Twist and Shout: Single-Molecule Mechanochemistry Hongbin Li† and Gilbert C. Walker*,‡ †
Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada
‡
ABSTRACT: Chemical reactions can be accelerated by various means, including applied mechanical forces. If the direction of the force does not project well onto the desired reaction coordinate, then only poor acceleration is achieved. Recent developments in single polymer mechanics illustrate how to overcome this limitation, in a simple cis−trans isomerization reaction. Generalizing the approach, synthetic chemistry can be used to attach tethers to different parts of reacting molecular fragments to direct the force usefully. This Perspective explores the prospects for using applied mechanical forces to create exciting new chemistries. For example, it is possible to imagine making polymers that sense mechanical forces within hard-to-reach places, like biological cells, or using mechanical forces to make nanoscale electrical devices using conjugated polymers.
Hints of this effect were seen before,14−16 but Huang et al. have provided a richly supported example.
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mechanical stretching force applied to the ends of a polymer chain can accelerate a chemical reaction within it.1−7 This finding has developed in the background of work that explored the effect of applying tension on the ends of polyprotein molecules to unfold them,8 isomerization reactions that did not involve bond breaking,9 and other mainly noncovalent bonding changes,10 with a few exceptions. Applied force is able both to lower the activation energy of a reaction and to change the shape of the potential energy surface. A number of surprises have emerged in the new field of mechanochemistry. Among them, it has been found that thermally forbidden reactions may efficiently occur under mechanical stress. Early explanations were that the polymer can provide a “lever arm” to amplify forces and to reduce reaction barriers.11 In this issue of ACS Nano, Huang et al.12 examine the forceinduced cis-to-trans rotation of carbon−carbon double bonds in a synthetic polymer. To some extent, the rotation might have been anticipated because the polymer chain with a predominantly trans configuration of double bonds is longer than the cis form. On the other hand, theoretical simulations indicated that the single bonds in the polymer backbone would break before cis−trans isomerization would occur. On doing the experiment, Huang et al. and Radiom et al.13 in concurrent work found not only that isomerization occurred but Huang et al. also found that the reaction rate could be accelerated a billion times over the thermally activated rate, due to the applied force. Not anticipated was the finding that the tension applied to the polymer ends gave rise to an added torsional force to facilitate the rotation of cis bonds. In other words, the applied force was not simply parallel to the double bond direction. © XXXX American Chemical Society
In this issue of ACS Nano, Huang et al. examine the force-induced cis-to-trans rotation of carbon−carbon double bonds in a synthetic polymer. Huang et al. employed a number of important tools to support their claims. They prepared a polymer containing both cis and trans double bonds, with functional groups at either end of the chain to attach the chain to the surface and an atomic force microscope (AFM) tip (see Figure 1). In the force versus
Figure 1. Pulling and twisting greatly accelerate the thermally forbidden cis−trans isomerization of a carbon−carbon double bond.
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DOI: 10.1021/acsnano.6b08562 ACS Nano XXXX, XXX, XXX−XXX
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methods, to prepare molecules that are difficult to synthesize using traditional methods and to investigate their chemical reactivity.20 Going forward, mechanochemistry will likely present new opportunities and expose new frontiers in chemistry. Electron transfer (ET) is arguably the simplest chemical reaction, yet it has not been studied by single-polymer techniques. Yet, one can imagine a simple experiment where a conjugated polymer is suspended between a tip and a substrate. An isolated chain might have numerous sections over which the electronic wave functions of the polymer might be delocalized. Simple theory suggests that an electron traveling along the polymer backbone would hop between delocalized regions under a bias applied on either end of the polymer chain. If tension is applied to the polymer chain, then the chain will straighten and, on average, the conjugation lengths will increase and the resistance of the chain will drop (Figure 2a). Such an experiment would answer
extension profile, a transition force was observed that would not be present in a simple entropy-based explanation for the resistance of the polymer chain to applied force. The force transition gave rise to a percentage increase in the polymer contour length that matched, within experimental error, the percentage increase expected for conversion of the cis double bond to a single bond. The identity of the diradical nature of the singly bonded form was confirmed using 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO), which can capture diradical intermediates; TEMPO lowered the transition force, which indicated it could bias the reaction pathway. By theoretically modeling their data, Huang et al. were able to find an activation energy for the barrier in the TEMPO-free reaction under applied force. Interestingly, this activation energy provided an opportunity to probe the similarity of the force-induced reaction with a thermally induced reaction attempted by simple sample heating. Under such heating, cis−trans isomerization was not observed, indicating that the force-induced reaction followed a different reaction path than would a thermal reactionthat is, the force-induced reaction was “thermally forbidden”.17 Quantum chemical density functional theory calculations were made to determine why the barrier to isomerization could be reduced under applied force, and a role for rotation around the C−CC−C dihedral angle was identified. From the experimental data, the authors found that roughly 1/3 of the free energy barrier to the cis− trans isomerization was overcome by torque. Earlier studies, where carbon−carbon double bonds were well-aligned with the pulling direction (e.g., polyisoprene and polybutadiene), would not have been expected to generate such a torque.
By theoretically modeling their data, Huang et al. were able to find an activation energy for the barrier in the TEMPO-free reaction under applied force.
Figure 2. (a) Effect of polymer chain stretching on charge hopping between conjugated chain sections. (b) Single-molecule force sensors embedded within cells. (c) High-throughput flow cytometry of force sensors within cells. (d) Surface potential control to examine polymer chains less than 10 nm long.
fundamental questions about single-molecule conductivity and the role of temperature, which can be effectively changed by tension, and how chain dynamics couple to ET on the nanoscale. Combining mechanical force with optical spectroscopy is largely an open frontier. Consider extending the experiment in the last paragraph: one can imagine measuring the single-chain fluorescence of such a polymer, with red shifts associated with changes in the chain conjugation length. Such efforts may also offer the possibility of developing fluorescence-based sensors to quantify the applied stretching force (Figure 2b). Combining force, electrical resistance, and fluorescence data would be a powerful development and would help to resolve questions such as conductivity at polymeric interfaces to separate holes and electrons. By pulling more than one polymer chain at a time, the role of charge hopping between chains could be identified. The challenges to single-molecule mechanics within cells were reviewed a few years ago.12 More recent developments of plasmonic nanoparticles to replace micron-dimensioned force probes in optical tweezers are exciting for cellular studies. Plasmonic interactions give rise to useful gradient forces even for 30 nm particles. Not only do such platforms promise the
CHALLENGES AND FUTURE OUTLOOK As the central driving force in mechanochemistry, force is a vector. Thus, the amplitude as well as the direction along which the force is applied are critical to the force modulation of the free energy profile for a given molecule/reaction. The impact of a torque, as revealed by Huang et al. for the cis−trans isomerization of the CC, reflects another important aspect of force on a mechanochemical reaction. Most molecules are anisotropic. Mechanical anisotropy (i.e., the phenomenon whereby the same molecule displays different mechanical stability when stretched from different directions) has been well-documented for proteins18,19 as well as for molecular clusters. One can imagine that applying a stretching force to different atoms/substituents in the same molecule will lead to different pulling directions as well as torques and, consequently, different force-induced mechanochemical reactivity. Thus, by combining force spectroscopy with synthesis, it will be possible to make use of such mechanical anisotropy to screen and ultimately strategically to tailor force application to the molecule to accelerate some reactions that are otherwise difficult to realize. Along these lines, one can also imagine using force spectroscopy, in combination with high-throughput B
DOI: 10.1021/acsnano.6b08562 ACS Nano XXXX, XXX, XXX−XXX
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(5) Wang, J.; Kouznetsova, T. B.; Craig, S. L. Activated antiWoodward−Hoffmann−DePuy Reaction. J. Am. Chem. Soc. 2015, 137, 11554−11557. (6) Larsen, M. B.; Boydston, A. J. Flex-Activated″ Mechanophores: Using Polymer Mechanochemistry to Direct Bond Bending Activation. J. Am. Chem. Soc. 2013, 135, 8189−8192. (7) Zheng, P.; Chou, C. C.; Guo, Y.; Wang, Y.; Li, H. Single Molecule Force Spectroscopy Reveals the Molecular Mechanical Anisotropy of the Fes4Metal Center in Rubredoxin. J. Am. Chem. Soc. 2013, 135, 17783−17792. (8) Rief, M.; Gautel, M.; Oesterhelt, F.; Fernandez, J. M.; Gaub, H. E. Reversible Unfolding of Individual Titin Immunoglobulin Domains by AFM. Science 1997, 276, 1109−1112. (9) Marszalek, P. E.; Oberhauser, A. F.; Pang, Y. P.; Fernandez, J. M. Polysaccharide Elasticity Governed by Chair-Boat Transitions of the Glucopyranose Ring. Nature 1998, 396, 661−664. (10) Li, I. T.; Walker, G. C. Signature of Hydrophobic Hydration in a Single Polymer. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 16527−16532. (11) Wang, J.; Kouznetsova, T. B.; Kean, Z. S.; Fan, L.; Mar, B. D.; Martinez, T. J.; Craig, S. L. A Remote Stereochemical Lever Arm Effect in Polymer Mechanochemistry. J. Am. Chem. Soc. 2014, 136, 15162−15165. (12) Huang, W.; Zhu, Z.; Wen, J.; Wang, X.; Qin, M.; Cao, Y.; Ma, H.; Wang, W. Single Molecule Study of Force-Induced Rotation of Carbon−Carbon Double Bonds in Polymers. ACS Nano 2016, DOI: 10.1021/acsnano.6b07119. (13) Radiom, M.; Kong, P.; Maroni, P.; Schafer, M.; Kilbinger, A. F. M.; Borkovec, M. Mechanically Induced Cis-to-Trans Isomerization of Carbon−Carbon Double Bonds Using Atomic Force Microscopy. Phys. Chem. Chem. Phys. 2016, 18, 31202−31210. (14) Valiaev, A.; Lim, D. W.; Oas, T. G.; Chilkoti, A.; Zauscher, S. Force-Induced Prolyl cis-trans Isomerization in Elastin-Like PolyPeptides. J. Am. Chem. Soc. 2007, 129, 6491−6497. (15) Rognoni, L.; Möst, T.; Ž oldák, G.; Rief. Force-Dependent Isomerization Kinetics of a Highly Conserved Proline Switch Modulates the Mechanosensing Region of Filamin. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 5568−5573. (16) Marszalek, P. E.; Pang, Y. P.; Li, H.; El Yazal, J.; Oberhauser, A. F.; Fernandez, J. M. Atomic Levers Control Pyranose Ring Conformations. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 7894−7898. (17) Wang, J.; Kouznetsova, T. B.; Niu, Z.; Ong, M. T.; Klukovich, H. M.; Rheingold, A. L.; Martinez, T. J.; Craig, S. L. Inducing and Quantifying Forbidden Reactivity with Single-Molecule Polymer Mechanochemistry. Nat. Chem. 2015, 7, 323−327. (18) Carrion-Vazquez, M.; Li, H. B.; Lu, H.; Marszalek, P. E.; Oberhauser, A. F.; Fernandez, J. M. The Mechanical Stability of Ubiquitin is Linkage Dependent. Nat. Struct. Biol. 2003, 10, 738−743. (19) Brockwell, D. J.; Paci, E.; Zinober, R. C.; Beddard, G. S.; Olmsted, P. D.; Smith, D. A.; Perham, R. N.; Radford, S. E. Pulling Geometry Defines the Mechanical Resistance of a Beta-Sheet Protein. Nat. Struct. Biol. 2003, 10, 731−737. (20) Dufrêne, Y. F.; Evans, E.; Engel, A.; Helenius, J.; Gaub, H. E.; Müller, D. J. Five Challenges To Bringing Single-Molecule Force Spectroscopy into Living Cells. Nat. Methods 2011, 8, 123−127. (21) Yang, D.; Ward, A.; Halvorsen, K.; Wong, W. P. Multiplexed Single-Molecule Force Spectroscopy Using a Centrifuge. Nat. Commun. 2016, 7, 11026. (22) Halvorsen, K.; Wong, W. P. Massively Parallel Single-Molecule Manipulation Using Centrifugal Force. Biophys. J. 2010, 98, L53−L55.
opportunity to work on smaller length scales, they also promise the ability to undertake single-molecule mechanics within cells. Higher-throughput methods for measuring single-molecule mechanics are desirable. One approach, pioneered at Harvard, is to use a centrifugal device.21,22 Improving the length resolution of such centrifugation microscopy to be close to that of AFM holds the key to their wider application in singlepolymer mechanics as well as single-molecule biophysics. Flow cytometry is a common high-throughput method for examining cells one at a time. Plasmonic nanoparticles have already been integrated into flow cytometry for a variety of applications, including receptor labeling, nanoparticle uptake, etc. An interesting question is how optical tweezers using nanoparticles or something similar could enable force measurements on the fly within a flowing stream of cells (Figure 2c). At present, the majority of single-molecule mechanics measurements employ a force sensor that is sensitive in a single direction (sometimes two are used). Examining correlated forces simultaneously in orthogonal directions will provide deeper insight into the anisotropy of the tensions in mechanosensitive polymers. At present, torsional forces can be detected, but the sensitivity range is quite distinct from that of normal forces that are typically detected. Furthermore, for polymer chains longer than 10 nm, the torsional forces from chemical reactions may be too weak to detect. One strategy for overcoming this limitation may be to modify the instrumentation such that shorter polymer chains may be studied. Studying shorter polymer chains requires the ability to probe forces over short distances where the tip usually snaps free from van der Waals contact with the surface; this unstable jump from contact (or jump into contact) creates a blackout where no data are currently available. It may be possible to apply time-dependent voltages to the tip to create surface interaction potentials that are minimized, hence allowing shorter chains to be studied (Figure 2d). This type of modification would make the instrument better for studying short chains and more sensitive to lateral displacements of the chain.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Hongbin Li: 0000-0001-7813-1332 Gilbert C. Walker: 0000-0002-5248-5498 Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acsnano.6b08562 ACS Nano XXXX, XXX, XXX−XXX
