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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.


Initially the system is balanced (in proportion to thesizes ofthe two compartments), with an equal density of particles in the leftand the right compartments. By changing the volume of the left-handcompartment, the system becomes statistically unbalanced. Openingthe door allows the particle to redistribute according to the newsize of the compartments. Closing the door ratchets the new distributionof the particles. Restoring the left compartment to its original sizeresults in a concentration gradient of the Brownian particles acrossthe two compartments. Here, the size of the compartment representsthe energy level of the macrocycle-station system.75
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fig29: Initially the system is balanced (in proportion to thesizes ofthe two compartments), with an equal density of particles in the leftand the right compartments. By changing the volume of the left-handcompartment, the system becomes statistically unbalanced. Openingthe door allows the particle to redistribute according to the newsize of the compartments. Closing the door ratchets the new distributionof the particles. Restoring the left compartment to its original sizeresults in a concentration gradient of the Brownian particles acrossthe two compartments. Here, the size of the compartment representsthe energy level of the macrocycle-station system.75

Mentions: In this system, the exchange of the macrocycle between stationscould be controlled by the introduction of a barrier in the middleof the thread (kinetic control) as well as by altering the bindingaffinity of the macrocycle to the two different stations (thermodynamiccontrol). Initially, 85% of the macrocycle bound the fumaramide stationand 15% the succinamide station. The system was still unlinked becauseof the kinetic barrier and therefore not in equilibrium. A balance-breakingstimulus (photoisomerization at 312 nm) generated a 49:51 E:Z photostationary state. Removal of the barrier, the linkingstimulus, allowed the balance to be restored by the macrocycle equilibration.Restoring the barrier unlinked the system. The last step was Z → E olefin isomerization, whichmade the system statistically unbalanced, unlinked, and not in equilibrium.After one operational cycle of the machine, 56% of the macrocycleswere located on the succinamide station. The thread had thereforechanged the net position of the macrocycle. Because the succinamidestation binds the macrocycle less strongly than the fumaramide station,this process has moved the macrocycle energetically uphill; that is,it has performed a ratcheting operation, transporting a particle awayfrom equilibrium. This system and its function is an experimentalrealization of the transportation task required in Smoluchowski’strapdoor and Maxwell’s pressure demon, which are discussedin the introduction (section 1) and are powered by chemical and light energy. Figure 29 shows a schematicrepresentation of the four sequential steps performed by rotaxane 75 to transport the system energetically uphill: balance-breaking1, linking, unlinking, balance breaking 2 (resetting the machine butnot the substrate).


Artificial Molecular Machines.

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

Initially the system is balanced (in proportion to thesizes ofthe two compartments), with an equal density of particles in the leftand the right compartments. By changing the volume of the left-handcompartment, the system becomes statistically unbalanced. Openingthe door allows the particle to redistribute according to the newsize of the compartments. Closing the door ratchets the new distributionof the particles. Restoring the left compartment to its original sizeresults in a concentration gradient of the Brownian particles acrossthe two compartments. Here, the size of the compartment representsthe energy level of the macrocycle-station system.75
© Copyright Policy
Related In: Results  -  Collection

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

fig29: Initially the system is balanced (in proportion to thesizes ofthe two compartments), with an equal density of particles in the leftand the right compartments. By changing the volume of the left-handcompartment, the system becomes statistically unbalanced. Openingthe door allows the particle to redistribute according to the newsize of the compartments. Closing the door ratchets the new distributionof the particles. Restoring the left compartment to its original sizeresults in a concentration gradient of the Brownian particles acrossthe two compartments. Here, the size of the compartment representsthe energy level of the macrocycle-station system.75
Mentions: In this system, the exchange of the macrocycle between stationscould be controlled by the introduction of a barrier in the middleof the thread (kinetic control) as well as by altering the bindingaffinity of the macrocycle to the two different stations (thermodynamiccontrol). Initially, 85% of the macrocycle bound the fumaramide stationand 15% the succinamide station. The system was still unlinked becauseof the kinetic barrier and therefore not in equilibrium. A balance-breakingstimulus (photoisomerization at 312 nm) generated a 49:51 E:Z photostationary state. Removal of the barrier, the linkingstimulus, allowed the balance to be restored by the macrocycle equilibration.Restoring the barrier unlinked the system. The last step was Z → E olefin isomerization, whichmade the system statistically unbalanced, unlinked, and not in equilibrium.After one operational cycle of the machine, 56% of the macrocycleswere located on the succinamide station. The thread had thereforechanged the net position of the macrocycle. Because the succinamidestation binds the macrocycle less strongly than the fumaramide station,this process has moved the macrocycle energetically uphill; that is,it has performed a ratcheting operation, transporting a particle awayfrom equilibrium. This system and its function is an experimentalrealization of the transportation task required in Smoluchowski’strapdoor and Maxwell’s pressure demon, which are discussedin the introduction (section 1) and are powered by chemical and light energy. Figure 29 shows a schematicrepresentation of the four sequential steps performed by rotaxane 75 to transport the system energetically uphill: balance-breaking1, linking, unlinking, balance breaking 2 (resetting the machine butnot the substrate).

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.