What makes a reaction “click chemistry”?
Click chemistry is a collective term for a class of chemical reactions that proceed rapidly, selectively and under mild conditions, with high yield and few unwanted by-products. The best-known variant in peptide research is azide-alkyne cycloaddition: an azide group and an alkyne group react with each other to form a stable triazole ring. For laboratories working with research peptides this is attractive, because the reaction is insensitive to most of the functional groups already present in a peptide, such as amines, carboxylic acids and thiols. As a result, a label, probe or surface can be coupled without disturbing the rest of the peptide structure.
CuAAC versus SPAAC: with or without a copper catalyst
Within azide-alkyne cycloaddition there are two main variants. The copper-catalysed version (CuAAC, Cu(I)-catalyzed azide-alkyne cycloaddition) is fast and inexpensive, but copper residues can damage peptides through oxidative processes and are difficult to remove completely — a point of attention for analytical control afterwards. The copper-free variant (SPAAC, strain-promoted azide-alkyne cycloaddition) uses a ring-strained cyclooctyne instead of a catalyst. This reaction proceeds more slowly, but avoids metal contamination entirely, which is relevant when the research sample is subsequently analysed with sensitive detection methods such as mass spectrometry. In practice, the choice between the two variants depends on the required reaction rate, the reagents available and the tolerance of the downstream research for traces of metal.
How does an azide or alkyne handle end up in a peptide?
To apply click chemistry, one of the two reactive groups must first be incorporated into the research peptide. This is usually done in one of two ways. During Fmoc solid-phase synthesis, a non-natural amino acid with an azide or alkyne side chain can be incorporated at a pre-selected position in the sequence, which gives full control over the location of the handle. Alternatively, an existing functional group — usually a free amine at the N-terminus or a lysine side chain — can be chemically converted after synthesis with a reagent that introduces an azide or alkyne group. Both routes have implications for the purity analysis: an additional synthesis step usually means an additional round of HPLC purification and a controlled confirmation of the correct mass before the coupling is carried out.
Applications of click chemistry in the research laboratory
In a laboratory context, click chemistry is mainly used to couple research peptides to detectable or functional units without affecting the peptide structure itself. Examples from the literature are the coupling of a fluorophore for imaging studies, the attachment of a peptide to a solid support or sensor surface for binding studies, and the formation of peptide-polymer conjugates for materials research. Because the resulting triazole ring is chemically very stable, the linkage remains intact under a wide range of analytical and experimental conditions, which benefits reproducibility.
Characterisation after the coupling reaction
As with any modification of a research peptide, characterisation after the reaction is essential. LC-MS confirms whether the mass shift corresponds to the expected triazole formation and whether unreacted starting material is still present. Analytical HPLC shows whether the conjugation reaction has proceeded quantitatively or whether an additional purification step is needed to separate the coupled product from unconverted peptide. With CuAAC reactions, residual traces of copper are often also checked for, for example with ICP-MS, to ensure that the final sample is suitable for the intended follow-up research.
In conclusion
Over twenty years, click chemistry has developed into a standard tool in the bioconjugation chemistry of peptides, precisely because the reaction is predictable, selective and readily characterised. For laboratories that combine research peptides with labels, surfaces or other functional units, the choice between CuAAC and SPAAC offers a direct means of weighing speed against the purity of the final product.
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