Active Transport in Rotaxanes

Every biology student learns about active versus passive transport in the context of transport of materials across cell membranes. The primary workhorse of biological active transport is ATP–the exothermicity of phosphate transfer drives the unfavorable movement of molecules across a cell membrane. Biology textbooks, in true biological fashion, represent this process with a bunch of boxes, circles, and pretty yellow starbursts. Stinkin’ biologists…

Rotaxane chemists have been attempting to harness the power of active transport for a few years now to create linear molecular motors, which use the energy of a chemical reaction to drive the movement of a wheel linearly down an axle. In 2006, David Leigh’s group reported the first example of a “molecular information ratchet,” in which the light-driven movement of the rotaxane wheel to a new position alters the kinetic barrier to movement back to the old position. In essence, Leigh’s rotaxane is an example of Maxwell’s Demon, driving a 50-50 equilibrium mixture of bistable rotaxanes to approximately a 70-30 distribution by kinetically “trapping” the molecules with the ring bound to one position versus the other. One can imagining polymerizing systems like this to create a linear motor. Because the movement is light driven, this is an example of active transport.

The elephant in the room with Leigh’s work is the 70-30 figure. The motor can be at best 70% efficient, and its efficiency should decrease exponentially as more stages are added. This problem has been solved in a new system designed by Makita and coworkers that exhibits essentially quantitative active transport. The Makita polymeric system consists of neopentyl end caps, an ammonium docking site (importantly, offset from the center of the monomer), acyl-protected amines on adjacent monomers, and a crown ether wheel. Treatment of the rotaxane with an amine base deprotonates the ammonium and drives the wheel toward the center of the rotaxane monomer, which is on the side of the distal cap (the “long end”). Super-fast acylation then traps the wheel on the long end. Deacylation and protonation of the next monomer drive the wheel past the short end of the next monomer and onto its newly formed ammonium group. Thus, the chemical energy of deprotonation and acylation push the wheel down the axle. By alternating acyl protecting groups, the wheel can be driven down the rotaxane in essentially quantitative yield!

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