Models from Nature and BottomUp Assembly

Blueprints for designing molecular machinery are found commonly in nature (Figure 11.2), which is rich with examples of molecules that are capable of performing complex functions.42,43 Kinesins, a class of motor proteins that shuttle various cargos along microtubules, achieve actuation by what has been referred to as a "walking" mechanism. The movement of myosin along actin filaments, driven by the hydrolysis of ATP, causes muscle contraction and expansion. Whip-like and sinusoidal movements of cilia and flagella, respectively, enable them to transport cells throughout the body. The rotary motion of Fj-ATPase, which is also powered by AT P, is capable of pumping protons across a membrane. In all these examples, the concerted movements of molecules or groups of molecules are used to perform vital biological functions. By drawing analogies between these natural motor proteins and macroscopic machines, many research groups have set out to design molecular machines capable of performing a veritable collection of functions.

In addition to gaining inspiration from nature, the drive to synthesize molecular actuators stems from the advantages provided by the bottom-up assembly1-5,44 of molecular components. The top-down approach to constructing functional devices and actuators reaches a number of fundamental limitations as device features approach the molecular scale. Lithographic techniques for fabricating devices become both less precise and increasingly costly as features become smaller.44,45 Building up from the molecular scale, however, has distinct advantages over the top-down approach. With the assistance of supramole-cular chemistry, molecular recognition, self-assembly, and template-directed synthesis, specific molecular architectures of varying complexity can be created. This approach benefits from the fact that by virtue

FIGURE 11.2 (a) A schematic representation of the motor protein kinesin, which is able to transport various cargos along microtubules. (b) The directional movements of myosin along actin filaments are responsible for muscle contraction and expansion. (c) Flagella and cilia (not shown) are able to propel materials across the cell surface as a result of their sinusoidal and whip-like movements, respectively. (d) F1-ATPase undergoes unidirectional rotation during the hydrolysis of ATP to ADP.

FIGURE 11.2 (a) A schematic representation of the motor protein kinesin, which is able to transport various cargos along microtubules. (b) The directional movements of myosin along actin filaments are responsible for muscle contraction and expansion. (c) Flagella and cilia (not shown) are able to propel materials across the cell surface as a result of their sinusoidal and whip-like movements, respectively. (d) F1-ATPase undergoes unidirectional rotation during the hydrolysis of ATP to ADP.

of molecular recognition and self-assembly, individual components do not have to be sequentially pieced together. In addition, template-directed synthesis can be particularly modular and the physical properties of constituent parts can be optimized to perform various desired functions. Moreover, the molecular nature of molecular machines makes them amenable to a wide variety of inputs, including pH changes, oxidation, reduction, ion addition, solvent polarity, and light. The characteristics of this chemical, bottom-up approach to the design of molecular-scale machinery with specific architectures, properties, and functions are some of the greatest motivators for research in this area.

11.1.5 Molecular Machine Taxonomy

Among the many designs for externally controllable molecular machines are molecular shuttles, switches, muscles, nanovalves, rotors, and surfaces with controlled wettability (Figure 11.3). Molecular shuttles are molecules wherein a ring component and a dumbbell component are mechanically interlocked and, thus, are considered [2]rotaxanes. The presence of two identical recognition sites that are capable of becoming complexed by the ring component through noncovalent interactions located on the dumbbell component results in what is referred to as a degenerate molecular shuttle. The thermally activated movement of the ring component between the identical sites along the dumbbell exchanges the two coconformations that the molecule is capable of adopting and is referred to as shuttling. The incorporation of two different recognition sites into the dumbbell gives rise to a molecular switch. With the appropriate external stimulus, a molecular switch is capable of being selectively stimulated to be in one of two identifiably different coconformations. Molecular muscles are compounds that can, when triggered by an external stimulus, be induced to expand or contract reversibly. This expansion and contraction is reminiscent of the same process that occurs in muscle fibers. Harnessing the ability to change molecular structures in such a way that they can cover or uncover porous materials, and thus either trap or release their content(s), forms the basis of a molecular nanovalve. Molecular rotors are molecules composed of two moieties that are able to rotate relative to each other in a controlled manner. Using structural changes in surface-bound molecules to induce changes in the hydrophobicity or hydrophilicity of a surface allows for selective control of surface wettability and has potential applications in microfluidic devices.

This chapter will highlight some of the recent accomplishments made in the design, synthesis, and modeling of synthetic organic molecular machines. Particular attention will be paid to those examples

Coconformer 1

Coconformer 2

Coconformer 1 Coconformer 2

Contracted .. p. Expanded

I Contracted Contracted

Coconformer 1 Coconformer 2

Contracted .. p. Expanded

I Contracted Contracted

Closed Open

Axle

Axle

Hydrophobic

Hydrophilic

FIGURE 11.3 (a) A molecular shuttle consisting of a ring component that is mechanically interlocked onto a dumbbell-shaped component and is able to shuttle between two recognition sites as a result of thermal activation. Here coconformers 1 and 2 are degenerate. (b) Molecular switches are compounds that can be externally stimulated to exist in either of two observably different states or conformers. (c) A molecular muscle is able to expand and contract reversibly upon external stimulation. (d) Molecular valves can be used to trap and release other molecules as the result of controlled molecular motions. (e) Molecular rotors undergo controlled rotational motion of a rotor unit relative to a stator, which are connected via an axle. (f) Surfaces with controlled wettability can be stimulated to be hydrophobic or hydrophilic.

that involve molecules in condensed phases or attached to surfaces, as they provide perhaps the greatest insight into the development of functional devices. The first section will focus on molecular shuttles and switches based on mechanically interlocking rotaxanes and noncovalently associated pseudorotaxanes based on the n-electron-rich cyclophane, cyclobis(paraquat-p-phenylene)vinylene46 (CBPQT4+), simply because they have been thoroughly studied in solution as well as in condensed phases and on solid substrates. The idea behind this agenda is twofold — (1) to provide specific examples of a class of molecular machines and (2) to introduce how such machines can be transferred from the solution phase onto surfaces in such a way that they retain their desirable physical and dynamic properties. From there on, the next three sections of this chapter will be devoted to recent advancements in designing organic molecules to function as molecular muscles, nanovalves, and rotors. The final section will feature some current research that takes advantage of the stimuli-responsive motions of surface-bound organic molecules to control surface wettability.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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