Molecular self-assembly of surfactant-like peptides to form nanotubes and nanovesicles

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Abstract
Several surfactant-like peptides undergo self-assembly to form nanotubes and nanovesicles having an average diameter of 30–50 nm with a helical twist. The peptide monomer contains 7–8 residues and has a hydrophilic head composed of aspartic acid and a tail of hydrophobic amino acids such as alanine, valine, or leucine. The length of each peptide is ≈2 nm, similar to that of biological phospholipids. Dynamic light-scattering studies showed structures with very discrete sizes. The distribution becomes broader over time, indicating a very dynamic process of assembly and disassembly. Visualization with transmission electron microscopy of quick-freeze/deep-etch sample preparation revealed a network of open-ended nanotubes and some vesicles, with the latter being able to “fuse” and “bud” out of the former. The structures showed some tail sequence preference. Many three-way junctions that may act as links between the nanotubes have been observed also. Studies of peptide surfactant molecules have significant implications in the design of nonlipid biological surfactants and the understanding of the complexity and dynamics of the self-assembly processes.