Click Chemistry and Its Future Application Prospects

The 2022 Nobel Prize in Chemistry was awarded to American scientists Carolyn R. Bertozzi, K. Barry Sharpless, and Danish scientist Morten Meldal in recognition of their contributions to the development of click chemistry and bioorthogonal chemistry. These three scientists pioneered a brand-new chemical concept that enables the building blocks of molecules to combine quickly and efficiently, much like Lego toys, where basic modules are used to create an infinite variety of shapes.

What is Click Chemistry? — A Chemical Technology for Precise Assembly Like a "Molecular Seatbelt"

Nomenclature and Core Definition of Click Chemistry

There was a slight misunderstanding in the Chinese translation of "click chemistry" — many people mistakenly associate "click" with the computer action of "clicking a mouse". In fact, "click" here is closer to the "click" sound made when a seatbelt buckle fastens. Professor Sharpless once used a vivid analogy to explain it: it is just like the quick and firm fastening of a seatbelt buckle with a "click". When a seatbelt buckle fastens with a "click", the two parts combine quickly and securely, which exactly aligns with the core feature of click chemistry: allowing two molecular modules to connect precisely under specific conditions without complex operations and without interfering with other molecules.

From the perspective of an authoritative definition, click chemistry is a concept of synthetic methodology formally proposed by Professor Sharpless in Angewandte Chemie International Edition in 2001. According to the definition of IUPAC (International Union of Pure and Applied Chemistry), it is "a set of fast and specific reactions that can efficiently assemble molecular fragments into more complex structures". Its core lies in the assembly of small molecular modules through carbon-heteroatom bonds (C-X-C, such as C-N and C-O bonds), avoiding the high difficulty and by-product problems associated with traditional carbon-carbon bond synthesis.

To be called a "qualified click reaction", it must meet 7 key criteria:

Modularity: Applicable to a variety of starting materials, just like Lego bricks that can be freely combined.

High Yield: The product accounts for more than 90%, with almost no waste.

Interference Resistance: Insensitive to water and oxygen, and can be carried out in an open environment or even under physiological conditions.

High Selectivity: Only a single structural product is generated, without the need to separate complex by-products.

Harmless By-Products: Only produces water or simple salts, which are friendly to the environment and biological systems.

Mild Conditions: No high temperature or high pressure is required, and the reaction can occur at room temperature.

Easy Separation: The product can usually be obtained through simple filtration or precipitation, without the need for complex column chromatography.

"Star Reactions" of Click Chemistry: From Classics to Innovations

Click chemistry is not a single reaction, but a collection of reactions that meet the above standards. Among them, the most core and mature one is the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction.

(1) Classic Core: CuAAC Reaction

In 2002, Meldal and Sharpless independently discovered this reaction: under the action of a copper (I) catalyst, the azide group (-N₃) and the terminal alkyne group (-C≡CH) combine like a "buckle" to form a stable 1,2,3-triazole ring structure. Its reaction mechanism can be simply understood in three steps:

1. Copper (I) combines with the terminal alkyne to form a "copper-acetylene complex", making the alkynyl group more susceptible to attack.

2. The terminal nitrogen atom of the azide group "actively" combines with the carbon atom in the complex to form a temporary "copper-containing six-membered ring intermediate".

3. The intermediate rearranges rapidly to release the 1,2,3-triazole product, and at the same time, the copper (I) catalyst is regenerated and can be recycled.

Recent studies have also found that the CuAAC reaction actually relies on "dinuclear copper species" (where two copper atoms act synergistically). This discovery allows scientists to further optimize the reaction efficiency, such as achieving a fast reaction at a lower copper concentration and reducing the toxicity of metals to biological systems.

(2) Copper-Free Innovation: "Tailored" for Biological Systems

Since copper ions have certain toxicity to living cells, scientists have developed copper-free click reactions to enable click chemistry to be used in living organisms:

Strain-Promoted Azide-Alkyne Cycloaddition (SPAAC) Reaction: It uses highly strained cyclooctynes (such as DBCO, dibenzocyclooctyne) instead of ordinary terminal alkynes. It can react quickly with azides without copper catalysis and is widely used in the labeling of cell surface molecules.

Tetrazine-Trans-Cyclooctene (TCO) Reaction: Its reaction rate is more than 100 times faster than that of the SPAAC reaction. It completes instantly like "molecular lightning" and is suitable for tracking dynamic processes in living organisms (such as tumor cell imaging).

(3) A New Generation of Breakthrough: Sulfur(VI) Fluoride Exchange (SuFEx) Reaction

In 2014, Sharpless' team developed the SuFEx reaction, which has become a representative of the new generation of click chemistry. It utilizes the specific reaction between hexavalent sulfur fluoride compounds (such as fluorosulfonyl, -SO₂F) and hydroxyl, amino groups, etc. It does not require a metal catalyst and the product has higher stability. It is not only suitable for material synthesis (such as the preparation of weather-resistant polymers) but also can be used for the modification of drug molecules — for example, adding "targeting groups" to antibody drugs to improve the accuracy of treatment.

"Cross-Border" Applications of Click Chemistry: From Laboratory to Daily Life

With its advantages of "high efficiency, precision, and mildness", click chemistry has penetrated into many fields such as materials science, biomedicine, and diagnostic technology, solving problems that are difficult to overcome with traditional methods.

Making Material Modification Simpler and More Precise with Click Chemistry

Functional Coatings: Introduce azide groups on the surface of plastics or textiles, and then connect antibacterial molecules (such as quaternary ammonium salts) and anti-ultraviolet molecules (such as benzotriazoles) through click reactions to prepare "antibacterial fabrics" and "anti-aging plastics".

Smart Materials: Polymers synthesized by the SuFEx reaction can undergo structural changes under specific conditions (such as pH changes and temperature increases), and can be used as "smart drug carriers" — releasing drugs only when they reach the lesion.

Electronic Materials: Precisely connect conductive molecules (such as thiophene derivatives) to the electrode surface through click reactions to improve the photoelectric conversion efficiency of solar cells.

Biomedicine: From "Drug Synthesis" to "Precision Diagnosis and Treatment"

(1) Therapeutic Field: Making Drugs More "Precise" and "Efficient"

Drug Conjugation: An "assembly tool" for antibody-drug conjugates (ADCs): ADCs are "targeted drug missiles" composed of "antibodies (navigation) + cytotoxic drugs (warheads)". Through CuAAC or SPAAC reactions, drug molecules can be precisely connected to specific sites of antibodies (such as lysine residues), avoiding the reduction in drug efficacy or increase in toxicity caused by "disordered connection" in traditional methods. Some key steps of currently marketed ADC drugs (such as trastuzumab-DM1 for the treatment of breast cancer) rely on click chemistry.

Prodrug Activation: "Unlocking" drugs in situ in tumors: Using bioorthogonal "cleavage" reactions (such as tetrazine-mediated bond cleavage), "prodrugs" can be designed — the drug is inactive outside the body, and only when it enters the tumor and undergoes a click reaction with the pre-injected "unlocking molecule" will the active drug be released, reducing damage to normal tissues. For example, encapsulate chemotherapeutic drugs in "azide-modified nanoparticles", and trigger "unlocking" with cyclooctyne when they reach the tumor to achieve local drug delivery.

(2) Diagnostic Field: Making "Invisible" Molecules "Visible"

Nucleic Acid Detection: Improving sensitivity: In the nucleic acid detection of the new coronavirus, some kits use "azide-modified primers" to amplify viral RNA, and then connect fluorescent molecules through click reactions to make the detection signal stronger and more stable, reducing the false negative rate.

Cell Imaging: Tracking life processes: Use the SPAAC reaction to label sugar molecules on the cell surface — first feed the cells with "azide-modified glucose", which will be used by the cells to synthesize glycoproteins and distributed on the cell membrane. Then add "fluorescently labeled cyclooctyne", and the two undergo a click reaction. This allows real-time observation of the glycosylation process of cells with a microscope, providing a basis for studying the mechanisms of diseases such as cancer and inflammation.

Future Application Prospects

Personalized Medicine: "Tailoring" Treatment Plans

In the future, click chemistry can be used to realize "patient-specific drugs": According to the gene mutation type of a patient's tumor, click reactions are used to quickly assemble different "targeting groups" and "pharmacophore groups". For example, design an "EGFR inhibitor-fluorescent probe" conjugate molecule for lung cancer patients, which not only achieves targeted therapy but also enables real-time monitoring of therapeutic effects through fluorescence.

Regenerative Medicine: Constructing "Bionic Tissues"

Click chemistry can be used to prepare "biocompatible scaffolds": Cross-link biological materials such as collagen and hyaluronic acid through click reactions to form a structure similar to human tissues. Then, plant stem cells on the scaffold and induce them to differentiate into bone, cartilage, or skin tissues for wound repair. For example, use the SPAAC reaction to "anchor" stem cells on the surface of the scaffold to avoid cell loss and improve the success rate of transplantation.

Aladdin: https://www.aladdinsci.com/