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 3-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, MOF-1. This MOF was synthesized by Zn-mediated complexation of a pyridyl-functionalized Aib hexamer, Py-Aib-Py, and 5-nitroisophthalate (nip). Single crystal X-ray diffraction of MOF-1 ([Zn(nip)(Py-Aib-Py)]) revealed that Py-Aib-Py in MOF-1 carried a C═O → NH hydrogen bonding array characteristic of α-helices, which is distinct from Py-Aib-Py alone adopting a 3-helical conformation with a C═O → NH hydrogen bonding array. The α-helical Py-Aib-Py units in MOF-1 assembled into a superhelical nanotubular architecture with a porous framework. When just the nitro group of the coligand nip was changed to a -butyl group, an MOF (MOF-2) with a completely different structure formed, where the constituent Py-Aib-Py units adopted only the 3-helical conformation, just like Py-Aib-Py in its crystalline structure.