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Artificial Molecular Machines.

Erbas-Cakmak S, Leigh DA, McTernan CT, Nussbaumer AL - Chem. Rev. (2015)

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.

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Hydrogen-bonding solvents have been shownto disrupt macrocycle–thread interactions in single stationrotaxanes, and here addition of 5% [D4]methanol increased the rate of shuttling 100-fold, consistentwith lowering the energy barrier to migration by disrupting station–macrocycleinteractions and thus raising ground-state energies... The effect of water on the rate of shuttlinghas been investigated and was found to be greatly superior to thatof other protic solvents... Therate of escape from the station energy well can then be modeled byan Arrhenius equation with a contribution from a distance-dependentdiffusion factor to the overall rate of shuttling... A quantum mechanicaltreatment of this system has found that, as the lengthening of thespacer has no effect on the activation for breaking the hydrogen bonds,the effect on the rate of shuttling is due to the widening of theoverall potential energy well... Molecular motion in mechanically interlocked and thus kineticallystable rotaxanes can be controlled using multiple binding sites withaffinities for the macrocycle that vary under different conditions.The conditions can be modified by electrochemical redox processes,light, pH, and environmental changes... Whenthe stilbene unit adopted the E form, the macrocyclecould move randomly along the full length of the thread by Brownianmotion, while when the Z form is adopted, the macrocyclewas trapped in one or the other of the two compartments... As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station... Tensionin the tether decreased as a result of the shuttling... Extracting useful work at the molecular scalerequires the restriction of the thermal movement of submolecular componentsor the exploitation of thermal motion with additional ratcheting.Shuttling, switching, and rotation processes in solution can be modulatedexternally, and the directionality of each motion can be controlledin single molecules... Third, to drive the walker away fromequilibrium, that is, to generate directional motion, a ratchetingprocess (either an energy or an information ratchet) must take place.In addition to the requirements of a Brownian motor, certain additionalcharacteristics are necessary for a motor to be defined as a walker... For the design of processive small molecule synthetic molecularwalkers, mechanically interlocked architectures are good candidates,because the walker (macrocycle) is mechanically bonded to the track(thread)... To perform tasks that cannot be accomplishedby conventional chemical means, it will be necessary to design systemswith multiple integrated parts, each component performing a dedicatedrole within the machine ensemble... This will not be straightforwardbecause unlike a watch where the second hand, say, does not interferewith the components in the escapement mechanism, the components ofa chemical machine are not easily isolated from each other (or theenvironment) and interference from one reactive part of a machinewith another will be a significant issue as complexity increases beyondthe current rather trivial systems. (iii) The machines we arefamiliar with in the macroscopic worldare generally stable, operating unchanged through many cycles, andby and large they do not make “mistakes”... Or it may bethat evolution just did not discover these solutions to such problemsand that mankind, with the whole of the periodic table and known syntheticchemistry to work with, can... Perhaps the most productive approachwill ultimately be found by following neither of these lines of investigationtoo closely, for example, by using chemical principles for “molecularrobotics” in which ratcheted motions of molecular components(i.e., biologically inspired mechanisms) are used to perform tasksthat have their origins in innovations introduced to advance developmentsin macroscopic technology (e.g., factory assembly lines).

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(a) Chemical structure of a rotaxane with fumaramide andsuccinamidestations depicted in green and orange, respectively. (b) Schematicrepresentation of macrocycle movement on a thread attached to a goldsurface as a result of the force exerted by an AFM probe. Applicationof force by cantilever movement (I,II) was followed by repositioningof the macrocycle (III) or detachment of the PEO tether (IV) dependingon the strength of the force. Reprinted with permission from ref (1143). Copyright 2011 NaturePublishing Group.
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fig41: (a) Chemical structure of a rotaxane with fumaramide andsuccinamidestations depicted in green and orange, respectively. (b) Schematicrepresentation of macrocycle movement on a thread attached to a goldsurface as a result of the force exerted by an AFM probe. Applicationof force by cantilever movement (I,II) was followed by repositioningof the macrocycle (III) or detachment of the PEO tether (IV) dependingon the strength of the force. Reprinted with permission from ref (1143). Copyright 2011 NaturePublishing Group.

Mentions: Hao et al. monitored the strength of metal–ligand complexationwith an AFM tip.1136 One terpyridine ligandwas attached to a gold surface while another was tethered to a gold-coatedAFM tip, and the bonding forces between osmium and the ligands wereanalyzed. The use of atomic tools to induce relative motion in aninterlocked molecular system is of particular interest in the developmentof molecular machines, because each component can be designed to functionlike the mechanical components of a macroscopic machine.1137,1138 In a polyrotaxane architecture adsorbed on MoS2, cyclodextrinring was translocated by a STM tip along a polyethylene glycol thread.1139 Such shuttling was thought to be responsiblefor the conductance switch observed upon the application of a voltagevia a STM tip in a bistable [2]rotaxane.1140 The potential use of interlocked molecular constructs in informationstorage was highlighted by Leigh et al., who showed that regular patternsof deformations could be successfully generated on a rotaxane monolayerwith the aid of a STM tip.1141 These arraysformed due to the relative ease of intercomponent mobility in thesemolecular constructs. The effect of electrostatic and steric forceson the shuttling of a bistable [2]rotaxane tethered to an AFM tiphas been investigated both experimentally and theoretically.1142 A molecular shuttle made up of fumaramideand succinamide stations has been attached to a gold surface by thiollinkers (Figure 41).1143 The macrocycle was tethered toan AFM probe using poly(ethylene oxide) (PEO). Strong hydrogen-bondinginteractions with the fumaramide station resulted in a 95:5 distributionof macrocycle in favor of this station. As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station. Tensionin the tether decreased as a result of the shuttling. Further movementof the cantilever resulted in the detachment of the PEO linker fromthe probe.


Artificial Molecular Machines.

Erbas-Cakmak S, Leigh DA, McTernan CT, Nussbaumer AL - Chem. Rev. (2015)

(a) Chemical structure of a rotaxane with fumaramide andsuccinamidestations depicted in green and orange, respectively. (b) Schematicrepresentation of macrocycle movement on a thread attached to a goldsurface as a result of the force exerted by an AFM probe. Applicationof force by cantilever movement (I,II) was followed by repositioningof the macrocycle (III) or detachment of the PEO tether (IV) dependingon the strength of the force. Reprinted with permission from ref (1143). Copyright 2011 NaturePublishing Group.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4585175&req=5

fig41: (a) Chemical structure of a rotaxane with fumaramide andsuccinamidestations depicted in green and orange, respectively. (b) Schematicrepresentation of macrocycle movement on a thread attached to a goldsurface as a result of the force exerted by an AFM probe. Applicationof force by cantilever movement (I,II) was followed by repositioningof the macrocycle (III) or detachment of the PEO tether (IV) dependingon the strength of the force. Reprinted with permission from ref (1143). Copyright 2011 NaturePublishing Group.
Mentions: Hao et al. monitored the strength of metal–ligand complexationwith an AFM tip.1136 One terpyridine ligandwas attached to a gold surface while another was tethered to a gold-coatedAFM tip, and the bonding forces between osmium and the ligands wereanalyzed. The use of atomic tools to induce relative motion in aninterlocked molecular system is of particular interest in the developmentof molecular machines, because each component can be designed to functionlike the mechanical components of a macroscopic machine.1137,1138 In a polyrotaxane architecture adsorbed on MoS2, cyclodextrinring was translocated by a STM tip along a polyethylene glycol thread.1139 Such shuttling was thought to be responsiblefor the conductance switch observed upon the application of a voltagevia a STM tip in a bistable [2]rotaxane.1140 The potential use of interlocked molecular constructs in informationstorage was highlighted by Leigh et al., who showed that regular patternsof deformations could be successfully generated on a rotaxane monolayerwith the aid of a STM tip.1141 These arraysformed due to the relative ease of intercomponent mobility in thesemolecular constructs. The effect of electrostatic and steric forceson the shuttling of a bistable [2]rotaxane tethered to an AFM tiphas been investigated both experimentally and theoretically.1142 A molecular shuttle made up of fumaramideand succinamide stations has been attached to a gold surface by thiollinkers (Figure 41).1143 The macrocycle was tethered toan AFM probe using poly(ethylene oxide) (PEO). Strong hydrogen-bondinginteractions with the fumaramide station resulted in a 95:5 distributionof macrocycle in favor of this station. As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station. Tensionin the tether decreased as a result of the shuttling. Further movementof the cantilever resulted in the detachment of the PEO linker fromthe probe.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

Hydrogen-bonding solvents have been shownto disrupt macrocycle–thread interactions in single stationrotaxanes, and here addition of 5% [D4]methanol increased the rate of shuttling 100-fold, consistentwith lowering the energy barrier to migration by disrupting station–macrocycleinteractions and thus raising ground-state energies... The effect of water on the rate of shuttlinghas been investigated and was found to be greatly superior to thatof other protic solvents... Therate of escape from the station energy well can then be modeled byan Arrhenius equation with a contribution from a distance-dependentdiffusion factor to the overall rate of shuttling... A quantum mechanicaltreatment of this system has found that, as the lengthening of thespacer has no effect on the activation for breaking the hydrogen bonds,the effect on the rate of shuttling is due to the widening of theoverall potential energy well... Molecular motion in mechanically interlocked and thus kineticallystable rotaxanes can be controlled using multiple binding sites withaffinities for the macrocycle that vary under different conditions.The conditions can be modified by electrochemical redox processes,light, pH, and environmental changes... Whenthe stilbene unit adopted the E form, the macrocyclecould move randomly along the full length of the thread by Brownianmotion, while when the Z form is adopted, the macrocyclewas trapped in one or the other of the two compartments... As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station... Tensionin the tether decreased as a result of the shuttling... Extracting useful work at the molecular scalerequires the restriction of the thermal movement of submolecular componentsor the exploitation of thermal motion with additional ratcheting.Shuttling, switching, and rotation processes in solution can be modulatedexternally, and the directionality of each motion can be controlledin single molecules... Third, to drive the walker away fromequilibrium, that is, to generate directional motion, a ratchetingprocess (either an energy or an information ratchet) must take place.In addition to the requirements of a Brownian motor, certain additionalcharacteristics are necessary for a motor to be defined as a walker... For the design of processive small molecule synthetic molecularwalkers, mechanically interlocked architectures are good candidates,because the walker (macrocycle) is mechanically bonded to the track(thread)... To perform tasks that cannot be accomplishedby conventional chemical means, it will be necessary to design systemswith multiple integrated parts, each component performing a dedicatedrole within the machine ensemble... This will not be straightforwardbecause unlike a watch where the second hand, say, does not interferewith the components in the escapement mechanism, the components ofa chemical machine are not easily isolated from each other (or theenvironment) and interference from one reactive part of a machinewith another will be a significant issue as complexity increases beyondthe current rather trivial systems. (iii) The machines we arefamiliar with in the macroscopic worldare generally stable, operating unchanged through many cycles, andby and large they do not make “mistakes”... Or it may bethat evolution just did not discover these solutions to such problemsand that mankind, with the whole of the periodic table and known syntheticchemistry to work with, can... Perhaps the most productive approachwill ultimately be found by following neither of these lines of investigationtoo closely, for example, by using chemical principles for “molecularrobotics” in which ratcheted motions of molecular components(i.e., biologically inspired mechanisms) are used to perform tasksthat have their origins in innovations introduced to advance developmentsin macroscopic technology (e.g., factory assembly lines).

No MeSH data available.


Related in: MedlinePlus