Bis-PEG7-acid: Properties, Handling & Application Guide

Bis-PEG7-acid is a symmetric, bifunctional polyethylene glycol reagent bearing two terminal carboxylic acid groups separated by a seven-unit ethylene oxide spacer, widely used as a hydrophilic linker in bioconjugation, PROTAC assembly, and drug-payload tethering applications.

Chemical Name

3,6,9,12,15,18,21-heptaoxatricosanedioic acid (Bis-PEG7-acid)

Functional Groups

Two terminal carboxylic acid (–COOH) groups

PEG Units

7 ethylene oxide (–CH2CH2O–) repeating units

Key Application

Bifunctional hydrophilic linker for PROTAC design, bioconjugation, and ADC synthesis

Solubility

Soluble in water, DMSO, DMF, and most polar organic solvents

Storage Recommendation

Store at –20 °C, protected from moisture and light; avoid repeated freeze-thaw cycles

Chemical Structure and Key Properties

Bis-PEG7-acid is a homodifunctional PEG reagent with the molecular backbone consisting of seven ethylene oxide repeat units flanked on both ends by acetic acid (–CH₂COOH) groups. This symmetric architecture means both termini present equivalent reactivity, enabling coupling to two amine-containing partners simultaneously via standard carbodiimide chemistry (e.g., EDC/NHS activation).

The extended PEG spacer imparts significant aqueous solubility and conformational flexibility to conjugated molecules. The hydrophilic chain reduces non-specific interactions with biological matrices, lowers immunogenicity risks in conjugate design, and improves pharmacokinetic profiles of attached payloads. The molecular weight is consistent with a discrete, monodisperse small-molecule PEG (not a polydisperse polymer), ensuring batch-to-batch reproducibility critical for research applications.

Key physicochemical properties include high solubility in water, DMSO, DMF, methanol, and dichloromethane, making Bis-PEG7-acid compatible with both aqueous bioconjugation buffers and anhydrous organic synthesis conditions.

Applications in PROTAC and Bioconjugation Research

PROTAC Linker Design: Bis-PEG7-acid is widely employed as a linker component in the assembly of Proteolysis-Targeting Chimeras (PROTACs). PROTACs are heterobifunctional small molecules that simultaneously bind a target protein and an E3 ubiquitin ligase, inducing targeted protein degradation. The PEG7 linker bridges the warhead and the E3 ligase ligand, providing the spatial geometry and aqueous solubility necessary for productive ternary complex formation. The dual carboxylic acid termini allow sequential or simultaneous amide bond formation with amine-functionalized warheads or ligands.

Bioconjugation and ADC Synthesis: In antibody-drug conjugate (ADC) research and general bioconjugation, Bis-PEG7-acid serves as a hydrophilic tether to attach small-molecule payloads, fluorescent dyes, or affinity tags to protein scaffolds. The PEG spacer increases the water solubility of hydrophobic payloads, reduces aggregation, and maintains antibody functionality. Activation of the carboxylic acid groups with NHS ester chemistry enables selective conjugation to lysine residues or N-terminal amines on proteins.

Crosslinking and Surface Functionalization: Bis-PEG7-acid can also be used to crosslink two amine-bearing biomolecules or to functionalize amine-coated surfaces and nanoparticles, introducing a flexible, hydrophilic spacer that reduces steric hindrance around the coupled entities.

Recommended Handling and Storage Protocol

Storage: Store Bis-PEG7-acid at –20 °C in a tightly sealed container, protected from moisture and light. Upon receipt, aliquot the material into single-use portions to avoid repeated freeze-thaw cycles, which can cause hydrolytic degradation of concentrated stock solutions and reduce reagent performance over time.

Preparation of Stock Solutions: Dissolve Bis-PEG7-acid in anhydrous DMSO or DMF to prepare concentrated stock solutions (typically 10–100 mM). For aqueous reactions, dilute the organic stock directly into the reaction buffer immediately before use. Avoid prolonged exposure of dissolved reagent to aqueous media at extreme pH values, as PEG-acid reagents can be susceptible to hydrolysis under strongly acidic or basic conditions.

Reaction Conditions for Amide Coupling: Activate the carboxylic acid groups using EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) in combination with NHS (N-hydroxysuccinimide) or sulfo-NHS at pH 5.5–6.0 (activation step), then adjust to pH 7.0–8.0 for conjugation with amine-bearing substrates. Perform reactions at room temperature or 4 °C to minimize side reactions. Remove excess reagent and by-products by dialysis, size-exclusion chromatography, or precipitation as appropriate.

Personal Protective Equipment: Handle with standard laboratory PPE including nitrile gloves, safety glasses, and a lab coat. Consult the Safety Data Sheet (SDS) prior to use for complete hazard information.

Quality, Purity and Analytical Verification

For research applications, particularly PROTAC and ADC development where linker identity and purity directly affect biological outcomes, analytical verification of Bis-PEG7-acid is recommended. Key analytical methods include:

• Mass Spectrometry (MS): Electrospray ionization (ESI-MS) or MALDI-TOF provides definitive molecular weight confirmation and reveals the presence of oligomeric impurities or truncated PEG species.
• NMR Spectroscopy: ¹H and ¹³C NMR confirm the symmetric structure, end-group integrity (–CH₂COOH), and absence of residual solvents or unreacted starting materials.
• HPLC Purity: Reverse-phase or normal-phase HPLC quantifies purity and detects low-level process impurities. For research-grade applications, purity ≥95% by HPLC is generally considered acceptable.
• Karl Fischer Titration: Determines residual moisture content, which is critical for moisture-sensitive coupling reactions.

Aladdin Scientific provides a Certificate of Analysis (CoA) with each lot, including purity data and structural confirmation. Researchers are encouraged to request the CoA and verify that lot-specific data meet their experimental requirements before use in critical assays.

Frequently asked questions

What is the difference between Bis-PEG7-acid and a heterobifunctional PEG linker?

Bis-PEG7-acid is a homodifunctional (symmetric) reagent with identical carboxylic acid groups at both termini, enabling coupling to two amine-bearing partners with equivalent chemistry at each end. Heterobifunctional PEG linkers carry two chemically distinct reactive groups (e.g., one carboxylic acid and one maleimide or azide), allowing orthogonal conjugation to different functional groups on two different molecules. The choice depends on whether the experimental design requires identical or different chemistries at each end of the linker.

Can Bis-PEG7-acid be used in aqueous buffer systems without organic co-solvents?

Yes, Bis-PEG7-acid is highly water-soluble and can be dissolved directly in aqueous buffers such as phosphate-buffered saline (PBS) or MES buffer for bioconjugation reactions. However, for precise dosing and to ensure complete dissolution of the initial solid material, it is common practice to prepare a concentrated stock solution in DMSO or DMF first, then dilute into aqueous buffer immediately before use. The final DMSO or DMF concentration should be kept below 10% (v/v) to maintain protein or biomolecule integrity.

How should I activate the carboxylic acid groups of Bis-PEG7-acid for amide bond formation?

The most common activation strategy uses EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) with NHS or sulfo-NHS. First, mix Bis-PEG7-acid with EDC and NHS in MES buffer (pH 5.5–6.0) and incubate for 15–30 minutes at room temperature to form the reactive NHS ester intermediate. Then add the amine-containing substrate in phosphate or borate buffer adjusted to pH 7.0–8.0 and incubate for 1–2 hours. Quench unreacted NHS ester with hydroxylamine or excess glycine. This two-step protocol improves reaction efficiency and reduces the formation of urea by-products from EDC hydrolysis.

What molar ratio of Bis-PEG7-acid to amine substrate is recommended for bioconjugation?

Typical protocols use a 2- to 10-fold molar excess of Bis-PEG7-acid (after NHS ester activation) relative to the amine-bearing substrate to drive complete coupling. The optimal ratio depends on the number of accessible amine groups on the substrate, the desired degree of labeling, and the experimental context. For controlled, site-specific conjugation requiring minimal crosslinking, lower molar excesses (2–3 fold) are preferred. Optimization with small-scale pilot reactions is recommended before scaling up.

Is Bis-PEG7-acid suitable for in vivo or clinical applications?

Bis-PEG7-acid from Aladdin Scientific is supplied as a research-grade reagent intended for in vitro research and preclinical studies only. It is not certified for clinical use, human therapeutic application, or as a component of any approved drug product without further regulatory evaluation and compliance with applicable GMP standards. Researchers intending to progress materials toward clinical development should consult regulatory guidelines and source materials with appropriate GMP documentation.

References

1. Cromm, P. M.; Crews, C. M. Targeted Protein Degradation: from Chemical Biology to Drug Discovery. Cell Chemical Biology, 2017, 24(9), 1181–1190. doi:10.1016/j.chembiol.2017.05.024
2. Hermanson, G. T. Bioconjugate Techniques, 3rd ed.; Academic Press: Waltham, MA, 2013.
3. Veronese, F. M.; Mero, A. The Impact of PEGylation on Biological Therapies. BioDrugs, 2008, 22(5), 315–329. doi:10.2165/00063030-200822050-00004
4. Kolodych, S. et al. Discovery and Assessment of Novel Sites for ADC Linker Attachment in Human IgG1. Bioconjugate Chemistry, 2015, 26(2), 197–203. doi:10.1021/bc500439t

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.