Twist–Turn–Twist Motif Chaperoned Inside Molecular Baskets

Two molecular baskets 1 6–, each with three (S)-glutamic acids at its rim, were found (NMR, ITC) to complex diammonium alkanes 2 2+ –5 2+ (+H3N­(CH2) n NH3 +, n = 7–10) giving ternary [2–5⊂1 2]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 1 6–, 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 1 6– capped [2–5⊂1]4– to give [2–5⊂1 2]10–, via interactions of the carboxylates at the rims with ammoniums on the guest. Long-range NOEs revealed that U-shaped 2 2+ and 5 2+ 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.