Click Chemistry

Click Chemistry[1] 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.[[2],[3],[4]] 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

These reactions are widely used in the field of bioscience,[[5],[6]] drug discovery[7] and material science.[8]


Thus these reactions have become new tools in the field of bioconjugation. The first bioconjugation reactions, such as polar reactions[[9],[10]] between aldehydes or ketones and hydrazides or oxyamines:

Figure 1 : Examples of reactions involving carbonyl groups

or the Staudinger-Bertozzi ligation[[11],[12]] between azide and phosphines:




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),[[13],[14]] 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.[[15],[16]] 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.[17]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.[[18],[19],[20],[21]] 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

The group of Leeper described the reaction of tetrazines with isonitrile derivatives[[22],[23]as also a new tool for bioconjugation:



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.


[1] Kolb H. C., Finn M. G., Sharpless K. B., Click chemistry: Diverse chemical function from a few good reactions, Angew. Chem. Int. Ed., 2001, 40, 2004.

[2] Sletten E. M., Bertozzi C. R., Bioorthogonal Chemistry: Fishing for Selectivity in a Sea of Functionality, Angew. Chem. Int. Ed., 2009, 48, 6974.

[3] Best M. D., Click Chemistry and Bioorthogonal Reactions: Unprecedented Selectivity in the Labeling of Biological Molecules, Biochemistry, 2009, 48, 6571.

[4] Lallana E., Riguera R., Fernandez-Megia E., Reliable and Efficient Procedures for the Conjugation of Biomolecules through Huisgen Azide-Alkyne Cycloadditions, Angew. Chem. Int. Ed., 2011, 50, 8794.

[5] Grammel M., Hang H. C., Chemical reporters for biological discovery, Nat. Chem. Biol., 2013, 9, 475.

[6] Su Y., Ge J., Zhu B., Zheng Y.-G. Zhu, Q., Yao S. Q., Target identification of biologically active small molecules via in situ methods, Curr. Opin. Chem. Biol., 2013, 17, 768.

[7] Zeng D., Zeglis B. M., Lewis J. S., Anderson C. J., The Growing Impact of Bioorthogonal Click Chemistry on the Development of Radiopharmaceuticals, J. Nucl. Med., 2013, 54, 829.

[8] Evans R. A., The rise of azide-alkyne 1,3-dipolar ‘click’ cycloaddition and its application to polymer science and surface modification, Aust. J. Chem., 2007, 60, 384.

[9] Mahal L. K., Yarema K. J., Bertozzi C. R., Engineering chemical reactivity on cell surfaces through oligosaccharide biosynthesis, Science, 1997, 276, 1125.

[10] Zeng Y., Ramya T. N. C., Dirksen A., Dawson P. E., Paulso, J. C., High-efficiency labeling of sialylated glycoproteins on living cells, Nat. Methods, 2009, 6, 207.

[11] Saxon E., Bertozzi C. R., Cell surface engineering by a modified Staudinger reaction, Science, 2000, 287, 2007.

[12] Prescher J. A., Dube D. H., Bertozzi C. R., Chemical remodelling of cell surfaces in living animals, Nature, 2004, 430, 873.

[13] Tornoe C. W., Christensen C., Meldal, M., Peptidotriazoles on solid phase: 1,2,3 -triazoles by regiospecific copper(I)-catalyzed 1,3-dipolar cycloadditions of terminal alkynes to azides, J. Org. Chem., 2002, 67, 3057.

[14] Rostovtsev V. V., Green L. G., Fokin V. V., Sharpless K. B., A stepwise Huisgen cycloaddition process: Copper(I)-catalyzed regioselective “ligation” of azides and terminal alkynes, Angew. Chem. Int. Ed., 2002, 41, 2596.

[15] Dumont A., Malleron A., Awwad M., Dukan S., Vauzeilles B., Click-Mediated Labeling of Bacterial Membranes through Metabolic Modification of the Lipopolysaccharide Inner Core, Angew. Chem. Int. Ed., 2012, 51, 3143.

[16] Mas Pons J., Dumont A., Sautejeau G., Fugier E., Baron A., Dukan S., Vauzeilles B., Identification of Living Legionella pneumophila Using Species-Specific Metabolic Lipopolysaccharide Labeling, Angew. Chem. Int. Ed., 2014, 53, 1275.

[17] Agard N. J., Prescher J. A., Bertozzi C. R. (2004) A strain-promoted 3+2 azide-alkyne cycloaddition for covalent modification of biomolecules in living systems, J. Am. Chem. Soc., 2004, 126, 15046.

[18] Seckute J., Devaraj N. K., Expanding room for tetrazine ligations in the in vivo chemistry toolbox, Curr. Opin. Chem. Biol., 2013, 17, 761.

[19] Blackman M. L., Royzen M., Fox, J. M., Tetrazine ligation: Fast bioconjugation based on inverse-electron-demand Diels-Alder reactivity, J. Am. Chem. Soc., 2008, 130, 13518.

[20] Taylor M. T., Blackman M. L., Dmitrenko O., Fox J. M., Design and Synthesis of Highly Reactive Dienophiles for the Tetrazine trans-Cyclooctene Ligation, J. Am. Chem. Soc., 2011, 133, 9646.

[21] Neves A. A., Stoeckmann H., Wainman Y. A., Kuo J. C. H., Fawcett S., Leeper F. J., Brindle, K. M., Imaging Cell Surface Glycosylation in Vivo Using “Double Click” Chemistry, Bioconj. Chem., 2013, 24, 934.

[22] Stairs S., Neves A. A., Stoeckmann H., Wainman Y. A., Ireland-Zecchini H., Brindle K. M., Leeper, F. J., Metabolic Glycan Imaging by Isonitrile-Tetrazine Click Chemistry, ChemBioChem, 2013, 14, 1063.

[23] Wainman Y. A., Neves A. A., Stairs S., Stoeckmann H., Ireland-Zecchini H., Brindle,K. M., Leeper F. J., Dual-sugar imaging using isonitrile and azido-based click chemistries, Org. Biomol. Chem., 2013, 11, 7297.