Limits...
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.

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.


(a) The structure and operation of walker 125, whichuses dynamic imine exchange chemistry. Amine footholds are highlightedin blue and red. Each molecule is labeled with one or two numbersin parentheses indicating the amine footholds to which the walkeris attached (foothold amines are assigned with numbers starting fromthe left). (b) 1H NMR spectra of indicated protons in CDCl3. H7 corresponds to a side product in which thewalker is detached from the track. Reprinted with permission fromref (1395). Copyright2012 American Chemical Society.
© Copyright Policy
Related In: Results  -  Collection

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

fig49: (a) The structure and operation of walker 125, whichuses dynamic imine exchange chemistry. Amine footholds are highlightedin blue and red. Each molecule is labeled with one or two numbersin parentheses indicating the amine footholds to which the walkeris attached (foothold amines are assigned with numbers starting fromthe left). (b) 1H NMR spectra of indicated protons in CDCl3. H7 corresponds to a side product in which thewalker is detached from the track. Reprinted with permission fromref (1395). Copyright2012 American Chemical Society.

Mentions: Reversible and processive migration of a Michaelacceptor along a protein was reported in 1979 by Lawton et al.1394 More recently, Lehn et al. used dynamic imineexchange to transport a salicylidene walker 125 alongan amine bearing track (Figure 49).1395 Deprotection of theamine at one side of the track using methoxyamine initiated the dynamicexchange reactions. The speed of exchange was shown to be modulatedwhen substitution, composition of the solvent, or temperature wasaltered. 1H NMR analysis in CDCl3 proved thatthe relative intensity of a characteristic signal corresponding tothe transported walker (H6) increased gradually and in2 days the transported walker was the dominant product in solution.Recently, the same group reported a modified version of their walkerwith thermodynamic sinks at each end of the track. These trap thestochastic walker at one end in acid as an imine, or as a lactoneat the other end under basic conditions.1396


Artificial Molecular Machines.

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

(a) The structure and operation of walker 125, whichuses dynamic imine exchange chemistry. Amine footholds are highlightedin blue and red. Each molecule is labeled with one or two numbersin parentheses indicating the amine footholds to which the walkeris attached (foothold amines are assigned with numbers starting fromthe left). (b) 1H NMR spectra of indicated protons in CDCl3. H7 corresponds to a side product in which thewalker is detached from the track. Reprinted with permission fromref (1395). Copyright2012 American Chemical Society.
© Copyright Policy
Related In: Results  -  Collection

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

fig49: (a) The structure and operation of walker 125, whichuses dynamic imine exchange chemistry. Amine footholds are highlightedin blue and red. Each molecule is labeled with one or two numbersin parentheses indicating the amine footholds to which the walkeris attached (foothold amines are assigned with numbers starting fromthe left). (b) 1H NMR spectra of indicated protons in CDCl3. H7 corresponds to a side product in which thewalker is detached from the track. Reprinted with permission fromref (1395). Copyright2012 American Chemical Society.
Mentions: Reversible and processive migration of a Michaelacceptor along a protein was reported in 1979 by Lawton et al.1394 More recently, Lehn et al. used dynamic imineexchange to transport a salicylidene walker 125 alongan amine bearing track (Figure 49).1395 Deprotection of theamine at one side of the track using methoxyamine initiated the dynamicexchange reactions. The speed of exchange was shown to be modulatedwhen substitution, composition of the solvent, or temperature wasaltered. 1H NMR analysis in CDCl3 proved thatthe relative intensity of a characteristic signal corresponding tothe transported walker (H6) increased gradually and in2 days the transported walker was the dominant product in solution.Recently, the same group reported a modified version of their walkerwith thermodynamic sinks at each end of the track. These trap thestochastic walker at one end in acid as an imine, or as a lactoneat the other end under basic conditions.1396

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.