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.