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Artificial Molecular Machines.

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

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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.


Maxwell’sdemons. (a) A “temperature demon”sorts particles according to velocity, generating a temperature gradient.(b) A “pressure demon” sorts particles according totheir direction of movement, generating a pressure gradient.37
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fig1: Maxwell’sdemons. (a) A “temperature demon”sorts particles according to velocity, generating a temperature gradient.(b) A “pressure demon” sorts particles according totheir direction of movement, generating a pressure gradient.37

Mentions: The Second Law of Thermodynamics states that the entropy ofan isolated system tends to increase, leading to an equilibrium distributionwith maximum entropy. To achieve any distribution other than the thermodynamicequilibrium, work must be done on the system. Various thought experimentshave been proposed that attempt to violate this premise and drivea system away from equilibrium without expending work. In the Maxwell’sDemon thought experiment, particles in a container are sorted by an“intelligent gatekeeper”, the “demon”(Figure 1).32 Gas particles are distributed in a containerpartitioned into two sections in an isolated system. The spontaneousformation of a heat or pressure gradient would lead to a decreasein entropy and thus violate the Second Law. The gatekeeper is ableto detect the velocity of each particle and can control the gate accordingly.The demon allows particles with higher than average speeds (shownin red in Figure 1a)to pass from the right section to the left, but not from the leftto the right. The opposite holds for slower particles (shown in bluein Figure 1a). Thegatekeeper thus causes a nonuniform distribution of particles betweenthe two sections, creating a temperature gradient. Similarly, a demoncapable of detecting the direction of the particle and opening thegate accordingly can concentrate particles in one section and thusgenerate a pressure gradient (Figure 1b). If the gate is frictionless, then in both casesa nonequilibrium distribution has been achieved seemingly withoutdoing work, which would violate The Second Law. The solution to theapparent paradox lies in considering the information required by thedemon to know when to open the door. As the demon must measure thevelocity or direction of each particle approaching the door33 and cannot have infinite memory,34 at some point the demon must “forget”this information. The destruction of information has a minimum enthalpiccost associated with it35 as has been experimentallyverified,36 which always exceeds the decreasein entropy in the container. Thus, a local decrease in entropy ispaid for by a generalized increase upon information deletion.


Artificial Molecular Machines.

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

Maxwell’sdemons. (a) A “temperature demon”sorts particles according to velocity, generating a temperature gradient.(b) A “pressure demon” sorts particles according totheir direction of movement, generating a pressure gradient.37
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Maxwell’sdemons. (a) A “temperature demon”sorts particles according to velocity, generating a temperature gradient.(b) A “pressure demon” sorts particles according totheir direction of movement, generating a pressure gradient.37
Mentions: The Second Law of Thermodynamics states that the entropy ofan isolated system tends to increase, leading to an equilibrium distributionwith maximum entropy. To achieve any distribution other than the thermodynamicequilibrium, work must be done on the system. Various thought experimentshave been proposed that attempt to violate this premise and drivea system away from equilibrium without expending work. In the Maxwell’sDemon thought experiment, particles in a container are sorted by an“intelligent gatekeeper”, the “demon”(Figure 1).32 Gas particles are distributed in a containerpartitioned into two sections in an isolated system. The spontaneousformation of a heat or pressure gradient would lead to a decreasein entropy and thus violate the Second Law. The gatekeeper is ableto detect the velocity of each particle and can control the gate accordingly.The demon allows particles with higher than average speeds (shownin red in Figure 1a)to pass from the right section to the left, but not from the leftto the right. The opposite holds for slower particles (shown in bluein Figure 1a). Thegatekeeper thus causes a nonuniform distribution of particles betweenthe two sections, creating a temperature gradient. Similarly, a demoncapable of detecting the direction of the particle and opening thegate accordingly can concentrate particles in one section and thusgenerate a pressure gradient (Figure 1b). If the gate is frictionless, then in both casesa nonequilibrium distribution has been achieved seemingly withoutdoing work, which would violate The Second Law. The solution to theapparent paradox lies in considering the information required by thedemon to know when to open the door. As the demon must measure thevelocity or direction of each particle approaching the door33 and cannot have infinite memory,34 at some point the demon must “forget”this information. The destruction of information has a minimum enthalpiccost associated with it35 as has been experimentallyverified,36 which always exceeds the decreasein entropy in the container. Thus, a local decrease in entropy ispaid for by a generalized increase upon information deletion.

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.