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 2²⁺-5²⁺ (⁺H₃N(CH₂)NH₃⁺, 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 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⊂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.