Toward a translational molecular ratchet: face-selective translation coincident with deuteration in a pseudo-rotaxane

In the molecular world, molecular ratchets can realize the unidirectional movement in molecular machines. However, construction of artificial molecular ratchets has been still a great challenge. In this study, we investigate the formation of pseudo-rotaxane of a newly designed two-station axis molecule with α-cyclodextrin (α-CD) and the deuteration of acidic protons in the axis in D(2)O by (1)H NMR at varying temperatures. Using the NMR data, we roughly estimate apparent rate constants for association, dissociation, and translation of α-CD during the pseudo-rotaxane formation based on a simplified kinetic model. These rate constants are indicative of face-selective and ratchet-like translation of α-CD on the axis because of the 2-methylpyridinium residues in the axis. We also evaluate apparent first-order rate constants for the deuteration. Comparison of these rate constants indicates that the face-selective translation of α-CD somehow couples with the deuteration. On the basis of this study, it is concluded that a translational molecular ratchet can be constructed using a large energy gradient with appropriate energy barriers and an enthalpically-driven coupled reaction.