Acid-PEG3-NHS Ester: PEG Linker for Bioconjugation

Acid-PEG3-NHS ester is a heterobifunctional polyethylene glycol crosslinker bearing an amine-reactive N-hydroxysuccinimide (NHS) ester at one terminus and a free carboxylic acid at the other, enabling sequential conjugation of two distinct biomolecular targets through a hydrophilic three-unit PEG spacer.

Chemical Class

Heterobifunctional NHS-PEG crosslinker

Reactive Groups

NHS ester (amine-reactive) and carboxylic acid

PEG Unit Length

3 ethylene oxide units (PEG3 spacer)

Primary Application

Bioconjugation, ADC linker synthesis, surface functionalization

NHS Ester Reactivity

Reacts with primary amines (–NH2) at pH 7–9 to form stable amide bonds

Carboxylic Acid Utility

Can be activated (e.g., with EDC/NHS) for further conjugation to amine-bearing molecules

Aladdin SKU

A597108

What Is Acid-PEG3-NHS Ester?

Acid-PEG3-NHS ester is a heterobifunctional crosslinking reagent that combines two chemically orthogonal reactive handles on a short, hydrophilic polyethylene glycol (PEG) scaffold. The NHS (N-hydroxysuccinimide) ester terminus reacts selectively and efficiently with primary amines—such as lysine side chains or N-terminal amino groups of proteins—under mildly alkaline aqueous conditions (pH 7–9), forming a stable covalent amide bond and releasing N-hydroxysuccinimide as a byproduct. The opposing carboxylic acid terminus remains available for subsequent activation and coupling, most commonly via carbodiimide (EDC) chemistry, to introduce a second amine-bearing molecule.

The three-unit PEG spacer (–CH₂CH₂O–)₃ provides approximately 14 Å of flexible, water-soluble tether between the two conjugated species. This hydrophilic spacer reduces steric hindrance, improves aqueous solubility of the conjugate, and minimizes non-specific adsorption compared to purely hydrocarbon linkers. As a result, Acid-PEG3-NHS ester is widely adopted in academic and industrial research settings where controlled, stepwise bioconjugation is required.

Key Properties and Chemical Characteristics

Bifunctional reactivity: The NHS ester reacts rapidly with primary amines under standard aqueous buffer conditions. The carboxylic acid group is chemically inert until deliberately activated, allowing researchers to perform sequential, one-pot or two-step conjugation strategies without unwanted side reactions between the two functional groups.

Hydrolytic stability considerations: NHS esters are susceptible to hydrolysis in aqueous solution, competing with the desired aminolysis reaction. To maximize conjugation efficiency, reactions should be performed at the lowest practical pH within the effective range (pH 7.2–8.5), and the reagent should be dissolved immediately before use. Anhydrous stock solutions in DMSO or DMF can be stored at –20 °C and are more stable than aqueous solutions.

PEG spacer benefits: The ethylene glycol repeating units confer exceptional water solubility and flexibility. The PEG3 chain length is suitable for applications where a compact linker is preferred—for example, in PROTAC or small-molecule conjugate design—while still providing sufficient distance to reduce steric clashes between the two conjugated moieties.

Molecular characteristics: The compound contains one NHS ester and one free carboxylic acid separated by a triethylene glycol chain. Exact molecular weight and purity specifications should be confirmed against the certificate of analysis (CoA) provided with each lot.

Applications in Research and Drug Discovery

Antibody-drug conjugate (ADC) linker chemistry: Acid-PEG3-NHS ester is used in the construction of ADC payloads and linker-payload modules. The NHS ester can be attached to an antibody lysine residue, while the carboxylic acid terminus is subsequently activated for coupling to an amine-containing cytotoxic payload or additional PEG extension. The PEG3 unit improves the aqueous solubility of hydrophobic drug payloads.

PROTAC and targeted protein degrader synthesis: In proteolysis-targeting chimera (PROTAC) design, Acid-PEG3-NHS ester serves as a bifunctional tether linking a protein-of-interest (POI) ligand to an E3 ubiquitin ligase ligand. The NHS ester provides a convenient handle for attachment to amine-functionalized warheads, and the PEG3 linker region contributes to cell permeability and ternary complex formation.

Surface and nanoparticle functionalization: Amine-functionalized surfaces (e.g., amine-coated microplates, silica nanoparticles, quantum dots) can be conjugated via the NHS ester, leaving the carboxylic acid exposed for subsequent attachment of targeting ligands, detection probes, or additional biomolecules via EDC/NHS coupling.

Protein-protein and protein-small molecule conjugation: Researchers use this reagent to prepare defined, stable conjugates between enzymes, antibodies, peptides, and small-molecule probes for use in ELISA, flow cytometry, biosensing, and imaging applications.

Protocol: Conjugating a Primary Amine-Containing Molecule via NHS Ester

Materials required: Acid-PEG3-NHS ester (A597108), anhydrous DMSO or DMF, phosphate-buffered saline (PBS, pH 7.4) or HEPES buffer (pH 7.5–8.5), amine-containing target molecule, desalting column or dialysis membrane.

Step 1 – Prepare stock solution: Dissolve Acid-PEG3-NHS ester in anhydrous DMSO or DMF at a concentration of 10–50 mM immediately before use. Avoid aqueous stock solutions to prevent premature hydrolysis. Keep on ice.

Step 2 – Reaction setup: Add the NHS ester stock solution to the amine-containing molecule dissolved in reaction buffer (PBS or HEPES, pH 7.5–8.5). Use a 5- to 20-fold molar excess of the NHS ester reagent relative to the target amine. Keep DMSO/DMF content below 10% (v/v) to maintain protein solubility where applicable. Mix gently.

Step 3 – Incubation: Incubate the reaction at room temperature for 30–60 minutes, or at 4 °C for 2–4 hours with gentle mixing. Longer incubation at lower temperature can improve selectivity.

Step 4 – Quench and purify: Quench unreacted NHS ester by adding Tris-HCl buffer (pH 7.4, final concentration 20–50 mM) or ethanolamine. Remove excess reagent and reaction byproducts by gel filtration, dialysis, or size-exclusion chromatography.

Step 5 – Carboxylic acid activation (optional): If further conjugation via the carboxylic acid terminus is desired, activate with EDC and sulfo-NHS in MES buffer (pH 6.0) before adding the second amine-containing molecule.

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

Storage, Handling, and Stability

Storage conditions: Store Acid-PEG3-NHS ester (A597108) as supplied at –20 °C in a tightly sealed container, protected from moisture and light. Desiccation is recommended. Under proper conditions, the solid or neat reagent is stable for an extended period; consult the product CoA for specific shelf-life data.

Handling precautions: NHS esters are reactive electrophiles. Handle with appropriate personal protective equipment (gloves, lab coat, safety glasses). Avoid inhalation of dust or aerosols. Dispose of waste in accordance with institutional and regulatory guidelines.

Preparation for use: Allow the container to equilibrate to room temperature before opening to prevent condensation of atmospheric moisture onto the reagent, which would promote hydrolysis. Weigh out only the amount needed and reseal the container promptly under inert gas (nitrogen or argon) if possible.

Solvent compatibility: The reagent is soluble in polar aprotic solvents including DMSO, DMF, and acetonitrile. Aqueous solutions should be prepared fresh and used within 30 minutes of preparation.

Frequently asked questions

What is the difference between Acid-PEG3-NHS ester and a standard NHS ester crosslinker?

Standard NHS ester crosslinkers may be homobifunctional (two NHS esters) or contain non-PEG spacers, limiting their utility for sequential, orthogonal conjugation. Acid-PEG3-NHS ester is heterobifunctional—one NHS ester reacts with primary amines while the carboxylic acid terminus can be independently activated for a second conjugation step. The PEG3 spacer also adds hydrophilicity and flexibility not found in purely hydrocarbon linkers.

Can I use Acid-PEG3-NHS ester with thiol-containing molecules?

The NHS ester in Acid-PEG3-NHS ester reacts primarily with primary amines and is not selective for thiols under standard conditions. If thiol-selective conjugation is required, a maleimide- or iodoacetamide-based reagent would be more appropriate. However, the carboxylic acid terminus can be functionalized post-conjugation to introduce thiol-reactive groups if a multi-step synthesis strategy is employed.

Why does my NHS ester conjugation yield appear low?

Low conjugation yields with NHS esters are most commonly caused by hydrolysis of the active ester before it reacts with the target amine. Ensure the reagent is dissolved in dry organic solvent immediately before use, keep reaction pH between 7.5 and 8.5, use the reagent in molar excess, and avoid amine-containing buffers such as Tris. Performing the reaction at 4 °C for a longer incubation period can also improve outcomes by reducing hydrolysis rates while maintaining aminolysis.

Is Acid-PEG3-NHS ester suitable for PROTAC synthesis?

Yes, Acid-PEG3-NHS ester is well-suited for PROTAC linker assembly. The PEG3 unit provides a water-solubilizing, flexible tether of appropriate length for many PROTAC scaffolds, and the dual functionality allows sequential attachment of a POI ligand and an E3 ligase ligand. The compact size of the PEG3 spacer is particularly useful when a shorter linker is required for optimal ternary complex geometry.

How should I quench excess Acid-PEG3-NHS ester after conjugation?

Add Tris-HCl (pH 7.4, 20–50 mM final concentration) or ethanolamine to quench unreacted NHS ester by reacting with its primary amine groups. After quenching for 10–15 minutes, remove small-molecule reagents and byproducts by gel filtration, dialysis, or size-exclusion chromatography appropriate for your conjugate size.

References

1. Hermanson, G.T. Bioconjugate Techniques, 3rd ed.; Academic Press: London, 2013. [Standard reference for NHS ester chemistry and PEG crosslinker protocols]
2. Saito, F. et al. Synthesis of bioconjugates using heterobifunctional PEG crosslinkers. [General reference — specific author names and publication details require verification]
3. Bondeson, D.P. et al. Catalytic in vivo protein knockdown by small-molecule PROTACs. Nature Chemical Biology, 2015. [Cited as example of PEG linker use in PROTAC context — verify DOI and PMID]

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