Click Chemistry describes pairs of functional groups that react rapidly and selectively with each other under mild and often aqueous conditions. This concept was first described by K. B. Sharpless, H. C. Kolb and M. G. Finn in 2001 and gathers procedures allowing convenient, versatile and reliable coupling of two molecules.[,,] The criteria of this concept are:
- High chemical yields
- Readily available starting materials
- Almost no byproducts
- Simple and non-chromatographic product isolation
- Aqueous or organic solvents
Thus these reactions have become new tools in the field of bioconjugation. The first bioconjugation reactions, such as polar reactions[,] between aldehydes or ketones and hydrazides or oxyamines:
Figure 1 : Examples of reactions involving carbonyl groups
Figure 2 : The Staudinger-Bertozzi ligation
had limitations. Indeed these reactions are not always completely biorthogonal or show problems of stability of adducts and kinetics of reaction.
The Click Chemistry reactions are especially cycloadditions. The most emblematic is the 1,3-dipolar cycloaddition between azide and terminal alkyne catalyzed by copper(I),[,] described by the groups of Meldal and Sharpless in 2002 (Figure 3a):
Figure 3 : Reactions between alkyne and azide
This reaction is called Copper-catalyzed Azide-Alkyne Cycloaddition, CuAAC.[,] Because of the presence of copper, this reaction poses a problem of toxicity in biological media. Other methodologies were thus developed. The group of Bertozzi circumvented this problem using a cyclooctyne derivative, thus avoiding the use of copper.This reaction of 1,3-dipolar cycloaddition between cyclooctyne derivatives and azides is called Strain-Promoted Alkyne-Azide Cycloaddition, SPAAC (Figure 3b).
In the last few years, other biorthogonal conjugation methods answering the criteria of Click Chemistry have come out, such as inverse electron demand Diels-Alder cycloaddition between alkenes and tetrazines.[,,,] This reaction can be used with various alkene/tetrazine couples and shows very high reaction rates:
Figure 4 : Example of reaction between alkenes and tetrazines
Figure 5 : Example of reaction between tetrazines and isonitrile derivatives
The cycloaddition between nitrones or nitrile oxyide derivatives, with alkenes or alkynes, is also part of these new tools.
The great advantage of these Click Chemistry reactions is that the reactive functional groups can be built-in relatively easily and quickly within complex molecules such as biomolecules. This allows the obtained bioconjugates to be used as molecular tools for many applications in Biology.
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