Interconversion of single and double helices formed from synthetic molecular strands

Synthetic single-helical conformations are quite common, but the formation of double helices based on recognition between the two constituent strands is relatively rare. Known examples include duplex formation through base-pa ir-specific hydrogen bonding and stacking, as found in nucleic acids and th eir analogues, and polypeptides composed of amino acids with alternating L and D configurations(1,2). Some synthetic polymers(3) and self-assembled fi bres(4) have double-helical winding induced by van der Waals interactions. A third mode of non-covalent interaction, coordination of organic ligands t o metal ions(5-7), can give rise to double, triple and quadruple helices, a lthough in this case the assembly is driven by the coordination geometry of the metal and the structure of the ligands, rather than by direct inter-st rand complementarity. Here we describe a family of oligomeric molecules wit h bent conformations, which exhibit dynamic exchange between single and dou ble molecular helices in solution, through spiral sliding of the synthetic oligomer strands. The bent conformations leading to the helical shape of th e molecules result from intramolecular hydrogen bonding within 2'-pyridyl-2 -pyridinecarboxamide units(8-12), with extensive intermolecular aromatic st acking stabilizing the double-stranded helices that form through dimerizati on.