Unraveling Unidirectional Threading of α-Cyclodextrin in a [2]Rotaxane through Spin Labeling Approach

We present here the results of a CW-ESR investigation of a double spin labeled alfa-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled alfa-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on-off by changing the pH of the solution in the presence of a competing macrocyclic host.