Biological and biomimetic molecular machinesjuh17:...Biological molecular machines are molecular...

REVIEW

Biological and biomimetic molecular machines
Tony J Huang† & Bala K Juluri†Author for correspondenceThe Pennsylvania State University, Department of Engineering Science & Mechanics, University Park, PA 16802, USATel.: +1 814 863 4209;Fax: +1 814 865 9974;Email: [email protected]
part of

Keywords: biomimetic, bistable rotaxanes, bottom-up fabrication, molecular machines, nanotechnology

10.2217/17435889.3.1.107 © 2

The evolution of life facilitates the creation of biological molecular machines. In these so-called ‘nanomachines,’ nature elegantly shows that when precisely organized and assembled, simple molecular mechanical components can link motions efficiently from the nanometer scale to the macroscopic world, and achieve complex functions such as powering skeletal muscles, synthesizing ATP and producing DNA/RNA. Inspired by nature, researchers are creating artifical molecular machines with tailored structures and properties, with the aim of realizing man-made active nanosystems that operate with the same efficiency and complexity as biological nanomachines. It is anticipated that in the not-too-distant future, unique applications of biological and biomimetic molecular machines will emerge in areas such as biochemical instrumentation and nanomedicine.

A molecular machine (or ‘nanomachine’) is amechanical device that is measured in nanome-ters (millionths of a millimeter, or units of 10-9

meter; on the scale of a single molecule) and con-verts chemical, electrical or optical energy tocontrolled mechanical work [1,2]. The humanbody can be viewed as a complex ensemble ofnanomachines [3,4]. These tiny machines areresponsible for the directed transport of macro-molecules, membranes or chromosomes withinthe cytoplasm. They play a critical role in virtu-ally every biological process (e.g., muscle con-traction, cell division, intracellular transport,ATP production and genomic transcription).

Even though biological molecular machineshave a long history, the idea of constructing arti-ficial molecular machines is recent. In 1959,Richard Feynman originated the idea of artificialmolecular machines in his historic address to theAmerican Physical Society, ‘There is Plenty ofRoom at the Bottom’ [5]. The soon-to-be Nobellaureate contemplated, “What are the possibili-ties of constructing molecular-scale mechanicalmachines … What would be the utility of suchmachines? Who knows? I cannot see exactlywhat would happen, but I can hardly doubt thatwhen we have some control of the arrangementof things on a molecular scale we will get anenormously greater range of possible propertiesthat substances can have, and of the differentthings we can do.” In the early 1980s, the earliestexamples of synthetic molecular-level machineswere reported; these were based on the photoi-somerization of azobenzene [6]. Since then,research in the field of artificial molecularmachines has accelerated.

This review discusses biological molecularmachines and hybrid systems that integrate theseorganic nanomachines with inorganic man-made devices. Artificial (or ‘biomimetic’) molec-ular machines are then examined, and recentexamples, such as ‘molecular muscles,’ ‘nanocars’and ‘nanovalves’ for drug delivery, are presented.Finally, the review concludes with a prospectiveof developments in nanomachinery.

Biological molecular machines Biological molecular machines are molecularproteins that convert the chemical energyreleased by hydrolysis of ATP into mechanicalenergy. Ubiquitous in biological systems, thesenanomachines are responsible for the directedtransport of macromolecules, membranes orchromosomes within the cytoplasm, thus play-ing a critical role in cell behavior and architec-ture [3,4,6–8]. Over the past decade, interest inbiological molecular machines has blossomed.Atomic structures of various nanomachines havebeen solved by X-ray crystallography, newmolecular machines have been discovered, andforces, steps and speeds have been measured atthe molecular level. In nanotechnology, twomain fabrication approaches have been utilized.In the bottom-up approach, materials anddevices are built from some of the smallestbuilding blocks, atoms or molecules. In the ‘top-down’ approach, nano-objects are constructedfrom larger entities using etching and deposi-tion techniques. Recently, hybrid devices inte-grating ‘bottom-up’-based biological molecularmachines and top-down-based inorganic nanos-tructures have been realized. These scientific

008 Future Medicine Ltd ISSN 1743-5889 Nanomedicine (2008) 3(1), 107–124 107

k.rowland

Text Box

For reprint orders, please contact: [email protected]

REVIEW – Huang & Juluri

108

breakthroughs have not only led to a much-improved understanding of cell motility, butalso have a profound impact in the field ofnanoscience and nanotechnology.

Myosin & kinesinMyosin, kinesin and their relatives are linearmotors that convert the energy of ATP hydroly-sis into mechanical work along polymersubstrates – myosin along actin filaments inmuscles and cells, and kinesin along microtu-bules (Figure 1) [9,10]. In a kinesin molecule, twoheads, each about 8 nm long, are joined at thecoiled-coil neck. The neck then connects to along-coiled coil that terminates in the tail, regionand binds to the organelle cargo. Myosin's headsare approximately twice the size of kinesin’s andare composed of two major domains: the motordomain which binds to actin and nucleotides,and the regulatory domain, which is an 8-nm-long α-helix stabilized by two calmodulin-likelight chains. A long-coiled-coil rod connects tothe regulatory domain and oligomerizes to formthe thick filament in muscle.

The developments in DNA/RNA sequenceshave helped reveal the similarities between myosinand kinesin [11]. The core 180 amino acids withinthe myosin and kinesin head structures have thesame fold. Furthermore, the preserved sequencedirection and order of the α-helices and β-sheetswithin the core suggest that these two biomotorsevolved from a common ancestor. Yet, despitethese structural similarities, there are profound

functional differences between kinesin andmyosin. Conventional kinesin operates alone or insmall numbers to transport membrane-boundorganelles over large distances (up to a millimeter)along microtubules, whereas muscle myosin oper-ates in huge arrays of up to a billion molecules in alarge muscle fiber and moves relatively short dis-tances (up to approximately one micron) alongactin filaments (Figure 2) [12,13]. Myosin providesthe power for all of our voluntary motions (e.g.,running, walking and lifting) as well as for invol-untary motions (e.g., beating of the heart).

Recent years have witnessed a strong interest inthe exploration of kinesin-based biological molec-ular machines for nano- and microscale locomo-tion applications. Limberis and Stewartdemonstrated that kinesin could be engineeredand used to transport silicon microchips alongmicrotubules [14]. Negatively charged microtu-bules bound electrostatically to a positivelycharged coverslip were aligned in a microfluidicchannel. Next, genetically modified kinesinmotors attached with silicon microchips wereadded into the microfluidic channel. The micro-chips were observed to move in a variety of ways(Figure 3), depending on how they were bound tomicrotubules. If a microchip moved along parallelmicrotubules, it traveled along linearly at a maxi-mum speed of 0.8 µm/s (Figure 3A). When it inter-acted with antiparallel microtubules, it rotated(Figure 3B). A microchip with one end fixed flippedover (Figure 3C). This work proved that kinesinmotors had enough force to transport cargos thatare much larger than kinesin molecules, thus pav-ing the way for future development of biomotor-based nanosystems.

To improve the efficiency of kinesin-basednanosystems, the orientation of microtubulesmust be controlled and aligned. Hiratsuka et al.developed a lithographic process to restrict micro-tubules onto fabricated tracks and control thedirection of kinesin’s mechanical motion [15]. Theexperiment used two concentric circles witharrowhead patterns facing opposite directions (Fig-

ure 4). Without the arrowheads in the pattern, themicrotubules would travel either clockwise orcounterclockwise in either circle. However, oncethe arrowheads pattern was added, within a fewminutes the microtubules would travel in thesame direction that the arrows pointed towards.As microtubules enter the arrowhead from thecorrect direction, they could easily pass through;however, if microtubules entered from the reversedirection, they would usually bump against thewall at the base of the arrow and turn around.

Figure 1. Schematic drawing of two kinesin motors bound to a microtubule segment.

The microtubule is built up from tubulin dimers and has a diameter of 24 nm. Each kinesin motor molecule has two motor domains or heads which are connected by a neck region and a long stalk. The total length of the kinesin molecule is approximately 80 nm [13]. Reproduced with permission from [13].

Nanomedicine (2008) 3(1) future science groupfuture science group

Biological and biomimetic molecular machines – REVIEW

future science groupfuture science group

Figure 3. Kinesin-po

(A) The chip moves on preference point. (B) The reference dot was placedperpendicular to the surfScale bar: 5 µm. Reproduced with permiss

Four different versions of the arrowhead patternwere tested; the most efficient pattern had a 73%chance of reversing microtubules in the wrongdirection.

The alignment of microtubules for guidingthe long-range transportation of kinesin motorshas also been investigated. Platt et al. reported

the alignment of magnetic nanoparticle func-tionalized microtubules in magnetic fields (Figure

5) [16]. In this study, microtubules were labeledwith cobalt ferrite (CoFe2O4) nanoparticlesthrough the biotin–neutravidin binding. Whenplaced above a magnet, magnetized microtu-bules aligned to match the orientation of mag-netic line. It was also shown that microtubulescould respond to the changing of magnetic ori-entation and reorient them within several sec-onds. Recently, in order to increase thedirectional control of magnetic nanoparticlelabeled microtubules, Huchtins et al. selectivelylabeled magnetic nanoparticles to a segmentedmicrotubule [17].

Recently, van den Heuvel et al. reconstitutedthe kinesin–microtubule transport system inenclosed fluidic channels and realized the activeelectrical control of the direction of individualkinesin-propelled microtubule filaments atY junctions [18]. Using this technique, theydemonstrated molecular sorting of differentlylabeled microtubules (Figure 6). The steering ofmicrotubules is attributed to electric field-induced bending of the leading tip. From meas-urements of the orientation-dependent electro-

Figure 2. Myosin filament operation.

The sliding of myosin along actin filaments causes muscle contraction and extension.

wered silicon microchips.

arallel microtubules at 0.8 µm/s. The arrow is a fixed chip rotates clockwise on antiparallel microtubules. A on one corner of the chip. (C) The chip flips ace [14].

ion from [14].

109www.futuremedicine.com


110

phoretic motion of individual, freely suspendedmicrotubules, they estimate the net appliedforce on the tip to be in the pN range.

These developments in kinesin–microtubuletransport systems have enabled initial stepstowards directed assembly in micro- and nano-scale. Brunner et al. recently demonstrated engi-

neered cargo transport by functionalizingimmobilized cargoes and microtubules with spe-cific cross-linking pairs [19]. The molecular rec-ognition event between the cargo- andmicrotubule-induced necessary rupture force toload immobilized cargo onto the microtubuleand transport it.

Figure 4. Unidirectional movement of microtubules along circular tracks.

(A) Image of the arrowhead pattern. (B–F) Photographs demonstrating the microtubule rotational movement were taken at 72 s intervals beginning 20 min after the addition of ATP. Some filaments were dyed to facilitate tracking of microtubules: the red microtubules are moving clockwise and the green microtubules are moving counterclockwise [15]. Reproduced with permission from [15].

Figure 5. Alignment of microtubules in magnetic field.

(A) Schematic of the CoFe2O4 nanoparticle functionalized microtubules on a kinesin surface. (B) The 90° rotation of magnetic field causes the reorientation of previously aligned microtubules within 4 s [16]. Scale bar = 58 µm. Reproduced with permission from [16].




Locomotion is not the only attribute of bio-logical molecular machines. Hess et al. havedeveloped a forcemeter assembled from kinesinand microtubules (Figure 7) [20]. One microtubulewith known stiffness serves as a cantilever. Forceis applied by another microtubule propelled bykinesin motor proteins adsorbed to a surface.The two microtubules are attached by a linkwhose strength is to be determined. The movingmicrotubule continues to travel while bendingthe molecular cantilever until the link ruptures.The force on the link can be calculated based onthe bending of the cantilevered microtubule, thedistance traveled by the second microtubule andthe angle between the two microtubules. Sincethis forcemeter measures forces at pN range, it isideal for measuring the strength of biologicalreceptor/ligand pairs, such as streptavi-din–biotin. In similar approaches, Deiz et al. andDinu et al. have used kinesin–microtubule sys-tem to physically stretch biofunctionalized DNAmolecules aimed towards parallel formation ofstructured DNA templates [21,22].

ATPaseATP synthase (ATPase) is a ubiquitous proteinthat functions as a proton pump and the primarysource of ATP for most eukaryotic cells [23–27]. It

manufactures ATP from ADP and phosphate byusing the energy derived from a transmembraneproton gradient. It can also reverse itself andhydrolyse ATP to pump protons against an elec-trochemical gradient. ATPase carries out both itssynthetic and hydrolytic cycles by a rotary mech-anism. The general structure of a F0F1-ATP syn-thase is shown in Figure 8. It consists of twoportions: a soluble component, F1, containingthe catalytic sites; and a transmembrane compo-nent, F0, comprising the proton channel. Theentire F0F1 structure is arranged as a counter-rotating ‘rotor’ and ‘stator’ assembly. The statorportion consists of catalytic sites from the α3β3hexamer, together with subunits α, β2 and δ.The rotor consists of 9–12 c-subunits arrangedin a ring and connected to the β- and ε-subunitsthat form the central ‘shaft’.

Recently, the rotation of F1-ATPase wasdirectly observed during ATP hydrolysis byattaching a fluorescent actin filament to theγ-subunit and immobilizing the α3β3 subuniton a coverslip [28]. In another experiment [29],F1-ATPase was integrated with inorganic nano-engineered systems to form hybrid organic-inor-ganic–nanomechanical devices powered bybiological molecular motors (Figure 9). The deviceconsisted of three parts:

Figure 6. Demonstration of molecular sorting.

(A) Color image of a mixture of red- and green-labeled microtubules approaching a Y junction. Electrical force is used to steer microtubules carrying green and red fluorophores into the right and left reservoirs, respectively. (B) Example of successful sorting events for a green- and a red-labeled microtubule. (C) Steering efficiencies for 189 red and 110 green microtubules [18]. Reproduced with permission from [18].



112

Figure 7. Kinesin an

The cantilevered MT is atother end is suspended acauses the smeared-out apropelled by kinesin motoof beads. The snapshots s), (B) makes contact (10s) the cantilevered microtMT: Microtubule.Reproduced with permis

• Nanofabricated nickel posts

• Recombinant F1-ATPase motors that wereengineered to adhere with the nanofabricatedstructures

• Engineered nanopropellers

To integrate these components into one func-tional unit, the F1-ATPase biomolecular motorswere biotinylated and then attached to the Niposts using histidine tags engineered into theβ subunit. In addition, the Ni nanopropellerswere linked to the γ-subunit usingbiotin–streptavidin. The rotation of the propel-lers was tested with and without ATP to prove

the dependency on the F1-ATPase motor. WithATP, only five of the 400 propellers rotated con-tinuously. Approximately 80% of the propellersdid not move at all. It was thought that the pro-pellers might have been attached to more thanone motor or attached to the substrate in addi-tion to the motor. Rotational velocity of the pro-pellers varied depending on the point ofattachment and the length of the propeller.Although the device needed further improve-ment, it laid the foundation for the future devel-opment of nanosystems using F1-ATPase as theactive nanostructure.

DNA DNA stores the genetic information vital forthe development and functioning of all livingorganisms. Taking advantage of the extraordi-nary specificity between two complementaryDNA strands, researchers have created a varietyof nanoscale machinery. Yurke et al. con-structed a DNA-based molecular tweezer oper-ated by the addition of specific oligonucleotidesequences [30]. In its open status, the DNAtweezer consists of a flexible single-strand hingeconnecting to two double-stranded arms (Figure

10A). Upon the addition of a set strand (F), themolecular tweezer hybridizes with the setstrand and the arms are pulled together, chang-ing the molecular conformation from the openstatus to the closed one. The reversible processcan be achieved by adding an unset strand

complementary to the set strand (F).Results from dye quenching experiments con-firmed that the operation of the DNA-basedmolecular tweezer can be repeated for multiplecycles (Figure 10B).

Recently, Shin et al. reported a DNA walkerthat can travel along a linear track by sequentialaddition of different complementary DNAstrands (Figure 11) [31]. The DNA walker consistsof a hybridized DNA pair with two legs. Toobtain stepwise motion of the DNA walkeralong the linear track, a DNA strand (A1) isadded to bind one leg of the DNA walker to thefirst anchor strand of the linear track. In the nextstep, a second DNA strand (A2) attaches theother leg of the walker molecule to the track.This step is followed by the addition of A1’s com-plementary strand D1, which unbinds the firstleg of the DNA walker from the linear track.When repeated in a cyclic fashion, this procedurerealizes linear motion of the DNA walker mole-cule along the track. The operation of this DNAwalker requires a constant supply and removal of

d microtubule.

tached on one end to a clump of beads, while the bove the surface. Swiveling due to Brownian motion ppearance of the free end. The moving microtubule is r proteins adsorbed to the surface and carries a clump

show (A) how the moving microtubule approaches (0 s), (C & D) bends (20 s and 30 s) and (E) releases (40 ubule [20].

sion from [20].

F( )




Figure 8. Schematic

The transmembrane (F0) cyclically synthesize and Reproduced with permiss

different DNA strands. To overcome this limita-tion, autonomous DNA walkers were later pro-posed and demonstrated by Bath et al. [32] andYin et al. [33]. These works have demonstratedthat by carefully engineering DNA strands, it ispossible to harness the binding between comple-mentary DNA strands to accomplish complicatedtasks such as locomotion.

Biomimetic molecular machinesBiological molecular machines are intrinsicallydesigned for the physiological conditions (37°C,pH 7) within the human body. These restric-tions, in addition to the difficulties of modify-ing their structure and functionality, hinderintegration with man-made devices. In thisregard, the future of molecule-driven actuatorslies in the development of artificial molecularmachines as opposed to biomolecular machines.The ability to synthesize analogical molecularversions of macroscopic machinery is consideredto be an essential step towards the constructionof molecular devices capable of performingmechanical work and storing/transmittinginformation [2]. Similarily to, their macroscopiccounterparts, artificial molecular machines arecharacterized by:

• The form of energy input enabling activation• The kind of movement executed by their

components• The way in which their operation can be con-

trolled and monitored• The possibility to repeat the operation at will• The time scale needed to complete a cycle of

operation• The function performed

Recent advances in synthetic chemistry haveyielded analogs of rotors, gears, switches, shut-tles, turnstiles and ratchets [1]

Rotaxanes and catenanes, which are com-posed of mutually recognizable and intercom-municating ring and dumbbell-shapedcomponents, are considered some of the mostsuccessful artificial molecular machines con-structed to date [1,2,34–41]. When stimulated bylight, electricity or chemical reagents, thesemechanically interlocked molecules (Figure 12)

experience relative internal motions (rotary andlinear) of their components just like the movingparts of macroscopic machines. A rotaxane is amolecule composed of a macrocyclic and adumbbell-shaped component (Figure 12A). Themacrocycle encircles the linear rod-like portionof the dumbbell-shaped component and istrapped mechanically around it by two sizeablestoppers. Thus, the ring and dumbbell compo-nents cannot dissociate from one another, eventhough they are not covalently bonded to eachother. The structural formula and graphicalrepresentation of a bistable rotaxane R1 isshown in Figure 12C. R1 consists of a tetracati-onic cyclobis(paraquat-p-phenylene)(CBPQT4+) ring component and a linear rodsection that contains two different stations, atetrathiafulvalene (TTF) unit and a dioxynaph-thalene (DNP) unit. R1’s starting state has itsTTF unit encircled by the positively chargedring. Upon oxidation, the TTF unit becomesdicationic (TTF2+) and experiences electrostaticrepulsion with the ring, causing the ring toshuttle toward the DNP unit in the oxidizedstate. Conversely, reduction of the TTF2+ backto a neutral TTF unit causes R1 to return to itsstarting state. Via ring translation, this bistablerotaxane can therefore be equated as a linearmolecular motor.

By contrast, a catenane is a molecule com-posed of two interlocked macrocyclic compo-nents. The incorporation of two differentrecognition sites within the ‘asymmetric’macrocycle (Figure 12B) allows the ‘symmetric’

drawing of a F0F1-ATP synthase.

and soluble component (F1) of F0F1 synthase can hydrolyze ATP by a rotary mechanism [25]. ion from [25].



114

Figure 9. F1-ATPase

The device consists of (A(B) F1-ATPase motor, (C)150 nm) and (D) assembReproduced with permis

macrocycle component to preferentially residearound one of these two positions. By switch-ingthe properties at one of the two recognitionsites of the asymmetric macrocycle on or off ,the relative position of the two species can bereversibly controlled.

Capable of delivering mechanical motions atdramatically reduced scales in comparison to tra-ditional microscale actuators, artificial molecularmachines such as bistable rotaxanes can be thekey active nanostructures for many applications.For example, as actuation materials, bistablerotaxanes have at least seven advantages.

• They can maintain their actuation propertiesacross multiple scales (from single moleculelevel to nano to micro to meso to macro),while most actuation materials are not capa-ble of keeping their actuation properties insmaller scale, the functions of these polymersare bulk properties that require multiunitinteractions, and thus are not desirable formolecular-level devices [42–45];

• They can generate large strains up to 42%,while the strains generated by the gold-stand-ard actuation materials, such as piezoelectricmaterials, are typically 0.1–0.2% [42–45];

• They have high force density, for example, abistable rotaxane generates a 100 pN force[46,47], while a kinesin biomotor, which ismuch larger than a bistable rotaxane, can onlygenerate 6 pN [7,11];

• They can undergo controlled mechanicalmotion for a variety of external stimuli (chem-ical [37,39], electricity [38,40] and light [36,48]),while traditional actuation materials and bio-motors must both rely on a single stimulus;

• They can be made metastable, the moleculescan persist in their actuated state long after thestimulus has been removed, and are thusenergy-saving;

• They can be customized and optimized,therefore conferring the flexibility necessaryfor a multitude of engineering applications.For example, they can be derivatized withdisulfide tethers to facilitate the formation ofself-assembled monolayers (SAMs) [49] orprepared with amphiphilic properties tofacilitate the formation of Langmuir–Blodg-ett films [50,51], both of which are key bot-tom-up nanomanufacturing technologies forthe simultaneous self-organization of a mul-titude of molecules;

• They can survive in a wide range of tempera-tures (-30°C to 100°C) and pH values (5–9),while biomotors are restricted to physiologi-cal conditions (T ∼37°C, pH ∼7). Theseadvantages indicate that although biologicalnanomachines provide perfect actuation inhuman bodies, artificial molecular motorssuch as bistable rotaxanes are more suitablefor the development of molecular motor-based medical and engineering applications(Table 1).

Molecular valve for drug delivery Nguyen et al. have utilized the locomotion of thering in R1 to build an impressive molecular valvefor drug delivery purposes [52]. To build this sys-tem, one end of the R1 molecule was attached tothe pores of silica nanoparticles and the otherend was left standing free. Redox-controlled bist-ability of the ring in R1 molecule allowed thestoring and releasing of the drug (guest mole-cules) across the pores of silica nanoparticles. Inthis system, guest molecules were loaded andstored in the pores when the ring of R1 was awayfrom the silica surface. When subjected to anoxidizing environment, the ring of R1 travelstowards the silica surface, enabling the guest

-powered nanomechanical device.

) Ni post (height 200 nm, diameter 80 nm), nanopropellers (length 750–1400 nm, diameter led device [29]. sion from [29].




molecules to be stored within the nanoparticles(Figure 13A). To release the guest molecules, theseloaded nanoparticles were subjected to a reduc-ing environment. In the reducing environment,the ring moves away from the silica surface, caus-ing the controlled drug release. This system wastested for storing and releasing a luminescentguest molecule Ir(ppy)3. When the nanoparti-cles loaded with Ir(ppy)3 were subjected to areducing environment, a sudden increase in theluminescence (Figure 13B) was correlated to therelease of the guest molecules due to the move-ment of the ring away from the silica surface.Recently, a similar release of guest molecules wasdemonstrated by photoactive rotaxanes [53,54].Such systems can be potentially utilized in thefuture light-driven molecular-level drug deliverysystems.

Nanocars Researchers at Rice University have demon-strated a nanoscaled version of a macroscopic cartranslating on a gold surface [55]. This single-molecule nanocar was built by four fullerenemolecules that act as wheels of a car (Figure 14A).Fullerenes are football-shaped molecules consist-ing of 60 carbon atoms and are reported to rollon gold surfaces. These fullerene wheels wereconnected with a chassis synthesized by Pd-cata-lyzed coupling reactions, and the presence of

alkyne connections in the chassis enabled therotation of the wheels. When the temperature ofthe gold substrate was raised above 170 °C, scan-ning tunneling microscopy images revealed thatthe nanocar exhibits both pivotal and translatingmotions (Figure 14B–F). These two motions arehypothesized to occur owing to the fullerenesrolling in multiple directions on the gold sur-faces. The reported nanocar lacked the capabilityto carry a cargo and was not self-motorized.Towards the realization of motor-driven nano-cars, a light-driven nanocar has been recentlysynthesized and shown to operate in solutionphase [56]. The potential applications of thesenanosystems could be in transportation of cargosfrom one location to another in multiple bio-medical applications.

Powering a microcantilever actuator with molecular muscles Huang and colleagues created a molecularmachine-based mechanical actuator by utilizing ahybrid top-down/bottom-up manufacturingapproach [57]. Expanding upon a series of bistable[2]rotaxanes, a bistable [3]rotaxane R2 (Figure 15A

& 15B) was created as a molecular muscle tomimic the contraction and extension movementsof skeletal muscle. This design takes advantage ofwell-established recognition chemistry that selec-tively positions the CBPQT4+rings around the

Figure 10. Operation of DNA-based molecular tweezer.

Operation of a DNA based molecular tweezer. (A) The open status can be changed to the closed status by adding a set strand (F). This process can be reversed by adding an unset strand . (B) The operation of DNA tweezer over multiple cycles is recorded by dye quenching techniques.Reproduced with permission from [30].

F( )



116

two TTF units of R2, as opposed to the twoDNP units. Chemical oxidation of the TTF unitsto their dicationic forms (TTF2+) drives theCBPQT4+ rings to the DNP units. This power

stroke arises primarily from electrostaticcharge–charge repulsion between the CBPQT4+

rings and the TTF2+ units. Upon reduction of thetwo TTF2+ stations back to their neutral form,

Figure 11. Stepwise motion of DNA Walker on a linear track.

Stepwise motion of a DNA walker. (A) Walker DNA molecule and linear track for locomotion. (B) Addition of A1 strand causes the binding of the DNA walker to the first anchor point on the locomotive track. (C) Addition of A2 binds the walker to the adjacent anchor point on the track. (D) Addition of A1’s complementary strand D1 causes the release of walker from the first anchor point and simultaneous yielding of waste DNA pair. Reproduced with permission from [31].




the inter-ring distance increases as the CBPQT4+

rings return to the TTF stations by means of athermally activated diffusive stroke. Thus, thecycle of contraction and extension within R2mimics the motion that occurs inside naturalmuscle fibers as shown in Figure 2. The incorpora-tion of a disulfide tether onto each CBPQT4+

ring component provides an anchoring point bywhich the [3]rotaxane can be attached to a goldsurface as a SAM. Oxidation of R2 will generate atensile stress upon a gold surface through thecontractive action of its two disulfide-tetheredCBPQT4+ rings. If the substrate is sufficientlythin and flexible, such as a long cantilever beam,

Figure 12. Artificial molecular machines.

Schematic drawing of a bistable (A) rotaxane, (B) catenane and (C) Molecular structure and operation mechanism of a bistable rotaxane R1.

Table 1. Comparison between biomotors and bistable rotaxanes.

Biological molecular motors Bistable rotaxanes

Surviving Condition Physiological conditions only (37°C, pH 7) T: -30–100°C, pH: 5 9

Flexibility Difficult to modify structures/functions Easy to modify

Stimuli Chemical only Chemical, electricity or light

Size 30–100 nm3 8 nm3

Force ~6 pN (Kinesin) 40–100 pN



118

Figure 13. Drug deli

(A) R1 molecules are attais done in four steps (loathe molecule controls theupon the addition of ascIr(ppy)3 molecules.Reproduced with permis

the cumulative effect of each individual molecu-lar muscle will produce an upward mechanicalbending of the beam. Conversly, reduction of theoxidized and contracted R2 will return theCBPQT4+ rings to the TTF stations and, conse-quently, relieve the stress upon the beam, result-ing in a downward motion and a return to thebeam’s equilibrium position.

For the first time the authors reported thenanomechanical response of microcantileverbeams coated with a SAM of artificial molecularmotors for chemical stimulus [57]. A simple modelthat considers the mechanical movements of eachmolecule within the SAM verifieschemo–mechanical transduction as a likely mech-anism for cantilever bending. Thischemo–mechanical actuation response of artificialmolecule machines was realized with a gold-coated silicon cantilever array that was coatedwith a SAM of R2 (Figure 15C) and placed in atransparent fluid cell. Addition of the oxidantsolution was observed to cause the cantileverbeams to bend to an upward limit of approxi-mately 35 nm (Figure 15D, top series of traces).Entry of the reductant solution into this systemcaused the beams to bend back downward to theirstarting positions. This behavior was observed forall four cantilever beams for 25 cycles (the first

three complete cycles are shown here). The slightattenuation in beam deflection following eachcycle is attributed to a gradual chemical and/orphysical passivation of the SAM. Nevertheless,the movement of the cantilever beams is directlycorrelated with the cycling of the oxidant andreductant solutions and the experimental data(35 nm displacement) matches closely with thetheoretical quantitative analysis (48 nm displace-ment). Furthermore, control experiments wereperformed with a control compound TPD thatdid not have movable rings, and the results(Figure 15D, bottom series of traces) prove that theobserved actuation is due to the movement of therings. These observations support the idea thatcumulative nanoscale movements within surface-bound molecular muscles can be harnessed toperform larger-scale mechanical work. In thefuture, it can be expected in the future that similarnanomechanical work will be performed onmicrocantilevers through other activation meth-ods such as light and electricity [58].

Driving macroscopic liquid droplets with nanomachines Berna and colleagues demonstrated the use of amolecular machine-based photoresponsive sur-face to control macroscopic liquid transport

very device based on R1 molecular machine.

ched to porous silica nanoparticles to build a drug delivery system [52]. Loading and releasing of guest molecules ding, closing the valve, opening the value, release). Motion of the ring toward or away from the surface along storage and release of the drug. (B) Sudden increase in the emission intensity of luminescent guest molecule

orbic acid indicates the operation of molecular valve. (C) Inset shows the visible spectrum of the released

sion from [52].




across surfaces [59]. In this work, a SAM of11-mercapoundeconoic acid was assembledonto a gold surface, and photoactive rotaxanesR3 were physisorbed onto the SAM with thedumbbell of R3 parallel to the gold surface (Fig-

ure 16A & 16B). R3 is comprises a ring and adumbbell with a fluoroalkane station and aphoto-responsive fumaramide station, whichhas a high binding affinity for the ring. Photoi-somerization results in a transformation of thefumaramide into maleamide, which has a muchlower affinity for the ring component. Thus,irradiation of light reduces the binding affinityto the ring drastically and, as a result, the equi-librium position of the ring is in favor of thefluoroalkane station, instead of the fumaramidestation. Therefore, the ring movement can beused to expose or conceal the fluoroalkane sta-tion, thus changing the surface energy. Whensmall drops of low-volatility liquids (e.g.,CH2I2) are deposited onto the molecularmachine-based photoresponsive surface, the col-lective operation of the R3 monolayer was suffi-cient to power the movement of a microliter

droplet up a twelve degree incline (Figure 16 C–F).In this experiment, approximately 50% effi-ciency was achieved and approximately 50% ofthe light energy absorbed by R3 was used toovercome the effect of gravity.

Rotating a microscopic glass rod with nanomachinesAnother illustration of the power of artificialnanomachines was completed by Eelkema andcolleagues [60]. They proved that light-drivenunidirectional molecular rotary motors R4embedded in a liquid-crystal (LC) film arecapable of rotating a glass rod that is approxi-mately 10,000-times larger than R4. R4(Figure 17A) is comprised of a rotary part (a right-handed helical structure), an axle (a central car-bon–carbon double bond) and a stator part.Upon doping of a nonpolymeric LC film withR4, the helical organization induced by R4results in a polygonal fingerprint texture in thesurface of the LC film. When this R4-dopedsample is irradiated with UV light with a wave-length of 365 nm, photochemical isomerization

Figure 14. Locomotion of Nanocar on gold surface.

(A) Nanocar is comprised of four wheels made up of fullerenes and a chassis component. They exhibit both pivotal and translational motion on gold surface due to the rolling nature of fullerenes. (B-F) Pivoting and translating motion trajectory of nanocar on gold surface captured by STM [55].Reproduced with permission fron [55].



120

Figure 15. Realizatio

(A) Molecular structure amicrocantilever beams bywith the process shown Reproduced with permis

around the central carbon–carbon bond on R4occurs, which leads to the inversion of the helic-ity from being right handed to being left handedand the reorganization of the LC film in a rota-tional (clockwise) fashion. Removing the UVlight source causes a rotation in the oppositedirection (counterclockwise). The rotation of thetexture in the R4-doped LC film can be utilizedto manipulate a glass rod (5 µm by 28 µm) placedon top of the surface (Figure 17B). In other words,changes in the shape of R4 induce a remarkablerotational reorganization of the LC film in whichR4 is embedded, and cause the rotation of sub-millimeter-sized particles on the surface.Recently, they also demonstrated that thesemolecular rotary motors can be immobilizedonto a fixed substrate and rotation can beachieved in monolayers of molecules [61], a crucialstep towards harnessing energy at larger scales.

Conclusion In this article, we reviewed recent advances inboth biological and biomimetic molecularmachines. Biological nanomachines play essen-tial roles in virtually every biological process,from powering motion in the human body toproducing DNA and RNA. In stark contrast tobiology, none of mankind’s major-impact tech-nologies involves the exploitation of nanoma-chines, and the field of biomimetic molecularmachines is still in its infancy.

In the past two decades, research in the field ofbiological and biomimetic molecular machineshas accelerated. The mechanisms of novel biologi-cal molecular machines are being understood.Nanomachines have been integrated with man-made devices. Biomimetic molecular machineswith precisely tailored structures and functions(e.g., molecular analogs of rotors, gears, switches,

n of Nanoscale actuator powered by artificial molecular muscles.

nd schematic representation of bistable [3]rotaxane molecular muscle R2. (B) Schematic diagram of bending switching R2. (C) Molecular structure of control dumbbell compound (TPD). (D) Experimental data associated

in (B).sion from [57].




Figure 16. Macroscomolecular machines

(A) Schematic drawing o240–400 nm light causesdroplet against the gravitReproduced with permiss

pic transport of liquid droplets by surface-bound optically switchable .

f light-switchable rotaxanes R3 physisorbed onto a SAM of 11-MUA on Au(111). (B) Illumination with the rings shuttle to the fluoroalkane units, leaving a more polarophilic surface. (C–F) Locomotion of water ational forces on a surface coated with R3 molecules [59].ion from [59].



122

Figure 17. Demonst

(A) Chemical structure o45 s and show clockwiseReproduced with permis

Executive summary

• A molecular machine or optical energy to c

• Myosin, kinesin and AThey have been succemolecular sorters.

• With great strides mafunctions can be synt

• Although biological nrotaxanes, are more s

• When stimulated by lexperience relative int

• Cumulative nanoscalework (e.g., driving midroplets), mimicking w

shuttles, turnstiles and ratchets) have been syn-thesized. Cumulative nanoscale movementswithin biomimetic nanomachines have been har-nessed to perform large-scale mechanical work(e.g., driving microcantilever beams, rotating glassrods that are less than a millimeter in length andmanipulating microliter droplets), mimickingwhat biomotors such as myosin have achieved inhuman muscles. It is anticipated that biomimeticmolecular machines could soon possess the ele-gance, efficiency and complexity of their biologi-cal counterparts. With the capability to deliverprecisely controlled mechanical motions at themolecular level, these tiny machines could havetremendous impact in the field of nanomedicine.

Future perspectiveJust as individual molecules must organize intocoherent assemblies for larger-scale work, so

must chemists, biologists, material scientists andengineers cooperate to promote a better funda-mental understanding of biological and biomi-metic molecular machines in surface-bound andsolid-state environments. Major challenges inthis field remain, such as light-driven molecularmachines, unidirectional molecular rotors, fastermolecular motions over longer time frames andbio-compatible, synthetic molecular machines.Although challenges remain, we believe that withthe collaboration across different disciplines, wecan design, construct and apply molecularmachinery with multifunctions over multiscales(nano, micro, meso and macro) for numerousbiomedical applications.

Acknowledgements

The author wishes to thank Thomas Walker for his help withthe manuscript.

ration of molecular rotor at work.

f R4. (B) Rotation of a glass rod on a R4-doped LC film. Pictures (from left to right) were taken at 0, 15, 30, and rotations of 0, 28, 141, and 226°, respectively [60]. Scale bars: 50 μm.sion from [60].

(or ‘nanomachine’) is a mechanical device that is measured in nanometers and converts chemical, electrical ontrolled mechanical work.

TPase are biological nanomachines that perform locomotive and actuation functions in living organisms. ssfully used to power man-made nanomechanical systems such as cargo transporters, propellers and

de in the field of supramolecular chemistry, biomimetic molecular machines with tailored structures and hesized.

anomachines provide perfect actuation in human bodies, biomimetic molecular machines such as bistable uitable for the development of molecular machine-based medical and engineering applications.

ight, electricity or chemical reagents, mechanically interlocked molecules termed bistable rotaxanes ernal motions of their components just like the moving parts of macroscopic machines.

movements within biomimetic nanomachines have been harnessed to perform large-scale mechanical crocantilever beams, rotating glass rods that are less than a millimeter in length and manipulating microliter

hat biomotors have achieved in human muscles.




Financial and competing interests disclosureThis work was supported in part by the NSF NIRT grant(ECCS-0609128), and the start-up fund provided by theESM Department, College of Engineering, MaterialsResearch Institute, and Huck Institutes for the Life Sciencesat The Pennsylvania State University. The authors have no

other relevant affiliations or financial involvement with anyorganization or entity with a financial interest in or finan-cial conflict with the subject matter or materials discussed inthe manuscript apart from those disclosed.

No writing assistance was utilized in the production ofthis manuscript.

Bibliography1. Balzani V, Credi A, Raymo FM, Stoddart

JF: Artificial molecular machines. Angew. Chem. Int. Ed. 39(19), 3349–3391 (2000).

2. Balzani V, Credi A, Venturi M: Molecular Devices and Machines – A Journey into the Nano World. Wiley-VCH, Weinheim, Germany (2003).

3. Goodsell DS: Our Molecular Nature: The Body’s Motors, Machines, and Messages. Copernicus, NY, USA (1996).

4. Boyer PD: A research journey with ATP synthase. J. Biol. Chem. 277(42), 39045–39061 (2002).

5. Feynman RP: There’s Plenty of Room at the Bottom. Eng. Sci. 23, 22–36 (1960).

6. Shinkai S, Manabe O: Photocontrol of ion extraction and ion-transport by photofunctional crown ethers. Top. Curr. Chem. 121, 67–104 (1984).

7. Howard J: Molecular motors: structural adaptations to cellular functions. Nature 389(6651), 561–567 (1997).

8. Vale RD, Milligan RA: The way things move: looking under the hood of molecular motor proteins. Science 288(5463), 88–95 (2000).

9. Cross RL, Duncan TM: Subunit rotation in F0F1-ATP synthases as a means of coupling proton transport through F-0 to the binding changes in F-1. J. Bioenerg. Biomembr. 28(5), 403–408 (1996)

10. Howard J, Hudspeth AJ, Vale RD: Movement of microtubules by single kinesin molecules. Nature 342(6246), 154–158 (1989).

11. Kull FJ, Sablin EP, Lau R, Fletterick RJ, Vale RD: Crystal structure of the kinesin motor domain reveals a structural similarity to myosin. Nature 380(6574), 550–555 (1996).

12. Hackney DD: The kinetic cycles of myosin, kinesin, and dynein. Annu. Rev. Physiol. 58, 731–750 (1996).

13. Lipowsky R, Klumpp S: ‘Life is motion’: multiscale motility of molecular motors. Physica A 352(1), 53–112 (2005).

14. Limberis L, Stewart RJ: Toward kinesin-powered microdevices. Nanotechnology 11(2), 47–51 (2000).

15. Hiratsuka Y, Tada T, Oiwa K, Kanayama T, Uyeda TQP: Controlling the direction of

kinesin-driven microtubule movements along microlithographic tracks. Bio. Phy. J. 81(3), 1555–1561 (2001).

16. Platt M, Muthukrishnan G, Hancock WO, Williams ME: Millimeter scale alignment of magnetic nanoparticle functionalized microtubules in magnetic fields. J. Am. Chem. Soc. 127(45), 15686–15687 (2005).

17. Hutchins BM, Platt M, Hancock WO, Williams ME: Directing transport of CoFe2O4-functionalized microtubules with magnetic fields. Small 3(1), 4 (2007).

18. van den Heuvel MGL, De Graaff MP, Dekker C: Molecular sorting by electrical steering of microtubules in kinesin-coated channels. Science 312(5775), 910–914 (2006).

19. Brunner C, Wahnes C, Vogel V: Cargo pick-up from engineered loading stations by kinesin driven molecular shuttles. Lab. Chip. 7(10), 1263–1271 (2007).

20. Hess H, Howard J, Vogel V: A piconewton forcemeter assembled from microtubules and kinesins. Nano. Lett. 2(10), 1113–1115 (2002).

21. Diez S, Reuther C, Dinu C et al.: Stretching and transporting DNA molecules using motor proteins. Nano. Lett. 3(9), 1251–1254 (2003).

22. Dinu CZ, Opitz J, Pompe W, Howard J, Mertig M, Diez S: Parallel manipulation of bifunctional dna molecules on structured surfaces using kinesin-driven microtubules. Small 2(8–9), 927 (2006).

23. Allison WS: F-1-ATPase: a molecular motor that hydrolyzes ATP with sequential opening and closing of catalytic sites coupled to rotation of its γ-subunit. Acc. Chem. Res. 31(12), 819–826 (1998).

24. Block SM: Real engines of creation. Nature 386(6622), 217–219 (1997).

25. Elston T, Wang HY, Oster G: Energy transduction in ATP synthase. Nature 391(6666), 510–513 (1998).

26. Uptain SM, Kane CM, Chamberlin MJ: Basic mechanisms of transcript elongation and its regulation. Annu. Rev. Biochem. 66, 117–172 (1997).

27. Yin H, Wang MD, Svoboda K, Landick R, Block SM, Gelles J: Transcription against an applied force. Science 270(5242), 1653–1657 (1995).

28. Noji H, Yasuda R, Yoshida M, Kinosita K: Direct observation of the rotation of F-1-ATPase. Nature 386(6622), 299–302 (1997)

29. Soong RK, Bachand GD, Neves HP, Olkhovets AG, Craighead HG, Montemagno CD: Powering an inorganic nanodevice with a biomolecular motor. Science 290(5496), 1555–1558 (2000).

30. Yurke B, Turberfield A, Mills A, Simmel F, Neumann J: A DNA-fuelled molecular machine made of DNA. Nature 406(6796), 605–608 (2000).

31. Shin JS, Pierce NA: A synthetic DNA Walker for molecular transport. J. Am. Chem. Soc. 126(35), 10834–10835 (2004)

32. Bath J, Green S, Turberfield A: A free-running DNA motor powered by a nicking enzyme. Angew. Chem. Int. Ed. 44(28), 4358–4361 (2005).

33. Yin P, Yan H, Daniell X, Turberfield A,Reif J: A unidirectional DNA walker that moves autonomously along a track. Angew. Chem. Int. Ed. 43(37), 4906–4911 (2004).

34. Anelli PL, Asakawa M, Ashton PR et al.: Toward controllable molecular shuttles. Chem. Eur. J. 3(7), 1113–1135 (1997).

35. Anelli PL, Spencer N, Stoddart JF: A molecular shuttle. J. Am. Chem. Soc. 113(13), 5131–5133 (1991).

36. Ashton PR, Philp D, Spencer N, Stoddart JF: The self-assembly of (N) pseudorotaxanes. Chem. Soc. Chem. Commun. 1677–1679 (1991).

37. Bissell RA, Cordova E, Kaifer AE, Stoddart JF: A chemically and electrochemically switchable molecular shuttle. Nature 369(6476), 133–137 (1994).

38. Brouwer AM, Frochot C, Gatti FG et al.: Photoinduction of fast, reversible translational motion in a hydrogen-bonded molecular shuttle. Science 291(5511), 2124–2128 (2001).

39. Martinez Diaz MV, Spencer N, Stoddart JF: The self-assembly of a switchable [2]rotaxane. Angew. Chem. Int. Ed. 36(17), 1904–1907 (1997).

40. Raehm L, Kern JM, Sauvage JP: A transition metal containing rotaxane in motion: Electrochemically induced pirouetting of the ring on the threaded dumbbell. Chem. Eur. J. 5(11), 3310–3317 (1999).



41. Stoddart JF, Tseng HR: Chemical synthesis gets a fillip from molecular recognition and self-assembly processes. Proc. Natl Acad. Sci. USA 99(8), 4797–4800 (2002).

42. Gad-El-Hak M: The MEMS Handbook. CRC Press, FL, USA (2001).

43. Kovacs GTA: Micromachined Transducers Sourcebook. McGraw-Hill, NY, USA (1998)

44. Liu C: Foundations of MEMS. Pearson Education New Jersey, USA (2006).

45. Madou M: Fundamentals of Microfabrication. CRC Press, NY, USA (2002).

46. Brough B, Northrop BH, Schmidt JJ et al.: Evaluation of synthetic linear motor-molecule actuation energetics. Proc. Natl Acad. Sci. USA 103(23), 8583–8588 (2006).

47. Liu Y, Flood AH, Bonvallett PA et al.: Linear artificial molecular muscles. J. Am. Chem. Soc. 127(27), 9745–9759 (2005).

48. Balzani V, Clemente-Leon M, Credi A et al.: Autonomous artificial nanomotor powered by sunlight. Proc. Natl Acad. Sci. USA 103(5), 1178–1183 (2006).

49. Tseng HR, Wu DM, Fang NXL, Zhang X, Stoddart JF: The metastability of an

electrochemically controlled nanoscale machine on gold surfaces. Chemphyschem 5(1), 111–116 (2004).

50. Lee IC, Frank CW, Yamamoto T et al.: Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes. Langmuir 20(14), 5809–5828 (2004).

51. Petty MC: Langmuir-Blodgett Films: An Introduction. Cambridge University Press, Cambridge, UK (1996).

52. Nguyen TD, Tseng HR, Celestre PC et al.: A reversible molecular valve. Proc. Natl Acad. Sci. USA 102(29), 10029–10034 (2005).

53. Nguyen TD, Ken CFL, Liong M, Liu Y, Stoddart JF, Zink JI: Versatile Supramolecular nanovalves reconfigured for light activation*. Adv. Funct. Mater. 17(13), 2101–2110 (2007).

54. Sourav Saha KC-FL, Nguyen TD, Stoddart JF, Zink JI: Nanovalves. Adv. Funct. Mater. 17(5), 685–693 (2007).

55. Shirai Y, Osgood AJ, Zhao YM, Kelly KF, Tour JM: Directional control in thermally driven single-molecule nanocars. Nano. Lett. 5(11), 2330–2334 (2005).

56. Morin JF, Shirai Y, Tour JM: En route to a motorized nanocar. Org. Lett. 8(8), 1713–1716 (2006).

57. Huang TJ, Brough B, Ho CM et al.: A nanomechanical device based on linear molecular motors. Appl. Phys. Lett. 85(22), 5391–5393 (2004).

58. Huang TJ, Flood, AH, Brough B et al.: Understanding and harnessing biomimetic molecular machines for nems actuation materials. IEEE T. Autom. Sci. Eng. 3(3), 254–259 (2006).

59. Berna J, Leigh DA, Lubomska M et al.: Macroscopic transport by synthetic molecular machines. Nat. Mater. 4(9), 704–710 (2005).

60. Eelkema R, Pollard MM, Vicario J et al.: Nanomotor rotates microscale objects. Nature 440(7081), 163–163 (2006)

61. Pollard M, Lubomska M, Rudolf P, Feringa B: Controlled rotary motion in a monolayer of molecular motors. Angew. Chem. Int. Ed. 46(8), 1278–1280 (2007).

124 Nanomedicine (2008) 3(1) future science groupfuture science group

Biological and biomimetic molecular machinesjuh17:...Biological molecular machines are molecular...

Documents

Transcript of Biological and biomimetic molecular machinesjuh17:...Biological molecular machines are molecular...