Twist-Turn-Twist Motif Chaperoned Inside Molecular Baskets

摘要

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 subset of 1(2)](10- )assemblies (K = 10(7)-10(9) M-2). From the magnetic perturbation of proton nuclei (H-1 NMR, NICS), we deduced that each guest assumed a U shape within the binary complex, [2-5 subset of 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 subset of 1](4-) forms first and then another 1(6- )capped [2-5 subset of 1](4-) to give [2-5 subset of 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.