Pumping between phases with a pulsed-fuel molecular ratchet

Abstract

The sorption of species from solution into and onto solids, surfaces, crystals, gels and other matrices, underpins the sequestering of waste and pollutants, the recovery of precious metals, heterogeneous catalysis, many forms of chemical and biological analysis and separation science, and numerous other technologies. [1][2][3] In such cases the transfer of the substrate between phases tends to proceed spontaneously, in the direction of equilibrium. Molecular ratchet mechanisms, 4-7 where kinetic gating 8-12 selectively inhibits or accelerates particular steps in a process, makes it possible to drive dynamic systems 13-16 out of equilibrium [17][18][19][20][21][22] . Here we report on a small-molecule pump 23 immobilised on and near the surface 24-26 of polymer beads, that uses an energy ratchet mechanism [18][19][20][21][22][27][28][29][30][31] to actively transport substrates from solution onto the beads away from equilibrium. One complete cycle of the pump occurs with each pulse of a chemical fuel, 20,32-36 synchronizing the ratchet dynamics so that the immobilised molecular machines all act in unison. Upon addition of the trichloroacetic acid (CCl3CO2H) fuel, 20,35,37,38 micrometre-diameter polystyrene beads functionalised with an average of ~8×10 10 molecular pumps per bead, 39 sequester from solution crown ethers appended with a fluorescent tag (Fig. 1). Following consumption of the fuel, the rings are mechanically trapped in a higher energy, out-of-equilibrium, state on the beads and cannot be removed by dilution nor by switching the binding interactions off. This differs from dissipative assembled materials that require a continuous supply of energy to persist. [13][14][15][16] Addition of a second pulse of fuel causes the uptake of more macrocycles, which can be labelled with a different fluorescent tag. This drives the system progressively further away from equilibrium and also confers sequence information 40 on the deposited structure. The polymer-bound substrates (and the stored energy) can subsequently be released back to the bulk on demand, 41-43 either emptying one compartment at a time or all at once. Non-equilibrium 44 sorption by using immobilised artificial molecular machines 45-51 to pump substrates from solution onto and into materials 52 , offers potential 53,54 for the transduction of energy from chemical fuels for the storage and release of energy and information.

Transamidation-Driven Molecular Pumps

“…This should enable the pump to be used to synthesize oligo- or polyrotaxanes with a single sequence of structurally distinct macrocycles pumped in a specific order. 21 , 29 , 30 , 50 …”

“…This constitutes active transport of the rings from bulk solution to the collection thread. 20 , 21 Accordingly, the pumping needs to be powered and to occur under kinetic control. The chemical structure of the pump is designed to promote macrocycle threading and inhibit dethreading.…”

Control on dissipative kinetics reveals the entropically-dominated essence of interlocked molecular machines

“…The dibenzyl ammonium recognition site is lost if a base is added, resulting in kinetic trapping of the threaded crown ether in a high-energy state. [23,41] In the reported architectures, this phenomenon is coupled with crown ether shuttling over a disulfide-controlled speed bump, resulting in the directional motion of macrocycles along the axle. [23,41] Here, we study the dethreading process from high-energy states in a minimalistic machine powered by acid-base stimuli.…”

“…[23,41] In the reported architectures, this phenomenon is coupled with crown ether shuttling over a disulfide-controlled speed bump, resulting in the directional motion of macrocycles along the axle. [23,41] Here, we study the dethreading process from high-energy states in a minimalistic machine powered by acid-base stimuli. Control on its operation was gained, obtaining kinetically-trapped or dissipative states on-demand, which allowed performing repeated ratcheting cycles.…”

“…[25] To advance the understanding of minimal molecular ratchets, we focused on a versatile molecular pump module developed by Leigh and coworkers, capable to accumulate multiple macrocycles in a high-energy interlocked structure, upon a sequence of acid-base stimuli. [23,41] The operating mechanism in action is based on the self-assembly of a crown ether macrocycle onto a dibenzyl ammonium ion, which is one of the most extensively studied recognition motifs in the domain of molecular machines. [42] A key design element is a benzyl aldehyde terminal in the axle component, which under acidic conditions can react with an hydrazide to form an hydrazone.…”