SPAAC

SPAAC stands for strain-promoted azide–alkyne cycloaddition, a copper-free variant of the azide–alkyne cycloaddition used in bioorthogonal chemistry. It enables rapid covalent labeling of biomolecules in living systems by reacting an azide with a strained cyclooctyne without copper catalysts, which can be cytotoxic.

Mechanism and reagents: The ring strain in cyclooctynes accelerates the formation of a 1,2,3-triazole with an azide under physiological conditions. Common strained alkynes include cyclooctynes such as BCN, DBCO (difluorinated cyclooctyne), and ADIBO; reagents like BARAC and related derivatives are also used. The reaction is chemoselective for azides and generally compatible with aqueous buffers, living cells, and organisms.

Applications: SPAAC is widely used for biomolecule labeling, including tracking proteins, antibodies, glycans, and lipids. It enables fluorescent imaging, protein tagging, and surface functionalization in biology and materials science. It is employed in diagnostic assays, drug delivery studies, and in the functionalization of hydrogels and nanoparticles for bioapplications.

Advantages and limitations: The copper-free aspect avoids cytotoxic copper, enabling in vivo use. Kinetics are typically slower than copper-catalyzed variants, but suitable strained-alkyne reagents provide rapid rates at micromolar concentrations. Some reagents can be bulky or hydrophobic, affecting solubility and cell permeability. Overall, the reaction is highly selective for azides, but reagent stability, toxicity, and non-specific interactions can vary with structure and context.

History: The approach emerged in the early 2000s as a copper-free alternative to the Huisgen cycloaddition. The term SPAAC was associated with work by Carolyn R. Bertozzi and colleagues in the development of bioorthogonal chemistry.