Design, Synthesis, and Structural Analysis of Turn Modified <i>cyclo</i>-(αβ³αβ²α)₂ Peptide Derivatives toward Crystalline Hexagon-Shaped Cationic Nanochannel Assemblies

Intrigued by the not fully extended hairpin monomeric structure of the previously reported cyclo-(αβ3αβ2α)2 peptide 2, as well as its nanochannel crystallographic assembly, we here investigate the structural properties of a series of turn derivatives (3–17). Five crystallographic monomeric structures of the symmetric and asymmetric cyclo-(αβ3αβ2α)2 peptides 3–17 are found; the novel saddlelike (4 and 16) and the twisted hairpin (6) conformers, as well as the not fully extended hairpins 7 and 14. The pentafluorophenyl/1-naphthyl and pentafluorophenyl/9-phenanthryl derivatives 7 and 14, respectively, adopt the anticipated hexagon-shaped crystalline nanotube assemblies, resembling the crystal packing of the parent peptide 2. The structural analysis of the compounds as described here can serve as a basis for biological applications, such as the design of β-sheet mimics or for the development of functional nanomaterials.