Dynamic chemical devices: Generation of reversible extension/contraction molecular motion by ion-triggered single/double helix interconversion

The polyheterocyclic strands 1-H and 2-H adopt a helical shape enforced by the pyridine-pyrimidine helicity codon. The crystal structure of 2-H shows the formation of stacks of dimers of right- and left-handed individual helices. Treatment of 1-H and 2-H with silver triflate results in the generation of double-helical entities 1-DH and 2-DH, containing two strands and two silver ions. NMR studies and determination of the crystal structure of 2-DH indicate that the duplex is stabilized by coordination of each Ag+ ion to two terminal bipyridine units, one from each strand, and by pronounced pi - pi stacking interactions between the internal heterocycles of the strands, yielding a very robust double helical structure. Reversible interconversion of the single and double helix may be achieved by addition of a cryptand capable of sequestering Ag+ and releasing it by protonation. Thus, successive addition of acid and base leads to reversible inter-conversion between the shorter (similar to3.6 Angstrom) single helix and the longer (similar to 10.3 Angstrom) double helix, resulting in the generation of pronounced extension/contraction motion. The system 1,2-H/1,2-DH represents a dynamic chemical device undergoing ionic modulation of reversible molecular mechanical motion fueled by acid/base neutralization.