β-sheet structures[10] yet the repetitive nature of these peptides also promotes nonspecific intermolecular aggregation. Recently, in several systems of de novo designed b-sheet peptides two-dimensional order has been demonstrated by grazing incidence X-ray diffraction[8, 9] and by scanning probe microscopy;[11] the extent of molecular registry has been associated with peptide composition and molecular chain length. Here we aim at formation of parallel β-sheet ordered assemblies at interfaces by using strands programmed to adopt distinct intermolecular electrostatic interactions

Characterization of peptide-guided polymer assembly at the air/water interface

An organo-soluble, peptide-polymer conjugate that combines poly(n-butyl acrylate) with a beta-sheet-forming peptide is spread at the water surface to investigate peptide-guided self-assembly in a two-dimensional environment. Single layers of the conjugate are studied to gain information on the packing, orientation, and structure of the conjugate molecules using standard monolayer techniques: isotherms, grazing incidence X-ray diffraction (GIXD), and infrared reflection absorption spectroscopy (IRRAS). At all conditions studied, the stabilizing beta-sheet network consists of antiparallel beta-sheets oriented parallel to the air/water interface. The incorporation of temporary switch defects in the peptide segment enables beta-sheet assembly to be triggered at different packing densities. Stable monolayers, with low compressibilities similar to peptide monolayers, form when beta-sheet assembly occurs in monolayers that contain closely packed conjugate molecules. Langmuir-Schaefer transfer of the switched monolayer seeded with 1/1000 part stearic acid results in a transferred monolayer containing ordered domains with 7 nm wide stripes, a width in agreement with the end-to-end distance of the conjugate molecule. In this interfacial system, high packing densities and a hydrophobic seed molecule play an important role in beta-sheet network and structure formation. Both effects likely direct the highly ordered beta-sheet structure because of beta-strand prealignment. Insights gained from self-assembly in this system can be applied to peptide aggregation mechanisms in more complex interfacial environments

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