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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).

No MeSH data available.


[2]Rotaxane 74 acts as an irreversiblemechanicalswitch. The silyl ether is too bulky to allow macrocycle shuttlingbetween the two succinamide stations.75
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fig27: [2]Rotaxane 74 acts as an irreversiblemechanicalswitch. The silyl ether is too bulky to allow macrocycle shuttlingbetween the two succinamide stations.75

Mentions: Stimuli-responsive molecular shuttles are rotaxanes in which thenet position of the macrocycle on the thread is controlled by externaltriggers (light, heat, electrons, chemical, pH, binding events, etc.).In general, these triggers change the properties of the binding sitesand subsequently have an influence on macrocycle/thread interactions.In this section, various stimuli-induced mechanisms to create andcontrol nonequilibrium conformations will be presented. Rotaxane succ1-74 has two identical stations, which are distinguishable dueto the differing stoppers. The thread is divided into two compartmentsby a bulky silyl ether barrier, preventing the macrocycle from shuttlingbetween the two stations.75 By removingthe silyl ether group, dynamic exchange of the macrocycle was enabledand the system moved toward equilibrium. This resulted in a 50:50distribution of the macrocycle between the two stations. The operationis irreversible, because the attachment of the bulky silyl group againdoes not change the average position of the macrocycle, and will notrestore the initial distribution of the macrocycle; the position ofthe macrocycle is not determined by the state of the machine. Themachine starts in a statistically unbalanced state (the bulky silylgroup prevented the macrocycle restoring balance by moving towardan equilibrium distribution). By “linking” the system,removing the bulky group, the machine could reach its equilibriumstate, because free movement of the macrocycle was no longer preventedby the barrier. Therefore, the macrocycle had “escaped”to a lower energy distribution (Figure 27).


Artificial Molecular Machines.

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

[2]Rotaxane 74 acts as an irreversiblemechanicalswitch. The silyl ether is too bulky to allow macrocycle shuttlingbetween the two succinamide stations.75
© Copyright Policy
Related In: Results  -  Collection

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

fig27: [2]Rotaxane 74 acts as an irreversiblemechanicalswitch. The silyl ether is too bulky to allow macrocycle shuttlingbetween the two succinamide stations.75
Mentions: Stimuli-responsive molecular shuttles are rotaxanes in which thenet position of the macrocycle on the thread is controlled by externaltriggers (light, heat, electrons, chemical, pH, binding events, etc.).In general, these triggers change the properties of the binding sitesand subsequently have an influence on macrocycle/thread interactions.In this section, various stimuli-induced mechanisms to create andcontrol nonequilibrium conformations will be presented. Rotaxane succ1-74 has two identical stations, which are distinguishable dueto the differing stoppers. The thread is divided into two compartmentsby a bulky silyl ether barrier, preventing the macrocycle from shuttlingbetween the two stations.75 By removingthe silyl ether group, dynamic exchange of the macrocycle was enabledand the system moved toward equilibrium. This resulted in a 50:50distribution of the macrocycle between the two stations. The operationis irreversible, because the attachment of the bulky silyl group againdoes not change the average position of the macrocycle, and will notrestore the initial distribution of the macrocycle; the position ofthe macrocycle is not determined by the state of the machine. Themachine starts in a statistically unbalanced state (the bulky silylgroup prevented the macrocycle restoring balance by moving towardan equilibrium distribution). By “linking” the system,removing the bulky group, the machine could reach its equilibriumstate, because free movement of the macrocycle was no longer preventedby the barrier. Therefore, the macrocycle had “escaped”to a lower energy distribution (Figure 27).

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.