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Protonation-induced transition between two distinct helical conformations of a synthetic oligomer via a linear intermediate.

in these so-called molecular machines is fueled by their relation to important natural phenomena and by the prospect of elaborating useful chemical devices. Molecular motions can be triggered by various means (e.g., ion binding, electron transfer, and light excitation) and can often be reverted by simply removing or reversing the triggering stimuli. [2] Here, we present an original system in which a large conformational change caused by protonation is formally reverted not only upon deprotonation but also upon further protonation: helical oligopyridine–dicarboxamide strands unfold to an extended linear conformation upon partial protonation, [3] which upon full protonation refolds to a helix related to (though distinct from) the initial state. Oligoamides prepared from 2,6-pyridinedicarboxylic acids and 2,6-diaminopyridines adopt stable helical conformations in the solid state as well as in solution in organic solvents and in water. [4–6] The bending of the strands into helices arises from the strongly preferred conformations of 2,6-bis(carbonylamino)pyridine units and 2,6-pyridinedicarboxamide units, which are both stabilized by intramolecular hydrogen bonds as in 1 and 2 (Scheme 1). We speculated that protonation at the pyridine nitrogen of these units should affect their conformational preferences and investigated their behavior in the presence of acids of various strengths: CF3CO2H (TFA, pKa = 0), CH3SO3H (MsOH, pKa =2), and CF3SO3H (TfOH, pKa =16). When 1 equiv MsOH is added to a solution of 1 in CDCl3, protonation is shown to occur quantitatively, leading to the conformation shown in Scheme 1, in which the carbonyl groups form hydrogen bonds with the pyridinium N + H and not with H 3