An autonomous chemically fuelled small-molecule motor

Beyond Single-Cycle Autonomous Molecular Machines: Light-Powered Shuttling in a Multi-Cycle Reaction Network

Biomolecular machines autonomously convert energy into functions, driving systems away from thermodynamic equilibrium. This energy conversion is achieved by leveraging complex, kinetically asymmetric chemical reaction networks that are challenging to characterize precisely. In contrast, all known synthetic molecular systems in which kinetic asymmetry has been quantified are well described by simple single-cycle networks. Here, we report on a unique light-driven [2]rotaxane that enables the autonomous operation of a synthetic molecular machine with a multi-cycle chemical reaction network. Unlike all prior systems, the present one exploits a photoactive macrocycle, which features a different photoreactivity depending on the binding sites at which it resides. Furthermore, E to Z isomerization reverses the relative affinity of the macrocycle for two binding sites on the axle, resulting in a multi-cycle network. Building on the most recent theoretical advancements, this work quantifies kinetic asymmetry in a multi-cycle network for the first time. Our findings represent the simplest rotaxane capable of autonomous shuttling developed so far and offer a general strategy to generate and quantify kinetic asymmetry beyond single-cycle systems.

Bidirectional Molecular Motors by Controlling Threading and Dethreading Pathways of a Linked Rotaxane

Artificial molecular motors have been presented as models for biological molecular motors. In contrast to the conventional artificial molecular motors that rely on covalent bond rotation, molecular motors with mechanically interlocked molecules (MIMs) have attracted considerable attention owing to their ability to generate significant rotational motion by dynamically shuttling macrocyclic components. The topology of MIM-type rotational molecular motors is currently limited to catenane structures, which require intricate synthetic procedures that typically produce a low synthetic yield. In this study, we develop a novel class of MIM-type molecular motors with a rotaxane-type topology. The switching of the threading/dethreading pathways of the linked rotaxane by protecting/deprotecting the bulky stopper group and changing the solvent polarity enables a net unidirectional rotation of the molecular motor. The threading/dethreading reaction rates were quantitatively evaluated through detailed spectroscopic investigations. Repeated net unidirectional rotation and switching of the direction of rotation were also achieved. Our findings demonstrate that linked rotaxanes can serve as MIM-type molecular motors with reversible rotational direction controlled by threading/dethreading reactions. These motors hold potential as components of molecular machinery.

Power Strokes in Molecular Motors: Predictive, Irrelevant, or Somewhere in Between?

For several decades, molecular motor directionality has been rationalized in terms of the free energy of molecular conformations visited before and after the motor takes a step, a so-called power stroke mechanism with analogues in macroscopic engines. Despite theoretical and experimental demonstrations of its flaws, the power stroke language is quite ingrained, and some communities still value power stroke intuition. By building a catalysis-driven motor into simulated numerical experiments, we here systematically report on how directionality responds when the motor is modified accordingly to power stroke intuition. We confirm that the power stroke mechanism generally does not predict motor directionality. Nevertheless, the simulations illustrate that the relative stability of molecular conformations should be included as a potential design element to adjust the motor directional bias. Though power strokes are formally unimportant for determining directionality, we show that practical attempts to alter a power stroke have side effects that can in fact alter the bias. The change in the bias can align with what power stroke intuition would have suggested, offering a potential explanation for why the flawed power stroke mechanism can retain apparent utility when engineering specific systems.

Transducing chemical energy through catalysis by an artificial molecular motor

Cells display a range of mechanical activities generated by motor proteins powered through catalysis1. This raises the fundamental question of how the acceleration of a chemical reaction can enable the energy released from that reaction to be transduced (and, consequently, work to be done) by a molecular catalyst2-7. Here we demonstrate the molecular-level transduction of chemical energy to mechanical force8 in the form of the powered contraction and powered re-expansion of a cross-linked polymer gel driven by the directional rotation of artificial catalysis-driven9 molecular motors. Continuous 360° rotation of the rotor about the stator of the catalysis-driven motor-molecules incorporated in the polymeric framework of the gel twists the polymer chains of the cross-linked network around one another. This progressively increases writhe and tightens entanglements, causing a macroscopic contraction of the gel to approximately 70% of its original volume. The subsequent addition of the opposite enantiomer fuelling system powers the rotation of the motor-molecules in the reverse direction, unwinding the entanglements and causing the gel to re-expand. Continued powered twisting of the strands in the new direction causes the gel to re-contract. In addition to actuation, motor-molecule rotation in the gel produces other chemical and physical outcomes, including changes in the Young modulus and storage modulus-the latter is proportional to the increase in strand crossings resulting from motor rotation. The experimental demonstration of work against a load by a synthetic organocatalyst, and its mechanism of energy transduction6, informs both the debate3,5,7 surrounding the mechanism of force generation by biological motors and the design principles6,10-14 for artificial molecular nanotechnology.

Rotaxane-catalyzed aerobic oxidation of primary alcohols

Nitroxide radicals are widely utilized as catalysts for the oxidation of primary alcohols. Here, the aerobic catalytic oxidation cycle of nitroxide radicals has been implemented within a mechanically interlocked rotaxane architecture consisting of a paramagnetic crown ether, which is confined by a molecular axle containing a dialkylammonium station and a 1,2,3-triazole unit. The rotaxane is engineered to exploit the oxidation of a primary alcohol: the primary catalyst is the wheel, a nitroxide radical capable of altering its oxidation state during the catalytic cycle, while the co-oxidant is the Cerium(IV)/O2 couple. The synthesis of the proposed rotaxane, along with its characterization using EPR, HRMS, voltammetry and NMR data, is reported in the paper. The aerobic catalytic oxidation cycle was further investigated using EPR, NMR and GC-MS analyses. This study can aid in the design of autonomously driven molecular machines that exploit the aerobic catalytic oxidation of nitroxide radicals.

Controlling carbodiimide-driven reaction networks through the reversible formation of pyridine adducts

Carbodiimides and pyridines form reversible adducts that slowly deliver carbodiimide "fuels" to out-of-equilibrium reaction networks, slowing activation kinetics and elongating transient state lifetimes. More-nucleophilic pyridines give more adduct under typical conditions. This approach can be used to extend the lifetimes of transient polymer hydrogels.

Insight from Electrochemical Analysis in the Radical Cation State of a Monopyrrolotetrathiafulvalene-Based [2]Rotaxane

Mechanically interlocked molecules are a class of compounds used for controlling directional movement when barriers can be raised and lowered using external stimuli. Applied voltages can turn on redox states to alter electrostatic barriers but their use for directing motion requires knowledge of their impact on the kinetics. Herein, we make the first measurements on the movement of cyclobis(paraquat-p-phenylene) (CBPQT4+) across the radical-cation state of monopyrrolotetrathiafulvalene (MPTTF) in a [2]rotaxane using variable scan-rate electrochemistry. The [2]rotaxane is designed in a way that directs CBPQT4+ to a high-energy co-conformation upon oxidation of MPTTF to either the radical cation (MPTTF⋅+) or the dication (MPTTF2+). 1H NMR spectroscopic investigations carried out in acetonitrile at 298 K showed direct interconversion to the thermodynamically more stable ground-state co-conformation with CBPQT4+ moving across the oxidized MPTTF2+ electrostatic barrier. The electrochemical studies revealed that interconversion takes place by movement of CBPQT4+ across both the MPTTF•+ (19.3 kcal mol-1) and MPTTF2+ (18.7 kcal mol-1) barriers. The outcome of our studies shows that MPTTF has three accessible redox states that can be used to kinetically control the movement of the ring component in mechanically interlocked molecules.

Directional Bias in Molecular Photogearing Evidenced by LED-Coupled Chiral Cryo-HPLC

Molecular gearing systems are technomimetic nanoscale analogues to complex geared machinery in the macroscopic world. They are defined as systems incorporating intermeshed movable parts which perform correlated rotational motions by mechanical engagement. Only recently, new methods to actively drive molecular gearing motions instead of relying on passive thermal activation have been developed. Further progress in this endeavor will pave the way for unidirectional molecular gearing devices with a distinct type of molecular machine awaiting its realization. Within this work an essential step towards this goal is achieved by evidencing directional biases for the light-induced rotations in our molecular photogear system. Using a custom-designed LED-coupled chiral cryo-HPLC setup for the in situ irradiation of enantiomeric analytes, an intrinsic selectivity for clockwise or counterclockwise rotations was elucidated experimentally. Significant directional biases in the photogearing processes and light-induced single bond rotations (SBRs) are observed for our photogear with directional preferences of up to 4.8 : 1. Harnessing these effects will allow to rationally design and construct a fully directional molecular gearing motor in the future.

Directionality Reversal and Shift of Rotational Axis in a Hemithioindigo Macrocyclic Molecular Motor

Molecular motors are central driving units for nanomachinery, and control of their directional motions is of fundamental importance for their functions. Light-driven variants use easy to provide, easy to dose, and waste-free fuel with high energy content, making them particularly interesting for applications. Typically, light-driven molecular motors work via rotations around dedicated chemical bonds where the directionality of the rotation is dictated by the steric effects of asymmetry in close vicinity to the rotation axis. In this work, we show how unidirectional rotation around a virtual axis can be realized by reprogramming a molecular motor. To this end, a classical light-driven motor is restricted by macrocyclization, and its intrinsic directional rotation is transformed into a directional rotation of the macrocyclic chain in the opposite direction. Further, solvent polarity changes allow to toggle the function of this molecular machine between a directional motor and a nondirectional photoswitch. In this way, a new concept for the design of molecular motors is delivered together with elaborate control over their motions and functions by simple solvent changes. The possibility of sensing the environmental polarity and correspondingly adjusting the directionality of motions opens up a next level of control and responsiveness to light-driven nanoscopic motors.

Repurposing a Catalytic Cycle for Transient Self-Assembly

Life operates out of equilibrium to enable various sophisticated behaviors. Synthetic chemists have strived to mimic biological nonequilibrium systems in such fields as autonomous molecular machines and dissipative self-assembly. Central to these efforts has been the development of new chemical reaction cycles, which drive systems out of equilibrium by conversion of chemical fuel into waste species. However, the construction of reaction cycles has been challenging due to the difficulty of finding compatible reactions that constitute a cycle. Here, we realize an alternative approach by repurposing a known catalytic cycle as a chemical reaction cycle for driving dissipative self-assembly. This approach can overcome the compatibility problem because all steps involved in a catalytic cycle are already known to proceed concurrently under the same conditions. Our repurposing approach is applicable to diverse combinations of catalytic cycles and systems to drive out of equilibrium, which will substantially broaden the scope of out-of-equilibrium systems.

A Light-Powered Single-Stranded DNA Molecular Motor with Colour-Selective Single-Step Control

Top-down control of small motion is possible through top-down controlled molecular motors in replacement of larger actuators like MEMS or NEMS (micro- or nano-electromechanical systems) in the current precision technology. Improving top-down control of molecular motors to every single step is desirable for this purpose, and also for synchronization of motor actions for amplified effects. Here we report a designed single-stranded DNA molecular motor powered by alternated ultraviolet and visible light for processive track-walking, with the two light colours each locking the motor in a full directional step to allow saturated driving but no overstepping. This novel nano-optomechanical driving mechanism pushes the top-down control of molecular motors down to every single step, thus providing a key technical capability to advance the molecular motor-based precision technology and also motor synchronization for amplified effects.

Dynamic control of circumrotation of a [2]catenane by acid-base switching

Dynamic control of the motion in a catenane remains a big challenge as it requires precise design and sophisticated well-organized structures. This paper reports the design and synthesis of a donor-acceptor [2]catenane through mechanical interlocking, employing a crown ether featuring two dibenzylammonium salts on its side arms as the host and a cyclobis(paraquat-p-phenylene) (CBPQT ⋅ 4PF6) ring as the guest molecule. By addition of external acid or base, the catenane can form self-complexed or decomplexed compounds to alter the cavity size of the crown ether ring, consequently affecting circumrotation rate of CBPQT ⋅ 4PF6 ring of the catenane. This study offers insights for the design and exploration of artificial molecular machines with intricate cascading responsive mechanisms.

Transient Covalent Polymers through Carbodiimide-Driven Assembly

Biochemical systems make use of out-of-equilibrium polymers generated under kinetic control. Inspired by these systems, many abiotic supramolecular polymers driven by chemical fuel reactions have been reported. Conversely, polymers based on transient covalent bonds have received little attention, even though they have the potential to complement supramolecular systems by generating transient structures based on stronger bonds and by offering a straightforward tuning of reaction kinetics. In this study, we show that simple aqueous dicarboxylic acids give poly(anhydrides) when treated with the carbodiimide EDC. Transient covalent polymers with molecular weights exceeding 15,000 are generated which then decompose over the course of hours to weeks. Disassembly kinetics can be controlled using simple substituent effects in the monomer design. The impact of solvent polarity, carbodiimide concentration, temperature, pyridine concentration, and monomer concentration on polymer properties and lifetimes has been investigated. The results reveal substantial control over polymer assembly and disassembly kinetics, highlighting the potential for fine-tuned kinetic control in nonequilibrium polymerization systems.

Guiding Transient Peptide Assemblies with Structural Elements Embedded in Abiotic Phosphate Fuels

Despite great progress in the construction of non-equilibrium systems, most approaches do not consider the structure of the fuel as a critical element to control the processes. Herein, we show that the amino acid side chains (A, F, Nal) in the structure of abiotic phosphates can direct assembly and reactivity during transient structure formation. The fuels bind covalently to substrates and subsequently influence the structures in the assembly process. We focus on the ways in which the phosphate esters guide structure formation and how structures and reactivity cross regulate when constructing assemblies. Through the chemical functionalization of energy-rich aminoacyl phosphate esters, we are able to control the yield of esters and thioesters upon adding dipeptides containing tyrosine or cysteine residues. The structural elements around the phosphate esters guide the lifetime of the structures formed and their supramolecular assemblies. These properties can be further influenced by the peptide sequence of substrates, incorporating anionic, aliphatic and aromatic residues. Furthermore, we illustrate that oligomerization of esters can be initiated from a single aminoacyl phosphate ester incorporating a tyrosine residue (Y). These findings suggest that activated amino acids with varying reactivity and energy contents can pave the way for designing and fabricating structured fuels.

Advancements and strategic approaches in catenane synthesis

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

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.

Using a biocatalyzed reaction cycle for transient and pH-dependent host-guest supramolecular hydrogels

The formation of transient structures plays important roles in biological processes, capturing temporary states of matter through influx of energy or biological reaction networks catalyzed by enzymes. These natural transient structures inspire efforts to mimic this elegant mechanism of structural control in synthetic analogues. Specifically, though traditional supramolecular materials are designed on the basis of equilibrium formation, recent efforts have explored out-of-equilibrium control of these materials using both direct and indirect mechanisms; the preponderance of such works has been in the area of low molecular weight gelators. Here, a transient supramolecular hydrogel is realized through cucurbit[7]uril host-guest physical crosslinking under indirect control from a biocatalyzed network that regulates and oscillates pH. The duration of transient hydrogel formation, and resulting mechanical properties, are tunable according to the dose of enzyme, substrate, or pH stimulus. This tunability enables control over emergent functions, such as the programmable burst release of encapsulated model macromolecular payloads.

Emergence of Dynamic Instability by Hybridizing Synthetic Self-Assembled Dipeptide Fibers with Surfactant Micelles

Supramolecular chemistry currently faces the challenge of controlling nonequilibrium dynamics such as the dynamic instability of microtubules. In this study, we explored the emergence of dynamic instability through the hybridization of peptide-type supramolecular nanofibers with surfactant micelles. Using real-time confocal imaging, we discovered that the addition of micelles to nanofibers induced the simultaneous but asynchronous growth and shrinkage of nanofibers during which the total number of fibers decreased monotonically. This dynamic phenomenon unexpectedly persisted for 6 days and was driven not by chemical reactions but by noncovalent supramolecular interactions between peptide-type nanofibers and surfactant micelles. This study demonstrates a strategy for inducing autonomous supramolecular dynamics, which will open up possibilities for developing soft materials applicable to biomedicine and soft robotics.

Kinetic Asymmetry and Directionality of Nonequilibrium Molecular Systems

Scientists have long been fascinated by the biomolecular machines in living systems that process energy and information to sustain life. The first synthetic molecular rotor capable of performing repeated 360° rotations due to a combination of photo- and thermally activated processes was reported in 1999. The progress in designing different molecular machines in the intervening years has been remarkable, with several outstanding examples appearing in the last few years. Despite the synthetic accomplishments, there remains confusion regarding the fundamental design principles by which the motions of molecules can be controlled, with significant intellectual tension between mechanical and chemical ways of thinking about and describing molecular machines. A thermodynamically consistent analysis of the kinetics of several molecular rotors and pumps shows that while light driven rotors operate by a power-stroke mechanism, kinetic asymmetry-the relative heights of energy barriers-is the sole determinant of the directionality of catalysis driven machines. Power-strokes-the relative depths of energy wells-play no role whatsoever in determining the sign of the directionality. These results, elaborated using trajectory thermodynamics and the nonequilibrium pump equality, show that kinetic asymmetry governs the response of many non-equilibrium chemical phenomena.

Theoretical Understanding of the Long-Range Proton Transfer Mechanism in 7-Hydroxy Quinoline-Based Molecular Switches: Varma's Proton Crane and Its Analogues

Herein, the detailed mechanism of intramolecular proton transfer in molecular switches, constructed from 7-hydroxy quinoline substituted in the eight-position C-C single axle, connected to three different proton cranes (morpholine, piperidine, and 1,3,5-dioxazine), was investigated by means of theoretical chemistry. The theoretical interpretation of the rotational mechanism and its stable structures were proposed for the well-known Varma's proton crane, based on morpholine molecule. The reliability of the theoretical simulations was confirmed by the available literature data from time-dependent IR measurements.

Stepwise Operation of a Molecular Rotary Motor Driven by an Appel Reaction

To date, only a small number of chemistries and chemical fueling strategies have been successfully used to operate artificial molecular motors. Here, we report the 360° directionally biased rotation of phenyl groups about a C-C bond, driven by a stepwise Appel reaction sequence. The motor molecule consists of a biaryl-embedded phosphine oxide and phenol, in which full rotation around the biaryl bond is blocked by the P-O oxygen atom on the rotor being too bulky to pass the oxygen atom on the stator. Treatment with SOCl2 forms a cyclic oxyphosphonium salt (removing the oxygen atom of the phosphine oxide), temporarily linking the rotor with the stator. Conformational exchange via ring flipping then allows the rotor and stator to twist back and forth past the previous limit of rotation. Subsequently, the ring opening of the tethered intermediate with a chiral alcohol occurs preferentially through a nucleophilic attack on one face. Thus, the original phosphine oxide is reformed with net directional rotation about the biaryl bond over the course of the two-step reaction sequence. Each repetition of SOCl2-chiral alcohol additions generates another directionally biased rotation. Using the same reaction sequence on a derivative of the motor molecule that forms atropisomers rather than fully rotating 360° results in enantioenrichment, suggesting that, on average, the motor molecule rotates in the "wrong" direction once every three fueling cycles. The interconversion of phosphine oxides and cyclic oxyphosphonium groups to form temporary tethers that enable a rotational barrier to be overcome directionally adds to the strategies available for generating chemically fueled kinetic asymmetry in molecular systems.

A rhythmically pulsing leaf-spring DNA-origami nanoengine that drives a passive follower

Molecular engineering seeks to create functional entities for modular use in the bottom-up design of nanoassemblies that can perform complex tasks. Such systems require fuel-consuming nanomotors that can actively drive downstream passive followers. Most artificial molecular motors are driven by Brownian motion, in which, with few exceptions, the generated forces are non-directed and insufficient for efficient transfer to passive second-level components. Consequently, efficient chemical-fuel-driven nanoscale driver-follower systems have not yet been realized. Here we present a DNA nanomachine (70 nm × 70 nm × 12 nm) driven by the chemical energy of DNA-templated RNA-transcription-consuming nucleoside triphosphates as fuel to generate a rhythmic pulsating motion of two rigid DNA-origami arms. Furthermore, we demonstrate actuation control and the simple coupling of the active nanomachine with a passive follower, to which it then transmits its motion, forming a true driver-follower pair.

Iminopyrrole-Based Self-Assembly: A Route to Intrinsically Flexible Molecular Links and Knots

The use of 2,5-diformylpyrrole in self-assembly reactions with diamines and Zn(II)/Cd(II) salts allowed the preparation of [2]catenane, trefoil knot, and Borromean rings. The intrinsically dynamic nature of the diiminopyrrole motif rendered all of the formed assemblies intramolecularly flexible. The presence of diiminopyrrole revealed new coordination motifs and influenced the host-guest chemistry of the systems, as illustrated by hexafluorophosphate encapsulation by Borromean rings.

Limits on the Precision of Catenane Molecular Motors: Insights from Thermodynamics and Molecular Dynamics Simulations

Thermodynamic uncertainty relations (TURs) relate precision to the dissipation rate, yet the inequalities can be far from saturation. Indeed, in catenane molecular motor simulations, we record precision far below the TUR limit. We further show that this inefficiency can be anticipated by four physical parameters: the thermodynamic driving force, fuel decomposition rate, coupling between fuel decomposition and motor motion, and rate of undriven motor motion. The physical insights might assist in designing molecular motors in the future.

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

Gold-hyaluranic acid micromotors and cold atmospheric plasma for enhanced drug delivery and therapeutic applications

Micro/nanomotors have emerged as promising platforms for various applications, including drug delivery and controlled release. These tiny machines, built from nanoscale materials such as carbon nanotubes, graphene, metal nanoparticles, or nanowires, can convert different forms of energy into mechanical motion. In the field of medicine, nanomotors offer potential for targeted drug delivery and diagnostic applications, revolutionizing areas such as cancer treatment and lab-on-a-chip devices. One prominent material used in drug delivery is hyaluronic acid (HA), known for its biocompatibility and non-immunogenicity. HA-based drug delivery systems have shown promise in improving the efficacy and reducing the toxicity of chemotherapeutic agents like doxorubicin (DOX). Additionally, micro/nanomotors controlled by external stimuli enable precise drug delivery to specific areas of the body. Cold atmospheric plasma (CAP) has also emerged as a promising technology for drug delivery, utilizing low-temperature plasma to enhance drug release and bioavailability. CAP offers advantages such as localized delivery and compatibility with various drug types. However, further research is needed to optimize CAP drug delivery systems and understand their mechanisms. In this study, gold-hyaluronic acid (Au-HA) micromotors were synthesized for the first time, utilizing acoustic force for self-motion. The release profile of DOX, a widely used anticancer drug, was investigated in pH-dependent conditions, and the effect of CAP on drug release from the micromotors was examined. Following exposure to the CAP jet for 1 min, the micromotors released approximately 29 μg mL-1 of DOX into the PBS (pH 5), which is significantly higher than the 17 μg mL-1 released without CAP. The research aims to minimize side effects, increase drug loading and release efficiency, and highlight the potential of HA-based micromotors in cancer therapy. This study contributes to the advancement of micro-motor technology and provides insights into the utilization of pH and cold plasma technology for enhancing drug delivery systems.

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.

Brownian motors powered by nonreciprocal interactions

Traditional models for molecular (Brownian) motors predominantly depend on nonequilibrium driving, while particle interactions rigorously adhere to Newton's third law. However, numerous living and natural systems at various scales seem to defy this well-established law. In this study, we investigated the transport of mixed Brownian particles in a two-dimensional ratchet potential with nonreciprocal interactions. Our findings reveal that these nonreciprocal interactions can introduce a zero-mean nonequilibrium driving force. This force is capable of disrupting the thermodynamic equilibrium and inducing directed motion. The direction of this motion is determined by the asymmetry of the potential. Interestingly, the average velocity is a peaked function of the degree of nonreciprocity, while the effective diffusion consistently increases with the increase of nonreciprocity. There exists an optimal temperature or packing fraction at which the average velocity reaches its maximum value. We share a mechanism for particle rectification, devoid of particle-autonomous nonequilibrium drive, with potential usage in systems characterized by nonreciprocal interactions.

Artificial Molecular Ratchets: Tools Enabling Endergonic Processes

Non-equilibrium chemical systems underpin multiple domains of contemporary interest, including supramolecular chemistry, molecular machines, systems chemistry, prebiotic chemistry, and energy transduction. Experimental chemists are now pioneering the realization of artificial systems that can harvest energy away from equilibrium. In this tutorial Review, we provide an overview of artificial molecular ratchets: the chemical mechanisms enabling energy absorption from the environment. By focusing on the mechanism type-rather than the application domain or energy source-we offer a unifying picture of seemingly disparate phenomena, which we hope will foster progress in this fascinating domain of science.

Mechanistic studies of isomeric [2]rotaxanes consisting of two different tetrathiafulvalene units reveal that the movement of cyclobis(paraquat-p-phenylene) can be controlled

Controlling the movement in artificial molecular machines is a key challenge that needs to be solved before their full potential can be harnessed. In this study, two isomeric tri-stable [2]rotaxanes 1·4PF6 and 2·4PF6 incorporating both a tetrathiafulvalene (TTF) and a monopyrrolotetrathiafulvalene (MPTTF) unit in the dumbbell component have been synthesised to measure the energy barriers when the tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) ring moves across either a TTF2+ or an MPTTF2+ dication. By strategically exchanging one of the thiomethyl barriers on either the TTF unit or the MPTTF unit with the bulkier thioethyl group, the movement of the CBPQT4+ ring in 14+ and 24+ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and 1H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 14+ and 24+, the CBPQT4+ ring moves first to a position where it is located between the TTF2+ and MPTTF2+ dications producing high-energy co-conformations which slowly interconvert into thermodynamically more stable co-conformations. The kinetics of the movement occurring in the tetra-oxidised [2]rotaxanes 18+ and 28+ were studied at different temperatures allowing the free energy of the transition state, when CBPQT4+ moves across TTF2+ (21.5 kcal mol-1) and MPTTF2+ (20.3 kcal mol-1) at 298 K, to be determined. These results demonstrate for the first time that the combination of a TTF and an MPTTF unit can be used to induce directional movement of the CBPQT4+ ring in molecular machines with a 90% efficiency.

Noncovalent Modification of Cycloparaphenylene by Catenane Formation Using an Active Metal Template Strategy

The active metal template (AMT) strategy is a powerful tool for the formation of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes, allowing the synthesis of a variety of MIMs, including π-conjugated and multicomponent macrocycles. Cycloparaphenylene (CPP) is an emerging molecule characterized by its cyclic π-conjugated structure and unique properties. Therefore, diverse modifications of CPPs are necessary for its wide application. However, most CPP modifications require early stage functionalization and the direct modification of CPPs is very limited. Herein, we report the synthesis of a catenane consisting of [9]CPP and a 2,2'-bipyridine macrocycle as a new CPP analogue that contains a reliable synthetic scaffold enabling diverse and concise post-modification. Following the AMT strategy, the [9]CPP-bipyridine catenane was successfully synthesized through Ni-mediated aryl-aryl coupling. Catalytic C-H borylation/cross-coupling and metal complexation of the bipyridine macrocycle moiety, an effective post-functionalization method, were also demonstrated with the [9]CPP-bipyridine catenane. Single-crystal X-ray structural analysis revealed that the [9]CPP-bipyridine catenane forms a tridentated complex with an Ag ion inside the CPP ring. This interaction significantly enhances the phosphorescence lifetime through improved intermolecular interactions.

The entropy-controlled strategy in self-assembling systems

Self-assembly of various building blocks has been considered as a powerful approach to generate novel materials with tailorable structures and optimal properties. Understanding physicochemical interactions and mechanisms related to structural formation and transitions is of essential importance for this approach. Although it is well-known that diverse forces and energies can significantly contribute to the structures and properties of self-assembling systems, the potential entropic contribution remains less well understood. The past few years have witnessed rapid progress in addressing the entropic effects on the structures, responses, and functions in the self-assembling systems, and many breakthroughs have been achieved. This review provides a framework regarding the entropy-controlled strategy of self-assembly, through which the structures and properties can be tailored by effectively tuning the entropic contribution and its interplay with the enthalpic counterpart. First, we focus on the fundamentals of entropy in thermodynamics and the entropy types that can be explored for self-assembly. Second, we discuss the rules of entropy in regulating the structural organization in self-assembly and delineate the entropic force and superentropic effect. Third, we introduce the basic principles, significance and approaches of the entropy-controlled strategy in self-assembly. Finally, we present the applications where this strategy has been employed in fields like colloids, macromolecular systems and nonequilibrium assembly. This review concludes with a discussion on future directions and future research opportunities for developing and applying the entropy-controlled strategy in complex self-assembling systems.

Autonomous DNA molecular motor tailor-designed to navigate DNA origami surface for fast complex motion and advanced nanorobotics

Nanorobots powered by designed DNA molecular motors on DNA origami platforms are vigorously pursued but still short of fully autonomous and sustainable operation, as the reported systems rely on manually operated or autonomous but bridge-burning molecular motors. Expanding DNA nanorobotics requires origami-based autonomous non-bridge-burning motors, but such advanced artificial molecular motors are rare, and their integration with DNA origami remains a challenge. Here, we report an autonomous non-bridge-burning DNA motor tailor-designed for a triangle DNA origami substrate. This is a translational bipedal molecular motor but demonstrates effective translocation on both straight and curved segments of a self-closed circular track on the origami, including sharp ~90° turns by a single hand-over-hand step. The motor is highly directional and attains a record-high speed among the autonomous artificial molecular motors reported to date. The resultant DNA motor-origami system, with its complex translational-rotational motion and big nanorobotic capacity, potentially offers a self-contained "seed" nanorobotic platform to automate or scale up many applications.

A transient vesicular glue for amplification and temporal regulation of biocatalytic reaction networks

Regulation of enzyme activity and biocatalytic cascades on compartmentalized cellular components is key to the adaptation of cellular processes such as signal transduction and metabolism in response to varying external conditions. Synthetic molecular glues have enabled enzyme inhibition and regulation of protein-protein interactions. So far, all the molecular glue systems based on covalent interactions operated under steady-state conditions. To emulate dynamic biological processes under dissipative conditions, we introduce herein a transient supramolecular glue with a controllable lifetime. The transient system uses multivalent supramolecular interactions between guanidinium group-bearing surfactants and adenosine triphosphate (ATP), resulting in bilayer vesicle structures. Unlike the conventional chemical agents for dissipative assemblies, ATP here plays the dual role of providing a structural component for the assembly as well as presenting active functional groups to "glue" enzymes on the surface. While gluing of the enzymes on the vesicles achieves augmented catalysis, oscillation of ATP concentration allows temporal control of the catalytic activities similar to the dissipative cellular nanoreactors. We further demonstrate temporal upregulation and control of complex biocatalytic reaction networks on the vesicles. Altogether, the temporal activation of biocatalytic cascades on the dissipative vesicular glue presents an adaptable and dynamic system emulating heterogeneous cellular processes, opening up avenues for effective protocell construction and therapeutic interventions.

A catalytically active oscillator made from small organic molecules

Oscillatory systems regulate many biological processes, including key cellular functions such as metabolism and cell division, as well as larger-scale processes such as circadian rhythm and heartbeat1-4. Abiotic chemical oscillations, discovered originally in inorganic systems5,6, inspired the development of various synthetic oscillators for application as autonomous time-keeping systems in analytical chemistry, materials chemistry and the biomedical field7-17. Expanding their role beyond that of a pacemaker by having synthetic chemical oscillators periodically drive a secondary function would turn them into significantly more powerful tools. However, this is not trivial because the participation of components of the oscillator in the secondary function might jeopardize its time-keeping ability. We now report a small molecule oscillator that can catalyse an independent chemical reaction in situ without impairing its oscillating properties. In a flow system, the concentration of the catalytically active product of the oscillator shows sustained oscillations and the catalysed reaction is accelerated only during concentration peaks. Augmentation of synthetic oscillators with periodic catalytic action allows the construction of complex systems that, in the future, may benefit applications in automated synthesis, systems and polymerization chemistry and periodic drug delivery.

A Doubly Dissipative System Driven by Chemical and Radiative Stimuli

The operation of a dissipative network composed of two or three different crown-ether receptors and an alkali metal cation can be temporally driven by the use (combined or not) of two orthogonal stimuli of a different nature. More specifically, irradiation with light at a proper wavelength and/or addition of an activated carboxylic acid, are used to modulate the binding capability of the above crown-ethers towards the metal ion, allowing to control over time the occupancy of the metal cation in the crown-ether moiety of a given ligand. Thus, application of either or both of the stimuli to an initially equilibrated system, where the metal cation is distributed among the crown-ether receptors depending on the different affinities, causes a programmable change in the receptor occupancies. Consequently, the system is induced to evolve to one or more out-of-equilibrium states with different distributions of the metal cation among the different receptors. When the fuel is exhausted or/and the irradiation interrupted, the system reversibly and autonomously goes back to the initial equilibrium state. Such results may contribute to the achievement of new dissipative systems that, taking advantage of multiple and orthogonal stimuli, are featured with more sophisticated operating mechanisms and time programmability.

Photochromic Azobenzene Inverse Opal Film toward Dynamic Anti-Fake Pattern

Azobenzene mesogens have garnered considerable research attention in the realm of photo-responsive materials due to their reversible trans-cis isomerization. In this paper, we demonstrate an azobenzene inverse opal film synthesized via photo-polymerization from a SiO2 opal template. The proposed design exhibits intriguing optical properties, including dynamic fluorescent features, distinct fluorescent enhancement, and an anti-fake micropattern with a switchable structure color. This work holds significant importance for advancing the development of novel optical devices.

Stimulus-Controlled Anion Binding and Transport by Synthetic Receptors

Anionic species are omnipresent and involved in many important biological processes. A large number of artificial anion receptors has therefore been developed. Some of these are capable of mediating transmembrane transport. However, where transport proteins can respond to stimuli in their surroundings, creation of synthetic receptors with stimuli-responsive functions poses a major challenge. Herein, we give a full overview of the stimulus-controlled anion receptors that have been developed thus far, including their application in membrane transport. In addition to their potential operation as membrane carriers, the use of anion recognition motifs in forming responsive membrane-spanning channels is discussed. With this review article, we intend to increase interest in transmembrane transport among scientists working on host-guest complexes and dynamic functional systems in order to stimulate further developments.

Alcohol imination catalyzed by carbon nanostructures synthesized by C(sp2)-C(sp3) free radical coupling

Imines are important intermediates for synthesizing various fine chemicals, with the disadvantage of requiring the use of expensive metal-containing catalysts. We report that the dehydrogenative cross-coupling of phenylmethanol and benzylamine (or aniline) directly forms the corresponding imine with a yield of up to 98%, and water as the sole by-product, in the presence of a stoichiometric base, using carbon nanostructures as the "green" metal-free carbon catalysts with high spin concentrations, which is synthesized by C(sp2)-C(sp3) free radical coupling reactions. The catalytic mechanism is attributed to the unpaired electrons of carbon catalysts to reduce O2 to O2·-, which triggers the oxidative coupling reaction to form imines, whereas the holes in the carbon catalysts receive electrons from the amine to restore the spin states. This is supported by density functional theory calculations. This work will open up an avenue for synthesizing carbon catalysts and offer great potential for industrial applications.

Mobile Molecules: Reactivity Profiling Guides Faster Movement on a Cysteine Track

We previously reported a molecular hopper, which makes sub-nanometer steps by thiol-disulfide interchange along a track with cysteine footholds within a protein nanopore. Here we optimize the hopping rate (ca. 0.1 s-1 in the previous work) with a view towards rapid enzymeless biopolymer characterization during translocation within nanopores. We first took a single-molecule approach to obtain the reactivity profiles of individual footholds. The pK a values of cysteine thiols within a pore ranged from 9.17 to 9.85, and the pH-independent rate constants of the thiolates with a small-molecule disulfide varied by up to 20-fold. Through site-specific mutagenesis and a pH increase from 8.5 to 9.5, the overall hopping rate of a DNA cargo along a five-cysteine track was accelerated 4-fold, and the rate-limiting step 21-fold.

Mobile Molecules: Reactivity Profiling Guides Faster Movement on a Cysteine Track

We previously reported a molecular hopper, which makes sub-nanometer steps by thiol-disulfide interchange along a track with cysteine footholds within a protein nanopore. Here we optimize the hopping rate (ca. 0.1 s-1 in the previous work) with a view towards rapid enzymeless biopolymer characterization during translocation within nanopores. We first took a single-molecule approach to obtain the reactivity profiles of individual footholds. The pKa values of cysteine thiols within a pore ranged from 9.17 to 9.85, and the pH-independent rate constants of the thiolates with a small-molecule disulfide varied by up to 20-fold. Through site-specific mutagenesis and a pH increase from 8.5 to 9.5, the overall hopping rate of a DNA cargo along a five-cysteine track was accelerated 4-fold, and the rate-limiting step 21-fold.

Inferring Subsystem Efficiencies in Bipartite Molecular Machines

Molecular machines composed of coupled subsystems transduce free energy between different external reservoirs, in the process internally transducing energy and information. While subsystem efficiencies of these molecular machines have been measured in isolation, less is known about how they behave in their natural setting when coupled together and acting in concert. Here, we derive upper and lower bounds on the subsystem efficiencies of a bipartite molecular machine. We demonstrate their utility by estimating the efficiencies of the F_{o} and F_{1} subunits of ATP synthase and that of kinesin pulling a diffusive cargo.

Evaluating the energy landscape of an out-of-equilibrium bistable [2]rotaxane containing monopyrrolotetrathiafulvalene

The unique redox properties of monopyrrolotetrathiafulvalene can be used to induce directional movement in interlocked molecules. In this study, the kinetics for the directional movement of cyclobis(paraquat-p-phenylene) across the dioxidised monopyrrolotetrathiafulvalene in a [2]rotaxane is quantified by time-resolved ¹H NMR spectroscopy.

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Kinetic Barrier Diagrams to Visualize and Engineer Molecular Nonequilibrium Systems

Molecular nonequilibrium systems hold great promises for the nanotechnology of the future. Yet, their development is slowed by the absence of an informative representation. Indeed, while potential energy surfaces comprise in principle all the information, they hide the dynamic interplay of multiple reaction pathways underlying nonequilibrium systems, i.e., the degree of kinetic asymmetry. To offer an insightful visual representation of kinetic asymmetry, we extended an approach pertaining to catalytic networks, the energy span model, by focusing on system dynamics - rather than thermodynamics. Our approach encompasses both chemically and photochemically driven systems, ranging from unimolecular motors to simple self-assembly schemes. The obtained diagrams give immediate access to information needed to guide experiments, such as states' population, rate of machine operation, maximum work output, and effects of design changes. The proposed kinetic barrier diagrams offer a unifying graphical tool for disparate nonequilibrium phenomena.

Beat of a current

The fluctuation relation, a milestone of modern thermodynamics, is only established when a set of fundamental currents can be measured. Here we prove that it also holds for systems with hidden transitions if observations are carried "at their own beat," that is, by stopping the experiment after a fixed number of visible transitions, rather than the elapse of an external clock time. This suggests that thermodynamic symmetries are more resistant to the loss of information when described in the space of transitions.

An Inbuilt Electronic Pawl Gates Orbital Information Processing and Controls the Rotation of a Double Ratchet Rotary Motor

A direct external input energy source (e.g., light, chemical reaction, redox potential, etc.) is compulsory to supply energy to rotary motors for accomplishing rotation around the axis. The stator leads the direction of rotation, and a sustainable rotation requires two mutual input energy supplies (e.g., light and heat, light and pH or metal ion, etc.); however, there are some exceptions (e.g., covalent single bond rotors and/or motors). On the contrary, our experiment suggested that double ratchet rotary motors (DRMs) can harvest power from available thermal noise, kT, for sustainable rotation around the axis. Under a scanning tunneling microscope, we have imaged live thermal noise movement as a dynamic orbital density and resolved the density diagram up to the second derivative. A second input energy can synchronize multiple rotors to afford a measurable output. Therefore, we hypothesized that rotation control in a DRM must be evolved from an orbital-level information transport channel between the two coupled rotors but was not limited to the second input energy. A DRM comprises a Brownian rotor and a power stroke rotor coupled to a -C≡C- stator, where the transport of information through coupled orbitals between the two rotors is termed the vibrational information flow chain (VIFC). We test this hypothesis by studying the DRM's density functional theory calculation and variable-temperature ¹H nuclear magnetic resonance. Additionally, we introduced inbuilt pawl-like functional moieties into a DRM to create different electronic environments by changing proton intercalation interactions, which gated information processing through the VIFC. The results show the VIFC can critically impact the motor's noise harvesting, resulting in variable rotational motions in DRMs.

Programmable Transport of C60 by Straining Graphene Substrate

Controlling the maneuverability of nanocars and molecular machines on the surface is essential for the targeted transportation of materials and energy at the nanoscale. Here, we evaluate the motion of fullerene, as the most popular candidate for use as a nanocar wheel, on the graphene nanoribbons with strain gradients based on molecular dynamics (MD), and theoretical approaches. The strain of the examined substrates linearly decreases by 20%, 16%, 12%, 8%, 4%, and 2%. MD calculations were performed with the open source LAMMPS solver. The essential physics of the interactions is captured by Lennard-Jones and Tersoff potentials. The motion of C60 on the graphene nanoribbon is simulated in canonical ensemble, which is implanted by using a Nose-Hoover thermostat. Since the potential energy of C60 is lower on the unstrained end of nanoribbons, this region is energetically more favorable for the molecule. As the strain gradient of the surface increases, the trajectories of the motion and the C60 velocity indicate more directed movements along the gradient of strain on the substrate. Based on the theoretical relations, it was shown that the driving force and diffusion coefficient of the C60 motion respectively find linear and quadratic growth with the increase of strain gradient, which is confirmed by MD simulations. To understand the effect of temperature, at each strain gradient of substrate, the simulations are repeated at the temperatures of 100, 200, 300, and 400 K. The large ratio of longitudinal speed to the transverse speed of fullerene at 100 and 200 K refers to the rectilinear motion of molecule at low temperatures. Using successive strain gradients on the graphene in perpendicular directions, we steered the motion of C60 to the desired target locations. The programmable transportation of nanomaterials on the surface has a significant role in different processes at the nanoscale, such as bottom-up assembly.

CO2 -Fueled Transient Breathing Nanogels that Couple Nonequilibrium Catalytic Polymerization

Here we present a "breathing" nanogel that is fueled by CO₂ gas to perform temporally programmable catalytic polymerization. The nanogel is composed of common frustrated Lewis pair polymers (FLPs). Dynamic CO₂ -FLP gas-bridging bonds endow the nanogel with a transient volume contraction, and the resulting proximal effect of bound FLPs unlocks its catalytic capacity toward CO₂ . Reverse gas depletion via a CO₂ -participated polymerization can induce a reverse nanogel expansion, which shuts down the catalytic activity. Control of external factors (fuel level, temperature or additives) can regulate the breathing period, amplitude and lifecycle, so as to affect the catalytic polymerization. Moreover, editing the nanogel breathing procedure can sequentially evoke the copolymerization of CO₂ with different epoxide monomers preloaded therein, which allows to obtain block-tunable copolycarbonates that are unachievable by other methods. This synthetic dissipative system would be function as a prototype of gas-driven nanosynthesizer.