BCN-carbamate-PEG2-Bromide: IEDDA Bioconjugation Linker

BCN-carbamate-PEG2-Bromide is a bifunctional bioorthogonal linker combining a strained bicyclo[6.1.0]nonyne (BCN) moiety for ultrafast inverse electron demand Diels-Alder (IEDDA) cycloaddition with a reactive alkyl bromide handle and a short PEG2 spacer that improves aqueous solubility.

CAS / Product SKU

E1529510 (Aladdin SKU)

Reactive Groups

BCN (IEDDA cycloaddition with tetrazines) and alkyl bromide (nucleophilic substitution)

Spacer

PEG2 (two ethylene glycol units) for improved aqueous solubility and reduced aggregation

Reaction Type

Inverse electron demand Diels-Alder (IEDDA); metal-free, bioorthogonal

Typical Applications

Antibody-drug conjugate (ADC) synthesis, protein labeling, cell-surface engineering, probe attachment

Storage Recommendation

Store under inert atmosphere at -20 °C, protected from light and moisture

What Is BCN-carbamate-PEG2-Bromide?

BCN-carbamate-PEG2-Bromide (Aladdin SKU: E1529510) is a heterobifunctional crosslinker designed for bioorthogonal and bioconjugation chemistry. The molecule integrates three functional domains into a single scaffold:

• BCN (bicyclo[6.1.0]nonyne): a strained cycloalkyne that reacts rapidly and selectively with 1,2,4,5-tetrazines via inverse electron demand Diels-Alder (IEDDA) cycloaddition. This reaction is metal-free, proceeds under physiological conditions and exhibits exceptional selectivity in complex biological matrices.
• Carbamate linkage: a stable urethane bond connecting the BCN group to the PEG spacer, providing hydrolytic stability superior to ester bonds while maintaining synthetic accessibility.
• PEG2 spacer: two ethylene glycol units that confer improved water solubility, reduce steric crowding at the conjugation site and help minimize non-specific interactions in biological assays.
• Alkyl bromide (–CH₂Br): an electrophilic handle for nucleophilic substitution (S_N2) reactions with thiols, amines, azides or other nucleophiles, enabling attachment to biomolecules or solid supports.

Together, these elements allow researchers to build defined conjugates in a two-step or one-pot strategy: first anchor the bromide end to a nucleophile-bearing molecule, then ligate the BCN end to a tetrazine-functionalized partner under mild, bioorthogonal conditions.

Mechanism: IEDDA Click Chemistry with BCN

The cornerstone reactivity of this linker is the inverse electron demand Diels-Alder (IEDDA) cycloaddition between the BCN dienophile and an electron-poor tetrazine diene. Key mechanistic features include:

1. Ring-strain driving force: The [6.1.0] bicyclic ring system imposes significant angle strain on the internal alkyne, dramatically accelerating cycloaddition kinetics relative to unstrained alkynes. Second-order rate constants for BCN–tetrazine reactions have been reported in the range of 10²–10⁴ M⁻¹s⁻¹ depending on the tetrazine substitution pattern, enabling rapid conjugation even at nanomolar concentrations.
2. Bioorthogonality: Neither BCN nor tetrazines react with canonical biological functional groups (amines, thiols, hydroxyls, carboxyls) under physiological pH and temperature, making them ideal for live-cell labeling and in vivo applications.
3. Metal-free conditions: Unlike copper-catalyzed azide-alkyne cycloaddition (CuAAC), IEDDA requires no transition metal catalyst, eliminating cytotoxicity concerns.
4. Stable cycloadduct: The initial [4+2] cycloadduct spontaneously releases N₂ to yield a stable dihydropyridazine product, completing the ligation irreversibly.

The alkyl bromide terminus provides orthogonal reactivity, allowing pre-functionalization of nucleophile-bearing biomolecules (e.g., cysteine thiols on antibodies or proteins) prior to the IEDDA step.

Applications in Bioconjugation and Chemical Biology

BCN-carbamate-PEG2-Bromide is a versatile reagent suited to several cutting-edge research applications:

Antibody–Drug Conjugate (ADC) Synthesis

The alkyl bromide can alkylate engineered cysteine or histidine residues on an antibody, while the BCN group is subsequently conjugated to a tetrazine-tagged cytotoxic payload. This two-step strategy provides site-specific conjugation with defined drug-to-antibody ratios (DAR), a critical parameter for ADC homogeneity and efficacy.

Protein and Peptide Labeling

Researchers can attach the bromide end to thiol-containing peptides or unnatural amino acid side chains, then react the BCN with tetrazine-functionalized fluorescent dyes, affinity handles (biotin) or PEG polymers. The PEG2 spacer reduces steric interference, preserving protein activity after labeling.

Cell-Surface Engineering

Metabolic labeling with tetrazine-modified sugars followed by BCN ligation enables rapid, selective decoration of cell-surface glycoproteins. Alternatively, BCN-functionalized surfaces can be coated with tetrazine-tagged ligands for controlled cell capture.

Solid-Phase and Material Science Applications

The alkyl bromide can be used in atom-transfer radical polymerization (ATRP) initiation or for functionalization of amine/thiol-bearing resins, nanoparticles or hydrogels, incorporating BCN handles for subsequent bioorthogonal surface modification.

Handling, Solubility and Storage Guidelines

Proper handling of BCN-carbamate-PEG2-Bromide is essential to preserve reagent integrity and ensure reproducible results.

Solubility

The PEG2 segment significantly improves the aqueous compatibility of this reagent compared with BCN reagents lacking PEG. Recommended solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and acetonitrile for stock solution preparation. For aqueous reactions, dilute DMSO stocks into phosphate-buffered saline (PBS) or other aqueous buffers, keeping the organic co-solvent below 5% (v/v) to avoid protein denaturation.

Storage

Store at −20 °C under an inert gas atmosphere (argon or nitrogen) in tightly sealed vials, protected from light and moisture. The alkyl bromide functionality can be susceptible to hydrolysis and should not be exposed to aqueous conditions before use. Repeated freeze-thaw cycles should be minimized; prepare single-use aliquots where practical.

Safety Considerations

Alkyl bromides are potential alkylating agents. Handle with appropriate personal protective equipment (gloves, lab coat, eye protection) in a well-ventilated fume hood. Consult the Safety Data Sheet (SDS) for full hazard information before use. Dispose of waste according to local regulations for halogenated organic compounds.

Reconstitution Tips

Allow the vial to equilibrate to room temperature before opening to prevent moisture condensation on the reagent. Prepare fresh working solutions for each experiment to maximize reactivity.

Comparison with Related BCN and Click Chemistry Linkers

Researchers selecting bioorthogonal linkers benefit from understanding how BCN-carbamate-PEG2-Bromide compares with related reagent classes:

Feature	BCN-carbamate-PEG2-Bromide	DBCO-PEG Linkers	TCO-PEG Linkers	BCN-NHS Ester
Bioorthogonal reaction	IEDDA (tetrazine)	SPAAC (azide)	IEDDA (tetrazine)	IEDDA (tetrazine)
Second handle	Alkyl bromide	Varies (NHS, maleimide)	Varies	NHS ester (amine)
IEDDA rate	Fast (BCN)	N/A	Very fast (TCO)	Fast (BCN)
PEG solubilizer	Yes (PEG2)	Often yes	Often yes	No
Metal-free	Yes	Yes	Yes	Yes
Thiol selectivity of 2nd handle	Yes (S-alkylation)	Depends on design	Depends on design	No (amine selective)

TCO (trans-cyclooctene) linkers generally exhibit faster IEDDA rates than BCN, but BCN offers greater stability under physiological conditions and is less prone to isomerization. The alkyl bromide handle of this reagent provides an alternative attachment mode to NHS esters, which is advantageous when lysine labeling would compromise protein function or when thiol-directed chemistry is preferred for site-specific conjugation.

Frequently asked questions

What nucleophiles react with the alkyl bromide group of BCN-carbamate-PEG2-Bromide?

The alkyl bromide undergoes SN2 reactions with soft nucleophiles, most efficiently with thiolates (e.g., cysteine residues, thiol-modified oligonucleotides). Primary amines can also react, but typically require harsher conditions or longer incubation times and may compete with hydrolysis. Reactions with azides or other hard nucleophiles are generally not favored under mild aqueous conditions.

Can BCN-carbamate-PEG2-Bromide be used for live-cell labeling experiments?

The BCN group is well suited for live-cell applications because IEDDA reactions with tetrazines are metal-free, fast and bioorthogonal. However, the free alkyl bromide is a reactive electrophile that could alkylate cellular nucleophiles non-specifically; therefore, the bromide should be fully reacted with the target biomolecule before introducing the conjugate into a live-cell context. Pre-formed BCN-labeled biomolecules can then be used safely for cell-surface IEDDA ligation.

How should I choose between BCN and TCO when designing an IEDDA-based conjugation strategy?

TCO generally reacts with tetrazines at higher rate constants than BCN, making it preferable when very fast kinetics are needed (e.g., in vivo pretargeting). BCN is more chemically stable—particularly resistant to thiol-induced isomerization—and may be preferred in reducing environments or when longer shelf-life of the functionalized biomolecule is required. Both are metal-free and bioorthogonal; the choice ultimately depends on the required reaction speed, stability constraints and the availability of compatible tetrazine partners.

What is the role of the PEG2 spacer in this linker?

The PEG2 spacer (two ethylene oxide units) improves the overall aqueous solubility of the reagent by introducing hydrophilicity between the relatively lipophilic BCN group and the alkyl bromide. It also provides molecular flexibility, reducing steric clashes between the two reactive termini and the biomolecules they address, which can improve conjugation efficiency and help preserve the biological activity of labeled proteins or antibodies.

Is BCN-carbamate-PEG2-Bromide compatible with standard HPLC purification?

Yes, conjugates bearing the BCN-PEG2 linker are generally amenable to reversed-phase or size-exclusion HPLC purification. However, the alkyl bromide intermediate should be purified under anhydrous conditions if possible, or kept in organic solvent until the conjugation step is complete, to minimize hydrolysis. Post-conjugation products (after IEDDA or alkylation) are typically more stable and can be purified using standard aqueous HPLC conditions.

References

1. Blackman, M. L.; Royzen, M.; Fox, J. M. Tetrazine ligation: fast bioconjugation based on inverse-electron-demand Diels-Alder reactivity. J. Am. Chem. Soc. 2008, 130, 13518–13519. doi:10.1021/ja8053805
2. Dommerholt, J.; Schmidt, S.; Temming, R.; Hendriks, L. J. A.; Rutjes, F. P. J. T.; van Hest, J. C. M.; Lefeber, D. J.; Friedl, P.; van Delft, F. L. Readily accessible bicyclononynes for bioorthogonal labeling and three-dimensional imaging of living cells. Angew. Chem. Int. Ed. 2010, 49, 9422–9425. doi:10.1002/anie.201005013
3. Oliveira, B. L.; Guo, Z.; Bernardes, G. J. L. Inverse electron demand Diels-Alder reactions in chemical biology. Chem. Soc. Rev. 2017, 46, 4895–4950. doi:10.1039/C7CS00184C
4. Devaraj, N. K. The future of bioorthogonal chemistry. ACS Cent. Sci. 2018, 4, 952–959. doi:10.1021/acscentsci.8b00251

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