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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) Chloride-dependent shuttling of tetralactammacrocycle, and(b) subsequent fluorescence enhancement in CHCl3 upon titrationwith tetrabutylammonium chloride. (c) Rotaxane solution in the absence(left) or presence (right) of chloride. Adapted with permission fromref (1310). Copyright2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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fig45: (a) Chloride-dependent shuttling of tetralactammacrocycle, and(b) subsequent fluorescence enhancement in CHCl3 upon titrationwith tetrabutylammonium chloride. (c) Rotaxane solution in the absence(left) or presence (right) of chloride. Adapted with permission fromref (1310). Copyright2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mentions: Indicator displacementassays involving supramolecular complexesand interlocked molecules are widely used for molecular sensing applications.1309 In these systems, a fluorescent molecule withcomplexation-dependent fluorescence properties is usually used asa guest. The optical change generated upon exchange of the fluorescentguest with the analyte is used as the reporting function. Smith etal. used a similar idea to develop a chloride anion sensor based ona rotaxane (Figure 45).1310,1311 A [2]rotaxane (113) was synthesizedusing a tetralactam macrocycle and a squaraine dye as building blocks.Tetralactam macrocycles are known to bind to and quench the emissionof red light from squaraines.1312 The exposureof this dye, upon chloride-induced displacement of the macrocycle,restored fluorescence. The process could be reversed by chloride precipitationas the NaCl salt. When the same rotaxane was adsorbed on a C-18 coatedreverse-phase silica gel plate and dipped into different concentrationsof aqueous solutions of chloride, the change in fluorescence was largeenough to be visually observable. Later, a ratiometric chloride sensorbased on the same squaraine rotaxane was reported.786 Interlocked systems are useful in such dye-displacementassays, as the displaced dye typically diffuses away preventing reversibility,whereas the stable rotaxane structure allows reversibility and reusability.In a similar example, the sodium ion-dependent shuttling of a [2.2.2]cryptandaway from a squaraine station enhanced squaraine emission.1312 Interlocked architectures have been mountedon metal nanoparticles and shown to keep their switching ability.1313


Artificial Molecular Machines.

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

(a) Chloride-dependent shuttling of tetralactammacrocycle, and(b) subsequent fluorescence enhancement in CHCl3 upon titrationwith tetrabutylammonium chloride. (c) Rotaxane solution in the absence(left) or presence (right) of chloride. Adapted with permission fromref (1310). Copyright2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
© Copyright Policy
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC4585175&req=5

fig45: (a) Chloride-dependent shuttling of tetralactammacrocycle, and(b) subsequent fluorescence enhancement in CHCl3 upon titrationwith tetrabutylammonium chloride. (c) Rotaxane solution in the absence(left) or presence (right) of chloride. Adapted with permission fromref (1310). Copyright2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mentions: Indicator displacementassays involving supramolecular complexesand interlocked molecules are widely used for molecular sensing applications.1309 In these systems, a fluorescent molecule withcomplexation-dependent fluorescence properties is usually used asa guest. The optical change generated upon exchange of the fluorescentguest with the analyte is used as the reporting function. Smith etal. used a similar idea to develop a chloride anion sensor based ona rotaxane (Figure 45).1310,1311 A [2]rotaxane (113) was synthesizedusing a tetralactam macrocycle and a squaraine dye as building blocks.Tetralactam macrocycles are known to bind to and quench the emissionof red light from squaraines.1312 The exposureof this dye, upon chloride-induced displacement of the macrocycle,restored fluorescence. The process could be reversed by chloride precipitationas the NaCl salt. When the same rotaxane was adsorbed on a C-18 coatedreverse-phase silica gel plate and dipped into different concentrationsof aqueous solutions of chloride, the change in fluorescence was largeenough to be visually observable. Later, a ratiometric chloride sensorbased on the same squaraine rotaxane was reported.786 Interlocked systems are useful in such dye-displacementassays, as the displaced dye typically diffuses away preventing reversibility,whereas the stable rotaxane structure allows reversibility and reusability.In a similar example, the sodium ion-dependent shuttling of a [2.2.2]cryptandaway from a squaraine station enhanced squaraine emission.1312 Interlocked architectures have been mountedon metal nanoparticles and shown to keep their switching ability.1313

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.