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 (nip2–). 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Oi → NHi+4 hydrogen bonding array characteristic
of α-helices, which is distinct from Py-Aib6-Py alone
adopting a 310-helical conformation with a COi → NHi+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 nip2– 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.