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Remote electrochemical modulation of pKa in a rotaxane by co-conformational allostery

Significance Rotaxanes are species in which a macrocyclic molecule—the ring—is interlocked with a dumbbell-shaped component—the axle. The translational motion of the ring along the axle provides the basis for constructing molecular machinery. In this paper we show that such a dynamic process enables the transfer of chemical information between two distant sites; as a result, the acidity of one site can be reversibly modulated by redox switching at the other site. Possibilities emerge not only for the rational design of species with tailor-made acid–base properties but also for the development of model systems to understand some of Nature’s most effective regulatory mechanisms—namely, allostery and proton-coupled electron transfer. Allosteric control, one of Nature’s most effective ways to regulate functions in biomolecular machinery, involves the transfer of information between distant sites. The mechanistic details of such a transfer are still an object of intensive investigation and debate, and the idea that intramolecular communication could be enabled by dynamic processes is gaining attention as a complement to traditional explanations. Mechanically interlocked molecules, owing to the particular kind of connection between their components and the resulting dynamic behavior, are attractive systems to investigate allosteric mechanisms and exploit them to develop functionalities with artificial species. We show that the pKa of an ammonium site located on the axle component of a [2]rotaxane can be reversibly modulated by changing the affinity of a remote recognition site for the interlocked crown ether ring through electrochemical stimulation. The use of a reversible ternary redox switch enables us to set the pKa to three different values, encompassing more than seven units. Our results demonstrate that in the axle the two sites do not communicate, and that in the rotaxane the transfer of information between them is made possible by the shuttling of the ring, that is, by a dynamic intramolecular process. The investigated coupling of electron- and proton-transfer reactions is reminiscent of the operation of the protein complex I of the respiratory chain.