Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimickingmacromolecular devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vitalliving processes such asDNA replication andATP synthesis.Kinesins andribosomes are examples of molecular machines, and they often take the form ofmulti-protein complexes. For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. The first example of an artificial molecular machine (AMM) was reported in 1994, featuring arotaxane with a ring and two different possiblebinding sites. In 2016 theNobel Prize in Chemistry was awarded toJean-Pierre Sauvage,Sir J. Fraser Stoddart, andBernard L. Feringa for the design and synthesis of molecular machines.
AMMs have diversified rapidly over the past few decades and their design principles, properties, andcharacterization methods have been outlined better. A major starting point for the design of AMMs is to exploit the existing modes of motion in molecules, such as rotation aboutsingle bonds orcis-trans isomerization. Different AMMs are produced by introducing various functionalities, such as the introduction ofbistability to create switches. A broad range of AMMs has been designed, featuring different properties and applications; some of these includemolecular motors,switches, andlogic gates. A wide range of applications have been demonstrated for AMMs, including those integrated intopolymeric,liquid crystal, andcrystalline systems for varied functions (such asmaterials research,homogenous catalysis andsurface chemistry).
Several definitions describe a "molecular machine" as a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli. The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level.[1] A few prime requirements for a molecule to be considered a "molecular machine" are: the presence of moving parts, the ability to consume energy, and the ability to perform a task.[2] Molecular machines differ from other stimuli-responsive compounds that can produce motion (such ascis-trans isomers) in their relatively larger amplitude of movement (potentially due tochemical reactions) and the presence of a clear external stimulus to regulate the movements (as compared torandom thermal motion).[1]Piezoelectric,magnetostrictive, and other materials that produce a movement due to external stimuli on a macro-scale are generally not included, since despite the molecular origin of the motion the effects are not useable on the molecular scale.[citation needed]
This definition generally applies to synthetic molecular machines, which have historically gained inspiration from the naturally occurring biological molecular machines (also referred to as "nanomachines"). Biological machines are considered to be nanoscale devices (such as molecularproteins) in a living system that convert various forms of energy to mechanical work in order to drive crucialbiological processes such asintracellular transport,muscle contractions,ATP generation andcell division.[3][4]
What would be the utility of such machines? Who knows? I cannot see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a molecular scale we will get an enormously greater range of possible properties that substances can have, and of the different things we can do.
Biological molecular machines have been known and studied for years given their vital role in sustaining life, and have served as inspiration for synthetically designed systems with similar useful functionality.[3][4] The advent of conformational analysis, or the study ofconformers to analyze complex chemical structures, in the 1950s gave rise to the idea of understanding and controlling relative motion within molecular components for further applications. This led to the design of "proto-molecular machines" featuring conformational changes such as cog-wheeling of thearomatic rings intriptycenes.[6] By 1980, scientists could achieve desired conformations using external stimuli and utilize this for different applications. A major example is the design of a photoresponsivecrown ether containing anazobenzene unit, which could switch betweencis andtrans isomers on exposure to light and hence tune the cation-binding properties of the ether.[7] In his seminal 1959 lectureThere's Plenty of Room at the Bottom,Richard Feynman alluded to the idea and applications of molecular devices designed artificially by manipulating matter at the atomic level.[5] This was further substantiated byEric Drexler during the 1970s, who developed ideas based onmolecular nanotechnology such as nanoscale "assemblers",[8] though their feasibility wasdisputed.[9]
The first example of an artificial molecular machine (a switchable molecular shuttle). The positively charged ring (blue) is initially positioned over thebenzidine unit (green), but shifts to thebiphenol unit (red) when the benzidine gets protonated (purple) as a result ofelectrochemicaloxidation or lowering of thepH.[10]
Though these events served as inspiration for the field, the actual breakthrough in practical approaches to synthesize artificial molecular machines (AMMs) took place in 1991 with the invention of a "molecular shuttle" bySir Fraser Stoddart.[10] Building upon the assembly of mechanically linked molecules such ascatenanes androtaxanes as developed byJean-Pierre Sauvage in the early 1980s,[11][12] this shuttle features a rotaxane with a ring that can move across an "axle" between two ends or possiblebinding sites (hydroquinone units). This design realized the well-defined motion of a molecular unit across the length of the molecule for the first time.[6] In 1994, an improved design allowed control over the motion of the ring bypH variation orelectrochemical methods, making it the first example of an AMM. Here the two binding sites are abenzidine and abiphenol unit; the cationic ring typically prefers staying over the benzidine ring, but moves over to the biphenol group when the benzidine gets protonated at low pH or if it gets electrochemicallyoxidized.[13] In 1998, a study could capture the rotary motion of a decacyclene molecule on a copper-base metallic surface using ascanning tunneling microscope.[14] Over the following decade, a broad variety of AMMs responding to various stimuli were invented for different applications.[15][16] In 2016, theNobel Prize in Chemistry was awarded to Sauvage, Stoddart, andBernard L. Feringa for the design and synthesis of molecular machines.[17][18]
Over the past few decades, AMMs have diversified rapidly and their design principles,[2] properties,[19] andcharacterization methods[20] have been outlined more clearly. A major starting point for the design of AMMs is to exploit the existing modes of motion in molecules.[2] For instance,single bonds can be visualized as axes of rotation,[21] as can bemetallocene complexes.[22] Bending or V-like shapes can be achieved by incorporatingdouble bonds, that can undergocis-trans isomerization in response to certain stimuli (typically irradiation with a suitablewavelength), as seen in numerous designs consisting ofstilbene and azobenzene units.[23] Similarly,ring-opening and -closing reactions such as those seen forspiropyran anddiarylethene can also produce curved shapes.[24] Another common mode of movement is the circumrotation of rings relative to one another as observed in mechanically interlocked molecules (primarily catenanes). While this type of rotation can not be accessed beyond the molecule itself (because the rings are confined within one another), rotaxanes can overcome this as the rings can undergo translational movements along a dumbbell-like axis.[25] Another line of AMMs consists of biomolecules such asDNA andproteins as part of their design, making use of phenomena likeprotein folding and unfolding.[26][27]
Some common types of motion seen in some simple components of artificial molecular machines. a) Rotation around single bonds and in sandwich-likemetallocenes. b) Bending due tocis-trans isomerization. c) Translational motion of a ring (blue) between two possible binding sites (red) along the dumbbell-like rotaxane axis (purple). d) Rotation of interlocked rings (depicted as blue and red rectangles) in a catenane.
AMM designs have diversified significantly since the early days of the field. A major route is the introduction ofbistability to produce molecular switches, featuring two distinct configurations for the molecule to convert between. This has been perceived as a step forward from the original molecular shuttle which consisted of two identical sites for the ring to move between without any preference, in a manner analogous to thering flip in an unsubstitutedcyclohexane. If these two sites are different from each other in terms of features likeelectron density, this can give rise to weak or strong recognition sites as in biological systems — such AMMs have found applications incatalysis anddrug delivery. This switching behavior has been further optimized to acquire useful work that gets lost when a typical switch returns to its original state.Inspired by the use ofkinetic control to produce work in natural processes, molecular motors are designed to have a continuous energy influx to keep them away fromequilibrium to deliver work.[2][1]
Various energy sources are employed to drive molecular machines today, but this was not the case during the early years of AMM development. Though the movements in AMMs were regulated relative to the random thermal motion generally seen in molecules, they could not be controlled or manipulated as desired. This led to the addition of stimuli-responsive moieties in AMM design, so that externally applied non-thermal sources of energy could drive molecular motion and hence allow control over the properties. Chemical energy (or "chemical fuels") was an attractive option at the beginning, given the broad array ofreversible chemical reactions (heavily based onacid-base chemistry) to switch molecules between different states.[28] However, this comes with the issue of practically regulating the delivery of the chemical fuel and the removal of waste generated to maintain the efficiency of the machine as in biological systems. Though some AMMs have found ways to circumvent this,[29] more recently waste-free reactions such based onelectron transfers or isomerization have gained attention (such as redox-responsiveviologens). Eventually, several different forms of energy (electric,[30] magnetic,[31] optical[32] and so on) have become the primary energy sources used to power AMMs, even producing autonomous systems such as light-driven motors.[33]
Various AMMs are tabulated below along with indicative images:[19]
Type
Details
Image
Molecular balance
A molecule that can interconvert between two or more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces,[34][35] such ashydrogen bonding,solvophobic or hydrophobic effects,[36]π interactions,[37] and steric and dispersion interactions.[38] The distinct conformers of a molecular balance can show different interactions with the same molecule, such that analyzing the ratio of the conformers and the energies for these interactions can enable quantification of different properties (such as CH-π or arene-arene interactions, see image).[39][40]
Molecular hinge
A molecular hinge is a molecule that can typically rotate in acrank-like motion around a rigid axis, such as a double bond or aromatic ring, to switch between reversible configurations.[41] Such configurations must have distinguishable geometries; for instance, azobenzene groups in a linear molecule may undergocis-trans isomerization[42] when irradiated withultraviolet light, triggering a reversible transition to a bent or V-shaped conformation (see image).[43][44][45] Molecular hinges have been adapted for applications such asnucleobase recognition,[46]peptide modifications,[47] and visualizing molecular motion.[48]
A molecule that performs a logical operation on one or more logic inputs and produces a single logic output.[49] Modelled onlogic gates, these molecules have slowly replaced the conventional silicon-based machinery. Several applications have come forth, such as water quality examination,food safety examination, metal ion detection, and pharmaceutical studies.[50][51] The first example of a molecular logic gate was reported in 1993, featuring a receptor (see image) where the emission intensity could be treated as a tunable output if the concentrations of protons and sodium ions were to be considered as inputs.[52]
A molecule that is capable of directional rotary motion around a single or double bond and produce useful work as a result (as depicted in the image).[53][54][55]Carbon nanotube nanomotors have also been produced.[56] Single bond rotary motors[57] are generally activated by chemical reactions whereas double bond rotary motors[58] are generally fueled by light. The rotation speed of the motor can also be tuned by careful molecular design.[59]
Molecular necklace
A class of mechanically interlocked molecules derived from catenanes where a large macrocycle backbone connects at least three small rings in the shape of a necklace (see image for example). A molecular necklace consisting of a large macrocycle threaded byn-1 rings (hence comprisingn rings) is represented as [n]MN.[60] The first molecular necklace was synthesized in 1992, featuring severalα-cyclodextrins on a singlepolyethylene glycol chain backbone; the authors connected this to the idea of a "molecular abacus" proposed by Stoddart and coworkers around the same time.[61] Several interesting applications have emerged for these molecules, such asantibacterial activity,[62]desulfurization of fuels,[63] andpiezoelectricity.[64]
A molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers (see schematic image on right). It has several molecular-scale blades attached at a certain pitch angle around the circumference of a nanoscale shaft.[65][66] Propellers have been shown to have interesting properties, such as variations in pumping rates for hydrophilic and hydrophobic fluids.[67]
A molecule capable of shuttling molecules or ions from one location to another. This is schematically depicted in the image on the right, where a ring (in green) can bind to either one of the yellow sites on the blue macrocyclic backbone.[68] A common molecular shuttle consists of a rotaxane where the macrocycle can move between two sites or stations along the dumbbell backbone; controlling the properties of either site and by regulating conditions like pH can enable control over which site is selected for binding. This has led to novel applications in catalysis and drug delivery.[68][69]
A molecule that can be reversibly shifted between two or more stable states in response to certain stimuli. This change of states influences the properties of the molecule according to the state it occupies at the moment. Unlike a molecular motor, any mechanical work done due to the motion in a switch is generally undone once the molecule returns to its original state unless it is part of a larger motor-like system. The image on the right shows ahydrazone-based switch that switches in response to pH changes.[70]
Host molecules capable of holding items between their two arms.[71] The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces,van der Waals forces,π interactions, orelectrostatic effects.[72] For instance, the image on the right depicts tweezers formed bycorannulene pincers clasping aC60 fullerene molecule, termed "buckycatcher".[73] Examples of molecular tweezers have been reported that are constructed from DNA and are consideredDNA machines.[74]
Single-molecule vehicles that resemble macroscopic automobiles and are important for understanding how to control molecular diffusion on surfaces. The image on the right shows an example with wheels made of fullerene molecules. The first nanocars were synthesized byJames M. Tour in 2005. They had an H-shaped chassis and 4 molecular wheels (fullerenes) attached to the four corners.[75] In 2011, Feringa and co-workers synthesized the first motorized nanocar which had molecular motors attached to the chassis as rotating wheels.[76] The authors were able to demonstrate directional motion of the nanocar on a copper surface by providing energy from a scanning tunneling microscope tip. Later, in 2017, the world's first-evernanocar race took place inToulouse.[77]
A ribosome performing theelongation and membrane targeting stages ofprotein translation. Theribosome is green and yellow, thetRNAs are dark blue, and the other proteins involved are light blue. The produced peptide is released into theendoplasmic reticulum.
Biological machines have potential applications innanomedicine.[82] For example, they could be used to identify and destroy cancer cells.[83][84]Molecular nanotechnology is aspeculative subfield of nanotechnology regarding the possibility of engineeringmolecular assemblers, biological machines which could re-order matter at a molecular or atomic scale.[citation needed]Nanomedicine would make use of thesenanorobots, introduced into the body, to repair or detect damages and infections, but these are considered to be far beyond current capabilities.[85]
Advances in this area are inhibited by the lack of synthetic methods.[86] In this context, theoretical modeling has emerged as a pivotal tool to understand theself-assembly or -disassembly processes in these systems.[87][88]
AMMs are gradually moving from the conventional solution-phase chemistry to surfaces and interfaces. For instance, AMM-immobilized surfaces (AMMISs) are a novel class of functional materials consisting of AMMs attached to inorganic surfaces forming features like self-assembled monolayers; this gives rise to tunable properties such as fluorescence, aggregation and drug-release activity.[99]
Most of these "applications" remain at the proof-of-concept level. Challenges in streamlining macroscale applications include autonomous operation, the complexity of the machines, stability in the synthesis of the machines and the working conditions.[1][100]
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