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A molecular machine, nanite, or nanomachine, refers to any discrete number of molecular components that produce quasi-mechanical movements (output) in response to specific stimuli (input). The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level. The term is also common in nanotechnology where a number of highly complex molecular machines have been proposed that are aimed at the goal of constructing a molecular assembler.
In 1959, physicist Richard Feynman popularized the idea of nanomachines in his talk entitled "There's Plenty of Room at the Bottom" at the California Institute of Technology. Since then, chemists and physicists alike have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. Molecular machines research is currently at the forefront with the 2016 Nobel Prize in Chemistry being awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Bernard L. Feringa for the design and synthesis of molecular machines.
Molecular machines can be divided into two broad categories; synthetic and biological. In general, synthetic molecular machines refer to molecules that are artificially designed and synthesized whereas biological molecular machines can commonly be found in nature.
A wide variety of rather simple molecular machines have been synthesized by chemists. They can consist of a single molecule; however, they are often constructed for mechanically-interlocked molecular architectures, such as rotaxanes and catenanes. Carbon nanotube nanomotors have also been produced.
- Molecular motors are molecules that are capable of unidirectional rotation motion powered by external energy input. A number of molecular machines have been synthesized powered by light or reaction with other molecules.
- A molecular propeller is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers. It has several molecular-scale blades attached at a certain pitch angle around the circumference of a nanoscale shaft. Also see molecular gyroscope.
- A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to changes in pH, light, temperature, an electric current, microenvironment, or the presence of a ligand.
- A molecular shuttle is a molecule capable of shuttling molecules or ions from one location to another. A common molecular shuttle consists of a rotaxane where the macrocycle can move between two sites or stations along the dumbbell backbone.
- A molecular balance is a molecule that can interconvert between two and more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces, such as hydrogen bonding, solvophobic/hydrophobic effects, π interactions, and steric and dispersion interactions.
- Molecular tweezers are host molecules capable of holding items between their two arms. The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π interactions, or electrostatic effects. Examples of molecular tweezers have been reported that are constructed from DNA and are considered DNA machines.
- A molecular sensor is a molecule that interacts with an analyte to produce a detectable change. Molecular sensors combine molecular recognition with some form of reporter, so the presence of the item can be observed.
- A molecular logic gate is a molecule that performs a logical operation on one or more logic inputs and produces a single logic output. Unlike a molecular sensor, the molecular logic gate will only output when a particular combination of inputs are present.
- A molecular assembler is a molecular machine able to guide chemical reactions by positioning reactive molecules with precision.
- A molecular hinge is a molecule that can be selectively switched from one configuration to another in a reversible fashion. Such configurations must have distinguishable geometries, for instance, Cis or Trans isomers of a V-shape molecule. Azo compounds perform Cis–trans isomerism upon receiving UV-Vis light.
The most complex molecular machines are proteins found within cells. These include motor proteins, such as myosin, which is responsible for muscle contraction, kinesin, which moves cargo inside cells away from the nucleus along microtubules, dynein, which produces the axonemal beating of motile cilia and flagella, and transmembrane ATPases such like ATP synthase. These proteins and their nanoscale dynamics are far more complex than any molecular machines that have yet been artificially constructed.
Probably the most significant biological machine known is the ribosome. Other important examples include motile cilia. A high-level-abstraction summary is that, "[i]n effect, the [motile cilium] is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines." Flexible linker domains allow the connecting protein domains to recruit their binding partners and induce long-range allostery via protein domain dynamics.
This protein flexibility allows the construction of biological machines. The first useful applications of these biological machines might be in nanomedicine. For example, they could be used to identify and destroy cancer cells. Molecular nanotechnology is a speculative subfield of nanotechnology regarding the possibility of engineering molecular assemblers, biological machines which could re-order matter at a molecular or atomic scale. Nanomedicine would make use of these nanorobots, introduced into the body, to repair or detect damages and infections. Molecular nanotechnology is highly theoretical, seeking to anticipate what inventions nanotechnology might yield and to propose an agenda for future inquiry. The proposed elements of molecular nanotechnology, such as molecular assemblers and nanorobots are far beyond current capabilities.
The construction of more complex molecular machines is an active area of theoretical and experimental research. A number of molecules, such as molecular propellers, have been designed, although experimental studies of these molecules are inhibited by the lack of methods to construct these molecules. In this context, theoretical modeling can be extremely useful to understand the self-assembly/disassembly processes of rotaxanes, important for the construction of light-powered molecular machines. This molecular-level knowledge may foster the realization of ever more complex, versatile, and effective molecular machines for the areas of nanotechnology, including molecular assemblers.
Although currently not feasible, some potential applications of molecular machines are transport at the molecular level, manipulation of nanostructures and chemical systems, high density solid-state informational processing and molecular prosthetics. Many fundamental challenges need to be overcome before molecular machines can be used practically such as autonomous operation, complexity of machines, stability in the synthesis of the machines and the working conditions.
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