Allosteric Regulation of Unidirectional Spring-like Motion of Double-Stranded Helicates

We report the unprecedented allosteric regulation of the extension and contraction motions of double-stranded spiroborate helicates composed of 4,4′-linked 2,2′-bipyridine (bpy) and its N,N′-dioxide units in the middle of ortho-linked tetraphenol strands. NMR and circular dichroism measurements and an X-ray crystallographic analysis along with theoretical calculations revealed that enantiomeric helicates contract and extend upon the binding and release of protons and/or metal ions at the covalently linked two binding bpy or N,N′-dioxide moieties without racemization, respectively, regulated by a cooperative anti–syn conformational change of the two bpy or N,N′-dioxide moieties. These anti–syn conformational changes that occurred at the linkages are amplified into a large-scale molecular motion of the helicates leading to reversible spring-like motions coupled with twisting in one direction in a highly homotropic allosteric fashion.