FT-IR, TEM, and AFM studies of supramolecular architecture formed by tripeptide-containing monoalkyl amphiphiles

Abstract

Assembling behavior of tripeptide-containing monoalkyl amphiphiles in organic solvents and in dried films were investigated using Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM). The amphiphiles have a tripeptide spacer (Ala-Ala-Ala, Phe-Phe-Phe, and Ile-Ile-Ile) between the C12 hydrophobic tail and the ammonium polar head. The FT-IR and TEM observations revealed that highly non-polar CCl4 induced the β-sheet-based supramolecular fibrous structures. In contrast, no spontaneous assembly was induced in the less non-polar CHCl3, and evaporation of the solvent was necessary to form hydrogen-bonded assemblies. The morphologies of the cast films both from the CCl4 and CHCl3 solutions were investigated by AFM observation. The morphology of the assembly in the dried cast films is essentially determined by the chemical structure of the tripeptide moiety. The phenylalanine-based amphiphile tends to form cast films with a flat morphology, but round fibrous structures are detected in the cast films of the isoleucine-based amphiphile. Both amphiphiles form fiber-like (or worm-like) structures in CCl4 as shown in TEM images. The collapse of the supramolecular structure of the phenylalanine-based amphiphile leads to formation of the flat morphology upon solvent-evaporation. In contrast, the fiber-like structure was maintained in the cast film of the isoleucine-based amphiphile. The shape of the supramolecular assembly depends somewhat on the casting solvent, although the solvent does not basically remain in the cast film. Subtle tuning of the assembling structure of the cast films is provided by the assembling process, i.e pre-assembling in solution (CCl4 system) and gradual-assembling upon solvent evaporation (CHCl3 system). Copyright © 2000 John Wiley & Sons, Ltd.