An α-Helically Folded α-Aminoisobutyric Acid (Aib) Oligomer That Assembles into a Metal-Peptide Superhelical Nanotube

α,α-Disubstituted α-amino acids such as α-aminoisobutyric acid (Aib), in their polymeric structures, are known to form a 3₁₀-helical conformation rather than an α-helical conformation, which is usually adopted by polymeric α-monosubstituted α-amino acids. Even α-helically folded Aib oligomers are unprecedented, although they have been predicted by theoretical calculations. In the present paper, we report the first α-helically folded Aib oligomer found in the course of our study on the construction of a metal-peptide framework, ^AibMOF-1. This MOF was synthesized by Zn²⁺-mediated complexation of a pyridyl-functionalized Aib hexamer, Py-Aib₆-Py, and 5-nitroisophthalate (nip^2-). Single crystal X-ray diffraction of ^AibMOF-1 ([Zn(nip)(Py-Aib₆-Py)]_n) revealed that Py-Aib₆-Py in ^AibMOF-1 carried a C═O_i → NH_i+4 hydrogen bonding array characteristic of α-helices, which is distinct from Py-Aib₆-Py alone adopting a 3₁₀-helical conformation with a C═O_i → NH_i+3 hydrogen bonding array. The α-helical Py-Aib₆-Py units in ^AibMOF-1 assembled into a superhelical nanotubular architecture with a porous framework. When just the nitro group of the coligand nip^2- was changed to a tert-butyl group, an MOF (^AibMOF-2) with a completely different structure formed, where the constituent Py-Aib₆-Py units adopted only the 3₁₀-helical conformation, just like Py-Aib₆-Py in its crystalline structure.