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Click chemistry

In chemical synthesis, 'click' chemistry is a class of biocompatible small molecule reactions commonly used in bioconjugation, allowing the joining of substrates of choice with specific biomolecules. Click chemistry is not a single specific reaction, but describes a way of generating products that follow examples in nature, which also generates substances by joining small modular units. In many applications, click reactions join a biomolecule and a reporter molecule. Click chemistry is not limited to biological conditions: the concept of a 'click' reaction has been used in pharmacological and various biomimetic applications. However, they have been made notably useful in the detection, localization and qualification of biomolecules. Agard, N. J.; Baskin, J. M.; Prescher, J. A.; Lo, A.; Bertozzi, C. R. (2006). 'A Comparative Study of Bioorthogonal Reactions with Azides'. ACS Chem. Biol. 1 (10): 644–648. doi:10.1021/cb6003228. In chemical synthesis, 'click' chemistry is a class of biocompatible small molecule reactions commonly used in bioconjugation, allowing the joining of substrates of choice with specific biomolecules. Click chemistry is not a single specific reaction, but describes a way of generating products that follow examples in nature, which also generates substances by joining small modular units. In many applications, click reactions join a biomolecule and a reporter molecule. Click chemistry is not limited to biological conditions: the concept of a 'click' reaction has been used in pharmacological and various biomimetic applications. However, they have been made notably useful in the detection, localization and qualification of biomolecules. Click reactions occur in one pot, are not disturbed by water, generate minimal and inoffensive byproducts, and are 'spring-loaded'—characterized by a high thermodynamic driving force that drives it quickly and irreversibly to high yield of a single reaction product, with high reaction specificity (in some cases, with both regio- and stereo-specificity). These qualities make click reactions particularly suitable to the problem of isolating and targeting molecules in complex biological environments. In such environments, products accordingly need to be physiologically stable and any byproducts need to be non-toxic (for in vivo systems). By developing specific and controllable bioorthogonal reactions, scientists have opened up the possibility of hitting particular targets in complex cell lysates. Recently, scientists have adapted click chemistry for use in live cells, for example using small molecule probes that find and attach to their targets by click reactions. Despite challenges of cell permeability, bioorthogonality, background labeling, and reaction efficiency, click reactions have already proven useful in a new generation of pulldown experiments (in which particular targets can be isolated using, for instance, reporter molecules which bind to a certain column), and fluorescence spectrometry (in which the fluorophore is attached to a target of interest and the target quantified or located). More recently, novel methods have been used to incorporate click reaction partners onto and into biomolecules, including the incorporation of unnatural amino acids containing reactive groups into proteins and the modification of nucleotides. These techniques represent a part of the field of chemical biology, in which click chemistry plays a fundamental role by intentionally and specifically coupling modular units to various ends. The term 'click chemistry' was coined by K. Barry Sharpless in 1998, and was first fully described by Sharpless, Hartmuth Kolb, and M.G. Finn of The Scripps Research Institute in 2001. Click chemistry is a method for attaching a probe or substrate of interest to a specific biomolecule, a process called bioconjugation. The possibility of attaching fluorophores and other reporter molecules has made click chemistry a very powerful tool for identifying, locating, and characterizing both old and new biomolecules. One of the earliest and most important methods in bioconjugation was to express a reporter on the same open reading frame as a biomolecule of interest. Notably, GFP was first (and still is) expressed in this way at the N- or C- terminus of many proteins. However, this approach comes with several difficulties. For instance, GFP is a very large unit and can often affect the folding of the protein of interest. Moreover, by being expressed at either terminus, the GFP adduct can also affect the targeting and expression of the desired protein. Finally, using this method, GFP can only be attached to proteins, and not post-translationally, leaving other important biomolecular classes (nucleic acids, lipids, carbohydrates, etc.) out of reach. To overcome these challenges, chemists have opted to proceed by identifying pairs of bioorthogonal reaction partners, thus allowing the use of small exogenous molecules as biomolecular probes. A fluorophore can be attached to one of these probes to give a fluorescence signal upon binding of the reporter molecule to the target—just as GFP fluoresces when it is expressed with the target. Now limitations emerge from the chemistry of the probe to its target. In order for this technique to be useful in biological systems, click chemistry must run at or near biological conditions, produce little and (ideally) non-toxic byproducts, have (preferably) single and stable products at the same conditions, and proceed quickly to high yield in one pot. Existing reactions, such as Staudinger ligation and the Huisgen 1,3–dipolar cycloaddition, have been modified and optimized for such reaction conditions. Today, research in the field concerns not only understanding and developing new reactions and repurposing and re-understanding known reactions, but also expanding methods used to incorporate reaction partners into living systems, engineering novel reaction partners, and developing applications for bioconjugation. For a reaction to be considered a click reaction, it must satisfy certain characteristics:

[ "Combinatorial chemistry", "Organic chemistry", "Inorganic chemistry", "Polymer chemistry", "Bioorthogonal chemistry", "Thionyl tetrafluoride", "Thiol-yne reaction", "Azide-alkyne Huisgen cycloaddition", "Thiol-ene reaction" ]
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