Chirality Induction and Protonation‐Induced Molecular Motions in Helical Molecular Strands

The long oligopyridinedicarboxamide strand 9 , containing 15 heterocyclic rings has been synthesized and its helical structure determined by X‐ray crystallography. It was shown that the shorter analogue 6 displays induced circular dichroism and amplification of induced chirality upon dissolution in an optically active solvent, diethyl‐ L ‐tartrate. A novel class of helical foldamers was prepared, strands 14 – 16 , based on two oligopyridine carboxamide segments linked through a L ‐tartaric acid derived spacer. These tartro strands display internal chirality induction as well as chirality amplification. NMR spectroscopy (on 8 and 9 ) and circular dichroism (on 16 ) studies show that the oligopyridine carboxamide strands undergo reversible unfolding/folding upon protonation. The protonation‐induced unfolding has been confirmed by X‐ray crystallographic determination of the molecular structure of the extended protonated heptameric form 8 + . The molecular‐scale mechano‐chemical motions of the protonation‐induced structural switching consist of a change of the length of the molecule, from 6 Å ( 6 , coiled form) to 29 Å ( 8 + , uncoiled form) for the heptamer and from 12.5 Å ( 9 , coiled form, X‐ray structure) to 57 Å ( 9 + , uncoiled form, from modeling) for the pentadecamer. Similar unfolding/folding motional processes take place in the L ‐tartro strands 15 and 16 upon protonation/deprotonation, with loss of helicity‐induced circular dichroism on unfolding as shown for the protonated form 16 + .