Modulated supramolecular assemblies composed of tripeptide derivatives: Formation of micrometer-scale rods, nanometer-size needles, and regular patterns with molecular-level flatness from the same compound

In this paper, we demonstrated that supramolecular structures of amphiphilic tripeptides can be effectively modulated by selecting the appropriate casting solvents. The tripeptide derivatives having an ammonium head and hydrophobic tails were dispersed in three kinds of solvents (nonpolar, medium nonpolar, and polar) followed by casting the solutions onto fleshly cleaved mica. The supramolecular structures in the cast films were observed using an atomic force microscope (AFM). The tripeptide derivative possessing the Ala-Ala-Glu sequence formed needles with nanometer-scale width, micrometer-scale rods, and patterns with molecular-level flatness by casting from H2O, CCl4, and CHCl3, respectively. Since the major driving forces of the assembly are significantly influenced by the polarity of the surrounding media, the assembling process can be altered depending on the nature of the solvents. In polar H2O, the hydrophobic interaction is dominant and the resulting assemblies are supported by hydrogen bonding. The strong hydrophobic interaction led to formation of a charged surface of unit structures and they were hardly fused upon solvent evaporation. As a result, thin needlelike structures remained in the cast film. In a nonpolar solvent such as CCl4, the assembly was driven by the insolubility of a polar part of the component and the hydrogen bonding, and solidification at the hydrophobic tails is induced only upon solvent evaporation. This two-step assembling process led to growth of the assembly, resulting in a larger structure. A medium nonpolar solvent;, i.e., CHCl3 is not good for both interactions, thus resulting in a nonassembling structure in the solution. Therefore, assembly occurred only when the solvent was evaporated. This process gave a two-dimensional pattern with molecular-level flatness. By using the solvents with a wide range of polarities, a variety of assembling processes leads to successful modulation of the supramolecular structures.