The design and synthesis of artificial materials, mimicking sophisticated and complex natural system, have been fundamental challenges in synthetic and biological field. Foldamers are considered promising artificial molecular frameworks that mimic the secondary structures of biological components. In light of the marvelous functional complexity and morphologies from tertiary and quaternary protein structures, foldamer-based higher-order architectures would also exhibit numerous functions and structural diversity. Importantly, structural differences between foldamers and biopolymers suggest that foldamer-based higher-order architectures carry topologies and functions unprecedented in nature. Foldamer-based higher-order architectures with various morphologies and scales have been reported in recent decades. Three-dimensional (3D) molecular architectures from self-assembly of peptide foldamers, termed foldectures, are one of the distinctive biomimetic materials. Their unique morphologies and well-defined interior and exterior structures imply that foldectures could be promising functional materials rivaling 3D biological architecture such as viral capsids. If one can display functional groups with spatial periodicity on the certain surface of the well-defined framework, this “surface-functionalized foldecture” would be a core material that further expands to advanced materials.
Herein foldectures with anisotropic surface characteristics that can potentially serve as a well-defined catalytic template were reported. Rhombic rod shaped foldectures with six facets were obtained by the aqueous self-assembly of helical $\beta$ -peptide foldamers with a C-terminal carboxylic acid. An analysis of the molecular packing by X-ray diffraction revealed that carboxylic acid groups were exposed exclusively on the two (001) rhombic facets due to antiparallel packing of the helical peptides. A surface energy calculation by molecular dynamics simulation was performed to provide a plausible explanation for the development of anisotropy during foldecture formation. The expected facet-selective surface properties of the foldecture were experimentally confirmed by selective deposition of metal nanoparticles on the (001) facets, leading to a new class of sequentially constructed, heterogeneous “foldecture core” materials.