Chloroacetamido-PEG4-NHS Ester: Uses, Properties & FAQs

Chloroacetamido-PEG4-NHS ester is a heterobifunctional polyethylene glycol crosslinker bearing an amine-reactive NHS ester at one terminus and a thiol-reactive chloroacetamide group at the other, enabling sequential, chemoselective conjugation of two biomolecules through a hydrophilic PEG4 spacer.

CAS / SKU

SKU C595062 (also known as BroadPharm BP-20519)

Functional Groups

NHS ester (amine-reactive) + Chloroacetamide (thiol-reactive)

PEG Chain Length

4 ethylene oxide units (PEG4 spacer)

Primary Application

Heterobifunctional bioconjugation, ADC linker synthesis, protein–protein and protein–small molecule crosslinking

Solubility

Soluble in common organic solvents (DMSO, DMF) and aqueous buffers at working concentrations

Recommended Storage

Store at –20 °C, desiccated, under inert atmosphere; avoid freeze-thaw cycles

What Is Chloroacetamido-PEG4-NHS Ester?

Chloroacetamido-PEG4-NHS ester (Aladdin SKU C595062; BroadPharm BP-20519) is a heterobifunctional crosslinker designed for sequential, chemoselective bioconjugation. The molecule contains two orthogonal reactive handles separated by a four-unit polyethylene glycol (PEG4) spacer:

• NHS ester terminus: reacts selectively with primary amines (–NH₂), such as lysine side chains or N-terminal amines on proteins and peptides, forming stable amide bonds under mild aqueous conditions (pH 7–9).
• Chloroacetamide terminus: reacts selectively with free thiols (–SH), such as cysteine residues or thiolated oligonucleotides, via an S-alkylation (thioether bond formation) mechanism.

Because the two reactive groups respond to entirely different nucleophiles under different conditions, researchers can perform a two-step, directed conjugation strategy—first reacting one end with a target amine, then reacting the other end with a thiol—minimising homo-bifunctional side products and maximising control over the final conjugate architecture.

Key Applications in Research and Drug Discovery

The orthogonal reactivity and hydrophilic PEG4 spacer of Chloroacetamido-PEG4-NHS ester make it suitable for a broad range of applications:

1. Antibody–Drug Conjugate (ADC) Synthesis: The reagent can be used to attach cytotoxic payloads or intermediate linker modules to antibodies. The NHS ester can couple to lysines on the antibody, while the chloroacetamide can engage an engineered or reduced cysteine on the payload, or vice versa.
2. Protein–Protein Crosslinking: Enables stable covalent linkage between two proteins bearing complementary amine and thiol groups, useful for creating fusion-like constructs or studying protein–protein interactions.
3. Protein–Oligonucleotide and Protein–Small-Molecule Conjugation: The bifunctional linker bridges amine-bearing proteins with thiolated nucleic acids or thiol-containing small molecules, supporting diagnostic probe and biosensor development.
4. Surface Functionalization: Amine-presenting surfaces (e.g., amino-silanised glass, aminodextran-coated beads) can be modified via the NHS ester, presenting reactive chloroacetamide groups for subsequent thiol capture.
5. PEGylation with a Reactive Handle: The PEG4 spacer increases aqueous solubility and reduces steric interference of the conjugate, while the terminal reactive groups permit further chemistry.

Reaction Conditions and Protocol Guidance

The following protocol guidance reflects standard practice for heterobifunctional NHS ester / chloroacetamide crosslinkers. Researchers should optimise conditions for their specific biomolecules.

Step 1 – NHS Ester Reaction (Amine Coupling)

• Dissolve Chloroacetamido-PEG4-NHS ester in anhydrous DMSO or DMF at 10–100 mM as a stock solution.
• Dilute into the amine-bearing biomolecule solution in PBS or HEPES buffer, pH 7.2–8.5.
• Use 5–20 molar equivalents of reagent relative to the target protein.
• React for 30–60 minutes at room temperature or 4 °C with gentle mixing.
• Remove unreacted NHS ester by gel filtration (e.g., Sephadex G-25) or dialysis.

Step 2 – Chloroacetamide Reaction (Thiol Coupling)

• Ensure the thiol-bearing partner has free, reduced thiols (use TCEP at 1–5 mM, then remove excess reductant before conjugation).
• Mix the NHS-ester-modified biomolecule from Step 1 with the thiol partner in PBS, pH 7.0–7.5.
• React for 2–16 hours at room temperature or 4 °C under inert atmosphere (N₂ or Ar) to prevent thiol oxidation.
• Quench unreacted chloroacetamide with excess β-mercaptoethanol or cysteine if required.

Note: Avoid primary amine-containing buffers (e.g., Tris, glycine) during the NHS ester reaction step, as these will compete with and quench the NHS ester.

Storage, Stability, and Handling Precautions

Storage: Store Chloroacetamido-PEG4-NHS ester at –20 °C in a tightly sealed container with desiccant. An inert atmosphere (nitrogen or argon) is recommended to prevent hydrolysis of the NHS ester and oxidation of the chloroacetamide functionality. Avoid repeated freeze-thaw cycles by preparing single-use aliquots.

Stability: NHS esters are susceptible to hydrolysis in aqueous solution; the half-life is pH- and temperature-dependent. Prepare working solutions fresh immediately before use. Dry solid material stored correctly is typically stable for 12–24 months (verify with lot-specific CoA).

Handling Precautions:

• Chloroacetamide-containing compounds may be irritants; handle in a well-ventilated fume hood and wear appropriate PPE (gloves, eye protection, lab coat).
• Dispose of waste according to local regulations for halogenated organic compounds.
• Consult the Safety Data Sheet (SDS) for full hazard information before use.

Frequently asked questions

What is the difference between the NHS ester and the chloroacetamide reactive groups on this linker?

The NHS ester reacts selectively with primary amines (e.g., lysine residues, N-terminal amines) at pH 7–9, forming a stable amide bond and releasing N-hydroxysuccinimide as a by-product. The chloroacetamide reacts with free thiols (e.g., cysteine residues) via an SN2-type alkylation to form a stable thioether bond. Because the two groups target different nucleophiles under different optimal conditions, they can be used sequentially in a two-step conjugation without cross-reactivity.

Why is a PEG4 spacer used, and can I substitute a shorter or longer PEG linker?

The PEG4 spacer (four ethylene oxide units, approximately 17–19 Å extended length) increases the aqueous solubility of the crosslinker and the resulting conjugate, reduces steric hindrance between the two conjugated biomolecules, and can improve binding accessibility. If a shorter spacer is needed (e.g., to place two domains in close proximity) or a longer spacer (e.g., to bridge large proteins with greater flexibility), alternative PEG-length variants should be selected. Consult Aladdin Scientific's linker catalogue for PEG2, PEG8, or PEG12 chloroacetamide–NHS ester analogues.

How do I prevent premature hydrolysis of the NHS ester during my conjugation experiment?

To minimise hydrolysis, dissolve the reagent in anhydrous DMSO or DMF as a concentrated stock and add it to the aqueous reaction mixture immediately before use. Keep the reaction pH at 7.2–8.5 (higher pH accelerates hydrolysis) and work at 4 °C if your biomolecule is sensitive to room-temperature incubation. Always prepare fresh working solutions; do not store diluted aqueous solutions of NHS ester reagents.

Is Chloroacetamido-PEG4-NHS ester suitable for use in ADC linker synthesis?

Yes. The reagent's NHS ester can couple to lysine residues on a monoclonal antibody, while the chloroacetamide group can subsequently react with thiol-bearing payloads or intermediate linker fragments. The PEG4 spacer confers aqueous solubility advantages important in ADC manufacturing. However, site-specific conjugation strategies (e.g., engineered cysteines, unnatural amino acids) may require careful optimisation of molar ratios and reaction conditions to achieve the desired drug-to-antibody ratio (DAR).

What buffers are compatible with the NHS ester reaction step?

PBS (pH 7.2–7.4), HEPES (pH 7.5–8.0), and sodium bicarbonate (pH 8.0–8.5) buffers are all compatible. Avoid any buffer containing primary amines, including Tris (tris(hydroxymethyl)aminomethane), glycine, and ethanolamine, as these nucleophiles will rapidly quench the NHS ester before it can react with your target biomolecule.

References

1. Hermanson GT. Bioconjugate Techniques. 3rd ed. Academic Press; 2013. Chapter 4: Zero-Length Crosslinkers and Chapters on Heterobifunctional Reagents.
2. BroadPharm Product Data Sheet: Chloroacetamido-PEG4-NHS ester, BP-20519.
3. Dozier JK, Distefano MD. Site-Specific PEGylation of Therapeutic Proteins. Int J Mol Sci. 2015;16(10):25831-25864. doi:10.3390/ijms161025831
4. Alley SC, Okeley NM, Senter PD. Antibody-drug conjugates: targeted drug delivery for cancer. Curr Opin Chem Biol. 2010;14(4):529-537. doi:10.1016/j.cbpa.2010.06.170

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