SPAAC — Strain-Promoted Azide-Alkyne Cycloaddition

Bioorthogonal Copper-Free Nobel Prize 2022

Overview

Strain-Promoted Azide-Alkyne Cycloaddition (SPAAC) is a copper-free variant of the azide-alkyne cycloaddition developed by Carolyn R. Bertozzi and colleagues in 2004^[4]. By exploiting the ring strain inherent in cyclooctynes — eight-membered rings containing an internal alkyne — SPAAC eliminates the need for cytotoxic copper catalysts, enabling click chemistry to be performed directly in living cells and organisms.

The driving force for SPAAC is the approximately 18 kcal/mol of ring strain stored in the bent alkyne of the cyclooctyne ring. This strain lowers the activation barrier for the [3+2] cycloaddition with azides sufficiently to allow the reaction to proceed at physiological temperature without any metal catalyst. The reaction yields a mixture of triazole regioisomers (1,4- and 1,5-disubstituted), unlike the strictly regioselective CuAAC.

Bertozzi's development of SPAAC and the broader field of bioorthogonal chemistry earned her a share of the 2022 Nobel Prize in Chemistry^[12], alongside Sharpless and Meldal.

Reaction Diagram

Key Properties

Key Cyclooctynes

The reactivity of SPAAC depends critically on the choice of cyclooctyne. Ring strain can be modulated by incorporating electron-withdrawing groups, fused ring systems, or exocyclic substituents that distort the alkyne bond angle further from linearity.

DIBO (Dibenzocyclooctyne)

The first cyclooctyne used in biological settings by Bertozzi (2004)^[4]. DIBO features two fused benzene rings that increase ring strain and improve reaction kinetics compared to unsubstituted cyclooctyne. However, its significant hydrophobicity limits aqueous solubility and cellular applications.

DBCO / DIBAC (Dibenzocyclooctyne with Amide)

One of the most widely used cyclooctynes. DBCO (also called DIBAC) incorporates an amide linkage that improves aqueous solubility while maintaining excellent reactivity (k ≈ 0.3–1.0 M⁻¹s⁻¹). It is commercially available in numerous conjugation kits and has been used extensively for glycan labeling, protein modification, and in vivo imaging.

BCN (Bicyclo[6.1.0]nonyne)

BCN features a fused cyclopropane ring that introduces additional strain. It reacts with azides at k ≈ 0.1–0.2 M⁻¹s⁻¹ and with tetrazines in IEDDA reactions, making it a versatile "dual-click" reagent. BCN is relatively compact and less hydrophobic than DBCO.

DIMAC (Dimethoxyazacyclooctyne)

DIMAC was designed to improve water solubility and reduce non-specific binding. The incorporation of methoxy groups and a nitrogen atom in the ring framework enhances hydrophilicity while maintaining useful reaction rates (k ≈ 0.03 M⁻¹s⁻¹).

BARAC (Biarylazacyclooctynone)

BARAC achieves the fastest SPAAC rates among cyclooctynes (k ≈ 2.0 M⁻¹s⁻¹) through a combination of ring strain and electronic activation from the biaryl and amide substituents. However, its synthesis is more complex, and the bulky structure may cause steric hindrance in some applications.

Cyclooctyne Comparison Table

Applications

Live-Cell Imaging

SPAAC enables the visualization of biomolecules in living cells without perturbing cellular function. Cells are metabolically labeled with azide-bearing sugar analogs (e.g., Ac₄ManNAz for sialic acid), followed by SPAAC conjugation with cyclooctyne-fluorophore probes. This two-step labeling strategy — pioneered by Bertozzi — allows selective imaging of cell-surface glycans, lipids, and proteins with subcellular resolution.

Glycan Labeling

Cell-surface glycans play critical roles in cell recognition, signaling, and immune response. SPAAC-based metabolic glycoengineering has become the standard approach for mapping the glycome. Azide-modified monosaccharides (ManNAz, GalNAz, GlcNAz, SiaNAz) are incorporated into glycans via native biosynthetic pathways, then labeled with DBCO-fluorophores or DBCO-biotin for detection and enrichment.

In Vivo Tumor Imaging

SPAAC has been used for pre-targeted in vivo imaging of tumors in mouse models. Azide-functionalized antibodies or nanoparticles accumulate at the tumor site, followed by systemic administration of a cyclooctyne-fluorophore conjugate. The bioorthogonal click reaction occurs selectively at the tumor, enabling high-contrast fluorescence or PET imaging with reduced background signal compared to directly labeled antibodies.

Protein Labeling & Modification

Site-specific protein modification via SPAAC avoids the non-selectivity of traditional lysine/cysteine bioconjugation. Non-canonical amino acids bearing azide groups (e.g., azidohomoalanine, p-azidophenylalanine) can be incorporated at defined positions via amber codon suppression or metabolic labeling, followed by SPAAC with cyclooctyne-PEG, cyclooctyne-drug, or cyclooctyne-fluorophore conjugates.

Hydrogel & Biomaterial Formation

SPAAC is used to form hydrogels under physiological conditions for cell encapsulation and tissue engineering. Multi-arm PEG derivatives functionalized with azide and cyclooctyne groups cross-link upon mixing, creating cytocompatible networks that can entrap living cells without exposure to UV light or toxic catalysts.

Advantages over CuAAC

Limitations

• Slower kinetics: SPAAC is typically ~100-fold slower than CuAAC. The rate constant for most cyclooctynes falls in the range of 0.01–2 M⁻¹s⁻¹, which may be insufficient for applications requiring rapid labeling at low concentrations.
• Bulky reagents: Cyclooctynes (MW ~200–500 Da) are significantly larger than simple terminal alkynes. This bulkiness can cause steric hindrance, affecting binding affinity, protein folding, or the ability to access sterically confined binding sites.
• Reduced cell penetration: The hydrophobic nature of many cyclooctynes (particularly DBCO and DIBO) can limit membrane permeability, restricting intracellular labeling applications. BCN offers somewhat better cell penetration due to its smaller, less hydrophobic structure.
• Regioisomer mixtures: Unlike CuAAC, SPAAC produces a mixture of 1,4- and 1,5-triazole regioisomers. For applications requiring a single isomer, this can be a disadvantage.
• Thiol reactivity: Some cyclooctynes (particularly BARAC) can undergo side reactions with biological thiols (e.g., glutathione, cysteine), leading to reduced labeling efficiency and potential off-target effects.

Related Reactions