Molecular switch

Source: Wikipedia, the free encyclopedia.

A molecular switch is a

molecular computers or responsive drug delivery systems.[3] Molecular switches are also important in biology because many biological functions are based on it, for instance allosteric regulation and vision. They are also one of the simplest examples of molecular machines
.

Biological molecular switches

In

intracellular signaling molecules by activating another protein in a signaling pathway. In order to do this, proteins can switch between active and inactive states, thus acting as molecular switches in response to another signal.[4] For example, phosphorylation of proteins can be used to activate or inactivate proteins. The external signal flipping the molecular switch could be a protein kinase, which adds a phosphate group to the protein, or a protein phosphatase, which removes phosphate groups.[4]

Acidochromic molecular switches

The capacity of some compounds to change in function of the

acid-base theory. Those are found in a wide range of plants like roses, cornflowers, primroses and violets. Robert Boyle was the first person to describe this effect, employing plant juices (in the forms of solution and impregnated paper).[5]

Molecular switches are most commonly used as pH indicators, which are molecules with acidic or basic properties. Their acidic and basic forms have different colors. When an acid or a base is added, the equilibrium between the two forms is displaced.[6]

Photochromic molecular switches

A widely studied class are

dithienylethenes, fulgides, stilbenes, spiropyrans
and phenoxynaphthacene quinones.

Switching in dithienylethene

Chiroptical molecular switches are a specific subgroup with photochemical switching taking place between an

circularly polarized light

Hindered Alkene Molecular Switch

Chiroptical molecular switches that show directional motion are considered

synthetic molecular motors:[8] When attached to the end of a helical poly (isocyanate) polymer, they can switch the helical sense of the polymer.[9]

TBu Helicenemolecularmotor

Host–guest molecular switches

In host–guest chemistry the bistable states of molecular switches differ in their affinity for guests. Many early examples of such systems are based on crown ether chemistry. The first switchable host is described in 1978 by Desvergne & Bouas-Laurent[10][11] who create a crown ether via photochemical anthracene dimerization. Although not strictly speaking switchable the compound is able to take up cations after a photochemical trigger and exposure to acetonitrile gives back the open form.

Anthracene Crown Desvergne 1978

In 1980 Yamashita et al.[12] construct a crown ether already incorporating the anthracene units (an anthracenophane) and also study ion uptake vs photochemistry.

Anthracene Crown Misumi 1980

Also in 1980 Shinkai throws out the anthracene unit as photoantenna in favor of an

trans-cis isomerization of the azo group which results in ring expansion. Thus in the trans form the crown binds preferentially to ammonium, lithium and sodium ions while in the cis form the preference is for potassium and rubidium (both larger ions in same alkali metal
group). In the dark the reverse isomerization takes place.

MolecularSwitchShinkay1980

Shinkai employs this devices in actual ion transport mimicking the biochemical action of monensin and nigericin:[14][15] in a biphasic system ions are taken up triggered by light in one phase and deposited in the other phase in absence of light.

Mechanically-interlocked molecular switches

Some of the most advanced molecular switches are based on

Stoddart[16] devices a molecular shuttle based on a rotaxane on which a molecular bead is able to shuttle between two docking stations situated on a molecular thread. Stoddart predicts that when the stations are dissimilar with each of the stations addressed by a different external stimulus the shuttle becomes a molecular machine. In 1993 Stoddart is scooped by supramolecular chemistry pioneer Fritz Vögtle who actually delivers a switchable molecule based not on a rotaxane but on a related catenane[17][18]

Photo switchable catenane Vögtle 1993 Molecular switch Kaifer and Stoddart 1994
Photo switchable catenane Vögtle 1993 Molecular switch Kaifer and Stoddart 1994

This compound is based on two ring systems: one ring holds the photoswichable azobenzene ring and two

NMR spectroscopy
shows that in the azo trans-form the polyether ring is free to rotate around its partner ring but then when a light trigger activates the cis azo form this rotation mode is stopped

Kaifer and Stoddart in 1994 modify their

radical ion
) and significantly both processes are reversible.

In 2007 molecular shuttles were utilized in an experimental

nanometer (nm) wide at 33 nm intervals) crossed by another 400 titanium top-nanowires with similar dimensions sandwiching a monolayer
of a bistable rotaxane depicted below:

Molecular Switch In Electronic memory

Each

chemical half-life of around one hour. The problem of defects is circumvented by adopting a defect-tolerant architecture also found in the Teramac project. In this way a circuit is obtained consisting of 160,000 bits on an area the size of a white blood cell
translating into 1011 bits per square centimeter.

References

  1. OCLC 428018682
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  2. PMID 28694943
    .
  3. S2CID 24175578
    .
  4. ^
    OCLC 1004752557
    .
  5. doi:10.1021/ed041p285
    .
  6. ^ Helmenstine, Anne Marie. "pH indicator definition and examples". ThoughtCo.
  7. doi:10.1002/9781118120392.ch8
  8. doi:10.1021/cr9900228
  9. S2CID 96957407
    .
  10. doi:10.1039/C39780000403
  11. ^ From Anthracene Photodimerization to Jaw Photochromic Materials and Photocrowns Henri Bouas-Laurent, Alain Castellan and Jean-Pierre Desvergne Pure Appl. Chem.5 Vol.52, pp.2633–2648. 1980 Link
  12. doi:10.1016/S0040-4039(01)85550-7
  13. doi:10.1021/ja00538a026
  14. doi:10.1021/ja00391a021
  15. ^ Switch-functionalized systems in biomimetic chemistry Seiji Shinkai Pure & App!. Chem., Vol. 59, No. 3, pp. 425-430, 1987 Link
  16. doi:10.1021/ja00013a096
  17. ^ Photoswitchable Catenanes Fritz Vögtle, Walter Manfred Müller, Ute Müller, Martin Bauer, Kari Rissanen
  18. doi:10.1002/anie.199314591
  19. doi:10.1038/369133a0
  20. ^ A 160-kilobit molecular electronic memory patterned at 1011 bits per square centimetre Jonathan E. Green, Jang Wook Choi1, Akram Boukai, Yuri Bunimovich, Ezekiel Johnston-Halperin, Erica DeIonno, Yi Luo, Bonnie A. Sheriff, Ke Xu, Young Shik Shin, Hsian-Rong Tseng, J. Fraser Stoddart and James R. Heath
    doi:10.1038/nature05462

Further reading
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