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.