A three-compartment chemically-driven molecular information ratchet

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Switched "On" Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site, and then dethreading), with the onset of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes.

A chemical reaction network implementation of a Maxwell demon

We study an autonomous model of a Maxwell demon that works by rectifying thermal fluctuations of chemical reactions. It constitutes the chemical analog of a recently studied electronic demon. We characterize its scaling behavior in the macroscopic limit, its performances, and the impact of potential internal delays. We obtain analytical expressions for all quantities of interest: the generated reverse chemical current, the output power, the transduction efficiency, and correlation between the number of molecules. Due to a bound on the nonequilibrium response of its chemical reaction network, we find that, contrary to the electronic case, there is no way for the Maxwell demon to generate a finite output in the macroscopic limit. Finally, we analyze the information thermodynamics of the Maxwell demon from a bipartite perspective. In the limit of a fast demon, the information flow is obtained, its pattern in the state space is discussed, and the behavior of partial efficiencies related to the measurement and feedback processes is examined.

Mechanically axially chiral catenanes and noncanonical mechanically axially chiral rotaxanes

Chirality typically arises in molecules because of a rigidly chiral arrangement of covalently bonded atoms. Less generally appreciated is that chirality can arise when molecules are threaded through one another to create a mechanical bond. For example, when two macrocycles with chemically distinct faces are joined to form a catenane, the structure is chiral, although the rings themselves are not. However, enantiopure mechanically axially chiral catenanes in which the mechanical bond provides the sole source of stereochemistry have not been reported. Here we re-examine the symmetry properties of these molecules and in doing so identify a straightforward route to access them from simple chiral building blocks. Our analysis also led us to identify an analogous but previously unremarked upon rotaxane stereogenic unit, which also yielded to our co-conformational auxiliary approach. With methods to access mechanically axially chiral molecules in hand, their properties and applications can now be explored.

A Co-conformationally "Topologically" Chiral Catenane

Catenanes composed of two achiral rings that are oriented (Cnh symmetry) because of the sequence of atoms they contain are referred to as topologically chiral. Here, we present the synthesis of a highly enantioenriched catenane containing a related but overlooked "co-conformationally 'topologically' chiral" stereogenic unit, which arises when a bilaterally symmetric Cnv ring is desymmetrized by the position of an oriented macrocycle.

Chemical fuels for molecular machinery

Chemical reaction networks that transform out-of-equilibrium 'fuel' to 'waste' are the engines that power the biomolecular machinery of the cell. Inspired by such systems, autonomous artificial molecular machinery is being developed that functions by catalysing the decomposition of chemical fuels, exploiting kinetic asymmetry to harness energy released from the fuel-to-waste reaction to drive non-equilibrium structures and dynamics. Different aspects of chemical fuels profoundly influence their ability to power molecular machines. Here we consider the structure and properties of the fuels that biology has evolved and compare their features with those of the rudimentary synthetic chemical fuels that have so far been used to drive autonomous non-equilibrium molecular-level dynamics. We identify desirable, but context-specific, traits for chemical fuels together with challenges and opportunities for the design and invention of new chemical fuels to power synthetic molecular machinery and other dissipative nanoscale processes.

Insights from an information thermodynamics analysis of a synthetic molecular motor

Information is physical, a realization that has transformed the physics of measurement and communication. However, the flow between information, energy and mechanics in chemical systems remains largely unexplored. Here we analyse a minimalist autonomous chemically driven molecular motor in terms of information thermodynamics, a framework that quantitatively relates information to other thermodynamic parameters. The treatment reveals how directional motion is generated by free energy transfer from chemical to mechanical (conformational and/or co-conformational) processes by 'energy flow' and 'information flow'. It provides a thermodynamic level of understanding of molecular motors that is general, complements previous analyses based on kinetics and has practical implications for machine design. In line with kinetic analysis, we find that power strokes do not affect the directionality of chemically driven machines. However, we find that power strokes can modulate motor velocity, the efficiency of free energy transfer and the number of fuel molecules consumed per cycle. This may help explain the role of such (co-)conformational changes in biomachines and illustrates the interplay between energy and information in chemical systems.

Chemical engines: driving systems away from equilibrium through catalyst reaction cycles

Biological systems exhibit a range of complex functions at the micro- and nanoscales under non-equilibrium conditions (for example, transportation and motility, temporal control, information processing and so on). Chemists also employ out-of-equilibrium systems, for example in kinetic selection during catalysis, self-replication, dissipative self-assembly and synthetic molecular machinery, and in the form of chemical oscillators. Key to non-equilibrium behaviour are the mechanisms through which systems are able to extract energy from the chemical reactants ('fuel') that drive such processes. In this Perspective we relate different examples of such powering mechanisms using a common conceptual framework. We discuss how reaction cycles can be coupled to other dynamic processes through positive (acceleration) or negative (inhibition) catalysis to provide the thermodynamic impetus for diverse non-equilibrium behaviour, in effect acting as a 'chemical engine'. We explore the way in which the energy released from reaction cycles is harnessed through kinetic selection in a series of what have sometimes been considered somewhat disparate fields (systems chemistry, molecular machinery, dissipative assembly and chemical oscillators), highlight common mechanistic principles and the potential for the synchronization of chemical reaction cycles, and identify future challenges for the invention and application of non-equilibrium systems. Explicit recognition of the use of fuelling reactions to power structural change in catalysts may stimulate the investigation of known catalytic cycles as potential elements for chemical engines, a currently unexplored area of catalysis research.

Interplay between a Foldamer Helix and a Macrocycle in a Foldarotaxane Architecture

The design and synthesis of a novel rotaxane/foldaxane hybrid architecture is reported. The winding of an aromatic oligoamide helix host around a dumbbell-shaped thread-like guest, or axle, already surrounded by a macrocycle was evidenced by NMR spectroscopy and X-ray crystallography. The process proved to depend on the position of the macrocycle along the axle and the associated steric hindrance. The macrocycle thus behaves as a switchable shield that modulates the affinity of the helix for the axle. Reciprocally, the foldamer helix acts as a supramolecular auxiliary that compartmentalizes the axle. In some cases, the macrocycle is forced to move along the axle to allow the foldamer to reach its best recognition site.

Mechanical bonding activation in rotaxane-based organocatalysts

Interlocked organocatalysts show enhanced catalytic performance when compared with their non-interlocked threads.The ring cooperatively activates the substrates, facilitating the formation and stabilization of catalytically active intermediates.

Selective Monofunctionalization Enabled by Reaction-History-Dependent Communication in Catalytic Rotaxanes

Selective monofunctionalization of substrates with distant, yet equally reactive functional groups is difficult to achieve, as it requires the second functional group to selectively modulate its reactivity once the first functional group has reacted. We now show that mechanically interlocked catalytic rings can effectively regulate the reactivity of stoppering groups in rotaxanes over a distance of about 2 nm. Our mechanism of communication is enabled by a unique interlocked design, which effectively removes the catalytic rings from the substrates by fast dethreading as soon as the first reaction has taken place. Our method not only led to a rare example of selective monofunctionalization, but also to a "molecular if function". Overall, the study presents a way to get distant functional groups to communicate with each other in a reaction-history-dependent manner by creating linkers that can ultimately perform logical operations at the molecular level.

Cycle Completion Times Probe Interactions with Environment

Recent measurements of the durations of nonequilibrium processes provide valuable information on microscopic mechanisms and energetics. Theory for corresponding experiments to date is well-developed for single-particle systems only. Little is known for interacting systems in nonequilibrium environments. Here we introduce and study a basic model for cycle processes interacting with an environment that can exhibit a net particle flow. We find a surprising richness of cycle time variations with environmental conditions. This manifests itself in unequal cycle times τ⁺ and τ^- in forward and backward cycle directions with both asymmetries τ^- < τ⁺ and τ^- > τ⁺, speeding up of backward cycles by interactions, and dynamical phase transitions, where cycle times become multimodal functions of the bias. The model allows us to relate these effects to specific microscopic mechanisms, which can be helpful for interpreting experiments.

Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane

We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely, an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state coconformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF²⁺ dication, the CBPQT⁴⁺ ring is forced toward the molecular rotor, leading to an increased energy barrier for the excited state to rotate the rotor into the state with a high nonradiative rate constant, resulting in an overall 3.4-fold fluorescence enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge-transfer state between the BODIPY rotor and the CBPQT⁴⁺ ring, movement of the ring toward the BODIPY rotor produces an unexpectedly strong fluorescence signal decrease as the result of photoinduced electron transfer from the BODIPY rotor to the CBPQT⁴⁺ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories, and molecular logic gates.

Synthesis of a Mechanically Planar Chiral Rotaxane Ligand for Enantioselective Catalysis

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed "mechanical chirality," have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction.

A [2]Rotaxane-Based Circularly Polarized Luminescence Switch

A rotaxane-based molecular shuttle has been synthesized in which the switching of the position of a fluorescent macrocycle on the thread turns “on” or “off” the circularly polarized luminescence (CPL) of the system while maintaining similar fluorescence profiles and quantum yields in both states. The chiroptical activity relies on the chiral information transfer from an ammonium salt incorporating d- or l-phenylalanine residues as chiral stereogenic covalent units to an otherwise achiral crown ether macrocycle bearing a luminescent 2,2′-bipyrene unit when they interact through hydrogen bonding. Each enantiomeric thread induces CPL responses of opposite signs on the macrocycle. Upon addition of base, the switching of the position of the macrocycle to a triazolium group disables the chiral information transfer to the macrocycle, switching “off” the CPL response. The in situ switching upon several acid/base cycles is also demonstrated.

Dynamic Control of Chiral Space Through Local Symmetry Breaking in a Rotaxane Organocatalyst

We report on a switchable rotaxane molecular shuttle that features a pseudo-meso 2,5-disubstituted pyrrolidine catalytic unit on the axle whose local symmetry is broken according to the position of a threaded benzylic amide macrocycle. The macrocycle can be selectively switched (with light in one direction; with catalytic acid in the other) with high fidelity between binding sites located to either side of the pyrrolidine unit. The position of the macrocycle dictates the facial bias of the rotaxane-catalyzed conjugate addition of aldehydes to vinyl sulfones. The pseudo-meso non-interlocked thread does not afford significant selectivity as a catalyst (2-14 % ee), whereas the rotaxane affords selectivities of up to 40 % ee with switching of the position of the macrocycle changing the handedness of the product formed (up to 60 % Δee).

Kinetic asymmetry allows macromolecular catalysts to drive an information ratchet

Molecular machines carry out their function by equilibrium mechanical motions in environments that are far from thermodynamic equilibrium. The mechanically equilibrated character of the trajectories of the macromolecule has allowed development of a powerful theoretical description, reminiscent of Onsager’s trajectory thermodynamics, that is based on the principle of microscopic reversibility. Unlike the situation at thermodynamic equilibrium, kinetic parameters play a dominant role in determining steady-state concentrations away from thermodynamic equilibrium, and kinetic asymmetry provides a mechanism by which chemical free-energy released by catalysis can drive directed motion, molecular adaptation, and self-assembly. Several examples drawn from the recent literature, including a catenane-based chemically driven molecular rotor and a synthetic molecular assembler or pump, are discussed.

The mechanism by which macromolecular catalysts use energy from exergonic reactions to move, adapt, and assemble has been unclear. In this Perspective article, R. Dean Astumian shows that in addition to disequilibrium of the catalyzed reaction, kinetic asymmetry is the essential feature required to drive non-equilibrium response by an information ratchet mechanism.

A Redox Strategy for Light-Driven, Out-of-Equilibrium Isomerizations and Application to Catalytic C-C Bond Cleavage Reactions

We report a general protocol for the light-driven isomerization of cyclic aliphatic alcohols to linear carbonyl compounds. These reactions proceed via proton-coupled electron-transfer activation of alcohol O-H bonds followed by subsequent C-C β-scission of the resulting alkoxy radical intermediates. In many cases, these redox-neutral isomerizations proceed in opposition to a significant energetic gradient, yielding products that are less thermodynamically stable than the starting materials. A mechanism is presented to rationalize this out-of-equilibrium behavior that may serve as a model for the design of other contrathermodynamic transformations driven by excited-state redox events.

Rotational isomerism of the amide units in rotaxanes based on a cyclic tetraamide and secondary ammonium ions

We describe the synthesis of [2]rotaxanes consisting of a macrocyclic tetraamide and mono- and bis-ammonium ions and their conformational isomerism.

Stereoselective Synthesis of Mechanically Planar Chiral Rotaxanes

Chiral interlocked molecules in which the mechanical bond provides the sole stereogenic unit are typically produced with no control over the mechanical stereochemistry. Here we report a stereoselective approach to mechanically planar chiral rotaxanes in up to 98:2 d.r. using a readily available α-amino acid-derived azide. Symmetrization of the covalent stereocenter yields a rotaxane in which the mechanical bond provides the only stereogenic element.

Chirality in rotaxanes and catenanes

Although chiral mechanically interlocked molecules (MIMs) have been synthesised and studied, enantiopure examples are relatively under-represented in the pantheon of reported catenanes and rotaxanes and the underlying chirality of the system is often even overlooked. This is changing with the advent of new applications of MIMs in catalysis, sensing and materials and the appearance of new methods to access unusual stereogenic units unique to the mechanical bond. Here we discuss the different stereogenic units that have been investigated in catenanes and rotaxanes, examples of their application, methods for assigning absolute stereochemistry and provide a perspective on future developments.

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 D₂O by ¹H 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.

Efficient Multicomponent Active Template Synthesis of Catenanes

We describe a simple and high yielding active template synthesis of [2]catenanes. In addition to mechanical bond formation using a single premacrocycle bearing an azide and alkyne moiety, our method is also suitable for the co-macrocyclization of readily available bis-alkyne and bis-azide comonomers and even short alkyne/azide components which oligomerize prior to mechanical bond formation.

Stochastically pumped adaptation and directional motion of molecular machines

Recent developments in synthetic molecular motors and pumps have sprung from a remarkable confluence of experiment and theory. Synthetic accomplishments have facilitated the ability to design and create molecules, many of them featuring mechanically bonded components, to carry out specific functions in their environment-walking along a polymeric track, unidirectional circling of one ring about another, synthesizing stereoisomers according to an external protocol, or pumping rings onto a long rod-like molecule to form and maintain high-energy, complex, nonequilibrium structures from simpler antecedents. Progress in the theory of nanoscale stochastic thermodynamics, specifically the generalization and extension of the principle of microscopic reversibility to the single-molecule regime, has enhanced the understanding of the design requirements for achieving strong unidirectional motion and high efficiency of these synthetic molecular machines for harnessing energy from external fluctuations to carry out mechanical and/or chemical functions in their environment. A key insight is that the interaction between the fluctuations and the transition state energies plays a central role in determining the steady-state concentrations. Kinetic asymmetry, a requirement for stochastic adaptation, occurs when there is an imbalance in the effect of the fluctuations on the forward and reverse rate constants. Because of strong viscosity, the motions of the machine can be viewed as mechanical equilibrium processes where mechanical resonances are simply impossible but where the probability distributions for the state occupancies and trajectories are very different from those that would be expected at thermodynamic equilibrium.

Spacer Length-Independent Shuttling of the Pillar[5]arene Ring in Neutral [2]Rotaxanes

For a series of neutral [2]rotaxanes consisting of a pillar[5]arene ring and axles possessing two stations separated by flexible spacers of different lengths, the free energies of activation for the ring shuttling between the stations were found to be independent of the spacer length. The constitution of the spacer affects the activation energies: replacement of CH₂ groups by repulsive oxygen atoms in the axle increases the barrier. The explanation for the observed length‐independence lies in the presence of a barrier for re‐forming the stable co‐conformation, which makes the ring travel back and forth along the thread in an intermediate state.

Mastering the non-equilibrium assembly and operation of molecular machines

In mechanically interlocked compounds, such as rotaxanes and catenanes, the molecules are held together by mechanical rather than chemical bonds. These compounds can be engineered to have several well-defined mechanical states by incorporating recognition sites between the different components. The rates of the transitions between the recognition sites can be controlled by introducing steric "speed bumps" or electrostatically switchable gates. A mechanism for the absorption of energy can also be included by adding photoactive, catalytically active, or redox-active recognition sites, or even charges and dipoles. At equilibrium, these Mechanically Interlocked Molecules (MIMs) undergo thermally activated transitions continuously between their different mechanical states where every transition is as likely as its microscopic reverse. External energy, for example, light, external modulation of the chemical and/or physical environment or catalysis of an exergonic reaction, drives the system away from equilibrium. The absorption of energy from these processes can be used to favour some, and suppress other, transitions so that completion of a mechanical cycle in a direction in which work is done on the environment - the requisite of a molecular machine - is more likely than completion in a direction in which work is absorbed from the environment. In this Tutorial Review, we discuss the different design principles by which molecular machines can be engineered to use different sources of energy to carry out self-organization and the performance of work in their environments.

Molecular machines with bio-inspired mechanisms

The widespread use of molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular machines-which by and large function as switches-and the machines of the macroscopic world, which utilize the synchronized behavior of integrated components to perform more sophisticated tasks than is possible with any individual switch. Should we try to make molecular machines of greater complexity by trying to mimic machines from the macroscopic world or instead apply unfamiliar (and no doubt have to discover or invent currently unknown) mechanisms utilized by biological machines? Here we try to answer that question by exploring some of the advances made to date using bio-inspired machine mechanisms.

Light-driven exchange between extended and contracted lasso-like isomers of a bistable [1]rotaxane

A photoactive hydrogen-bonded lasso having an amide-based [1]rotaxane structure has been constructed from acyclic precursors through a self-templating approach. The stability, structural integrity and switching are described.

Enantioselective Anion Recognition by Chiral Halogen-Bonding [2]Rotaxanes

The application of chiral interlocked host molecules for discrimination of guest enantiomers has been largely overlooked, which is surprising given their unique three-dimensional binding cavities capable of guest encapsulation. Herein, we combined the stringent linear geometric interaction constraints of halogen bonding (XB), the noncovalent interaction between an electrophilic halogen atom and a Lewis base, with highly preorganized and conformationally restricted chiral cavities of [2]rotaxanes to achieve enantioselective anion recognition. Representing the first detailed investigation of the use of chiral XB rotaxanes for this purpose, extensive ¹H NMR binding studies and molecular dynamics (MD) simulation experiments revealed that the chiral rotaxane cavity significantly enhances enantiodiscrimination compared to the non-interlocked free axle and macrocycle components. Furthermore, by examining the enantioselectivities of a family of structurally similar XB [2]rotaxanes containing different combinations of chiral and achiral macrocycle and axle components, the dominant influence of the chiral macrocycle in our rotaxane design for determining the effectiveness of chiral discrimination is demonstrated. MD simulations reveal the crucial geometric roles played by the XB interactions in orientating the bound enantiomeric anion guests for chiral selectivity, as well as the critical importance of the anions' hydration shells in governing binding affinity and enantiodiscrimination.

Enzyme-Mediated Directional Transport of a Small-Molecule Walker With Chemically Identical Feet

We describe a small-molecule "walker" that uses enzyme catalysis to discriminate between the relative positions of its "feet" on a track and thereby move with net directionality. The bipedal walker has identical carboxylic acid feet, and "steps" along an isotactic hydroxyl-group-derivatized polyether track by the formation/breakage of ester linkages. Lipase AS catalyzes the selective hydrolysis of the rear foot of macrocyclized walkers (an information ratchet mechanism), the rear foot producing an (R)-stereocenter at its point of attachment to the track. If the hydrolyzed foot reattaches to the track in front of the bound foot it forms an (S)-stereocenter, which is resistant to enzymatic hydrolysis. Only macrocyclic walker-track conjugates are efficiently hydrolyzed by the enzyme, leading to high processivity of the walker movement along the track. Conventional chemical reagents promote formation of the ester bonds between the walker and the track. Iterative macrocyclization and hydrolysis reactions lead to 68% of walkers taking two steps directionally along a three-foothold track.