Click Chemistry

What is Click Chemistry?

Click Chemistry is a novel developed by a Nobel Laureate in Chemistry. A chemical synthesis method was proposed by Barry Sharpless in 2001. The core idea is to quickly and easily link small molecules to form complex molecular structures through chemical reactions that are efficient, specific, and do not produce by-products. This method mimics the way biological macromolecules (e.g., proteins, nucleic acids, etc.) are constructed in nature, and aims to provide a set of fast, reliable, and simple chemical reaction tools for materials science, drug development, biochemistry, and other fields.

The History of Click Chemistry

In 1864, Germany chemist Johann Peter Griess (the discoverer of azide compounds) synthesized an organic azide compound-phenyl azide. Organic azide compounds are composed of three nitrogen atoms linked together, which can easily form a five-membered ring with two carbon atoms on the one hand. On the other hand, nitrogen molecules can also be removed, leaving a nitrogen atom to construct a new functionalized system.

In the 60 years after 1950, a large number of azide compounds were synthesized, and around 2000, scientists began to study azide nucleic acid, which began to study the conjugation reaction in organisms.

The concept of click chemistry was first proposed by Sharpless in 2001 and was inspired by the efficient molecular assembly process found in nature. In 2002, Sharpless and colleagues first applied the 1,3-dipole cycloaddition reaction (CuAAC), a copper-catalyzed alkyne cycloaddition reaction. This reaction has become a signature reaction in click chemistry and is widely used in various organic synthesis and biomarkers.

With the passage of time, click chemistry has gradually expanded to other types of reactions, such as Diels-Alder reactions, thiol-alkene reactions, mercaptan-alkyne reactions, Staudinger reactions, etc. These reactions all meet the standards of click chemistry, providing scientists with more tools and advancing the development of click chemistry in various fields.

In 2022, Morten Meldal and K. Barry Sharpless were awarded the 2022 Nobel Prize in Chemistry for their contributions to click chemistry and bioorthogonal chemistry for their independent reports on the copper-catalyzed azide-alkyne cycloaddition reaction, respectively, and Carolyn R. Bertozzi for expanding the application of "click chemistry" to the biological field.

Bioorthogonal Click Chemistry

Click chemistry is a chemical synthesis method that enables the rapid and efficient synthesis of useful new molecules based on carbon-heteroatom bond (C-X-C) linkage. Bioorthogonal is the use of the principle of click chemistry to produce chemical reactions in living organisms that do not interfere with their own biochemical reactions. Bioorthogonal click chemistry is an important branch of click chemistry that aims to perform efficient and specific chemical reactions in complex biological environments without interfering with natural biological processes.

One of the earliest bioorthogonal reactions is the Staudinger reaction, proposed by Carolyn Bertozzi in 2000. This reaction utilizes the specific reaction of azo formate with phosphorus to achieve the goal of labeling carbohydrate molecules on the cell surface. Subsequently, the CuAAC reaction was also improved for bioorthogonal chemistry, but its use in biological systems was limited due to the toxicity of copper ions to cells. To this end, scientists have developed copper-free alkyne cycloaddition (SPAAC), a strain-facilitated alkyne cycloaddition reaction. This reaction can be carried out under mild conditions without the need for a catalyst and is very biocompatible. Bioorthogonal click chemistry provides a powerful tool for studying life processes, and is widely used in cell labeling, protein modification, drug delivery, and other fields.

Click Chemistry Mechanism

The mechanism of click chemistry varies depending on the type of reaction, but all of them are characterized by high efficiency, strong selectivity, and mild conditions. Here are a few mechanisms of typical click chemistry:

Copper(I)-Catalyzed Alkyne-Azide Cycloaddition (CuAAC)

As the most classic click chemical reaction, the mechanism of CuAAC reaction includes the 1,3-dipole cycloaddition of alkyne and azide, and the copper ion acts as a catalyst to promote the cycloaddition reaction between alkyne and azide to generate a 1,2,3-triazole ring. The reaction reacts over a wide range of temperatures, is not sensitive to water, reacts in the pH range of 4 to 12, and is tolerant to many functional groups.

Strain-promoted alkyne Cycloaddition (SPAAC)

Bertozzi developed a strain-promoted cyclic addition reaction of azide-alkynes (SPAAC) that does not require the use of metal catalysts, reducing agents, or stabilizing ligands. The SPAAC reaction does not require a catalyst and facilitates the reaction through the release of strain energy. The alkyne ring structure (e.g., octuple cycloalkyne) undergoes a cycloaddition reaction with azide to form a 1,2,3-triazole ring. This reaction is highly selective and biocompatible, making it suitable for bioorthogonal chemistry.

Diels-Alder Reaction(IEDDA)

The Diels-Alder reaction is a [4+2] cycloaddition reaction between a diene and a diene to form a six-membered ring structure. The reaction does not require a catalyst and can be carried out at room temperature. This is a very rapid response for bioconjugation at low concentrations in labeling live cells, molecular imaging, and other bioconjugation applications.

Fig.1 Schematic diagram of three common click chemical reactions. (Yao Tingting, et al., 2021)

Thiol-Alkene Reaction and Thiol-Alkyne Reaction

These reactions utilize the addition of mercaptans to alkenes or alkynes to form thiol-based compounds. These reactions are usually catalyzed by ultraviolet light or free radicals, and have the advantages of high efficiency and mild conditions.

Advantages of Click Chemistry

Click chemistry offers several significant advantages:

High efficiency and yield: Click chemistry typically has high reaction efficiency and yield, enabling the generation of large amounts of target products in a short period of time, saving time and money.

Simplicity: Click chemistry is mild, often without the need for complex reaction equipment and post-processing, and is easy to operate. This makes click chemistry highly practical in both laboratory and industrial production.

Selectivity and specificity: Click chemistry has high selectivity and specificity, which can achieve selective ligation of specific molecules in complex reaction systems, reduce the formation of by-products, and improve the reliability of reactions.

Forgiving: Click chemistry has good compatibility with solvents and functional groups, can be performed in a wide range of solvents, and has a good tolerance to a wide range of functional groups. This makes click chemistry have a wide range of applications in organic synthesis, biochemistry, materials science, and other fields.

Biocompatibility: Bioorthogonal click chemistry is biocompatible and can be carried out efficiently in complex biological environments without interfering with natural biological processes. This makes click chemistry have important applications in biomedical fields such as biomarkers, drug delivery, protein modification, etc.

Summary

As an efficient, simple and selective chemical synthesis method, click chemistry has a wide range of application prospects in organic synthesis, biochemistry, materials science, and other fields. Its history shows the process of continuous innovation and improvement by scientists, and the emergence of bioorthogonal click chemistry provides a powerful tool for life science research. With the continuous progress of click chemistry technology and the continuous expansion of its application scope, click chemistry will play an increasingly important role in scientific research and practical application in the future.