Molecular ‘muscles

Muscles use energy to contract and expand in response to stimuli. They may act as an unconscious knee jerk reaction, as a consequence of neural activity arising from touching a hot surface, or in any number of conscious movements. Regardless of the reason for muscle activity the fundamental mechanism is a cycle between contraction and extension. 

On the human scale it is possible to copy this behaviour with pistons and hydraulic systems but can this be mimicked on the nanoscale If it can, then therein lies the potential to manufacture molecular scale pumps that could be incorporated into nanoscale devices. Furthermore if the contraction and expansion cycle could be linked to an external signal, such as the change in chemical environment associated with diseased tissue or tumour, the pump could form the basis of an in vivo drug delivery system that identified and treated diseases before they became manifest to a medical practitioner. 

On a simpler scale the Rybak-Akimova group found that when 4/-(amino-methylene)benzo[18]crown-6 was treated with an acid salt, mass spectrometric evidence indicated that dimers were the most stable species [13]. This was backed up by an X-ray crystal structure of the compound crystallized from methanol which revealed pairs of mutually interlocked crowns. The amine termini had become pro-tonated and the resultant ammonium group, illustrated in Fig. 8.4, formed a complex with a second crown. Although yet to be attempted, it would be intriguing to react the complex with a linear component containing a second amine and a bulky stopper group to generate a metal-free, interlocked pair of rotaxanes with two amine stations . The contraction and extension of the complex could then be controlled as a function of pH. 

Guan modelled the multidomain of titin by synthesizing a polymer composed of large macrocycles linked by alkene spacers, as seen in Fig. 8.5, that could 

---

Figure 11.41 A prototype pH triggered molecular muscle based on coiling of ligand 11.68 by Pb2+. The binding of Pb2+ is switched on or off according to the protonation state of the competitor ligand tren (reproduced by permission of The Royal Society of Chemistry).

Such oxidation-controlled molecular motion has also been used to produce molecular muscles out of [3]rotaxanes such as 108 (the wheel portion of which has been attached to a surface) which either expand or contract depending on the oxidation state (Scheme 11) <2005JA9745>. 

Cumulative nanoscale movements within surface-bound molecular muscles based on another type of rotaxanes have been harnessed to perform large-scale... 

Figure 17.8 A molecular muscle based on a self-assembled monolayer of the palindromic bistable rotaxane 9S+ (a), anchored on the gold surface of microcantilever beams (b).81 (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

Utilizing the Cu(I)/Cu(II) redox pair, Sauvage and his group designed a molecular muscle (Scheme 6.5.8) [24]. Two filaments are each connected to a macro-... 

Fig. 8.3 A molecular muscle in contracted (left) and extended (right) forms [12]...

Jimenez MC, Dietrich-Buchecker C, Sauvage JP (2000) Towards synthetic molecular muscles contraction and stretching of a linear rotaxane dimer. Angew Chem Int Ed 39 ... 

Would not it be amazing if the tiny nanomotions of molecular machines could be cooperatively scaled up to do work on macroscopic objects Toward that end, it has been demonstrated [226, 227] that a monolayer of molecular muscles - palindromic bistable [3]rotaxanes (Fig. 31a) immobilized on the surface of a thin... 

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Another area of NEMS that is receiving tremendous attention is the mimicry of biological systems, aptly referred to as biomimetics. For instance, in the development of linear molecular muscles that undergo contraction and extension movements. Initial work in this field utilized transition metal complexes containing rotaxanes and catenanes, due to the nondestructive redox processes occurring on the metal centers.Though these complexes were actuated by a chemical reaction, the movement was in a noncoherent manner. In order to better mimic skeletal muscle movement, one has to look at the mode of motion within the most efficient molecular machines - in our human bodies. 

The ability to create molecular level actuators has been taken even further in the example of thiophene-based molecular muscles by Madden and Hunter. Conformational rearrangement of the polymer backbone in molecules such as calix[4]arene-bithiophene was created at the molecular level [118]. This work has recently been reviewed and prospects for future research in this area considered [119]. 

Bruns CJ, Stoddart JF (2014) Rotaxane-based molecular muscles. Acc Chem Res 47 2186-2199. doi 10.1021/ai500138u... 

Fig. 8 (a) Depicting tetra(2,3-thienylene) and corresponding descriptors. and d (see Table 3). (b) Poly(tetra[2, 3-thienylene]). a putative molecular muscle, (c) Depicting redox-induced reversible carbon-carbon bond formation in octamethoxytetraphenylene. 

Fig. 5 Controlled molecular motion in rotaxanes light-driven shifting of the wheel along the rotaxane axle (top) and contraction of a molecular muscle stimulated hy electrochemical Cu(I)-Cu(II) interconversion (bottom).

---