Twist–Turn–Twist Motif Chaperoned Inside Molecular Baskets

Two
molecular baskets 16–, each
with three (S)-glutamic acids at its
rim, were found (NMR, ITC) to complex diammonium alkanes 22+–52+ (+H3N­(CH2)nNH3+, n = 7–10) giving
ternary [2–5⊂12]10– assemblies (K = 107–109 M–2). From the magnetic perturbation of proton nuclei (1H
NMR, NICS), we deduced that each guest assumed a U shape within the
binary complex, [2–5⊂1]4–. Two ammonium groups were bound in
the “anionic nest” at the top of 16–, while the hydrocarbon chain resided in its nonpolar
cavity. From detailed ITC analyses, we showed that the binary complex
[2–5⊂1]4– forms first and then another 16– capped
[2–5⊂1]4– to give [2–5⊂12]10–, via interactions of the carboxylates
at the rims with ammoniums on the guest. Long-range NOEs revealed
that U-shaped 22+ and 52+ coiled into highly strained twist–turn–twist formations,
for the first time observed within an abiotic host while curiously
resembling helix–turn–helix motif found in DNA binding
proteins.