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Short answer: PEGylation is the attachment of poly(ethylene glycol) (PEG) chains to a molecule or nanoparticle, usually to give it a hydrophilic “stealth” surface. In PEG names, the number (PEG 400, PEG 2000) usually means average molecular weight in g/mol.
This page is for lab and research staff who meet “PEG” on a reagent label, in a formulation or in a paper and need to read it correctly. It is general reference, not medical advice, and gives no dosing or procedures.
Definition and scope
Makharadze et al. (2025) define PEGylation as the attachment of PEG molecules to nanoparticles and describe the resulting “stealth effect”; the same term is used for conjugating PEG to proteins, peptides and lipids. The PEG terminal hydroxyl group, OH-(CH2CH2O)n-H, can be modified with various ligands, which is how PEG is coupled to other molecules.
What is the structure of polyethylene glycol (PEG)?
PEG is a hydrophilic, non-ionic polyether, written HO(CH2CH2O)nH, made by polymerizing ethylene oxide (Padin-Gonzalez et al., 2022). A Clariant technical brochure on its Polyglykol grades gives the same simple chain, HO-[CH2-CH2-O]n-H, where n is the number of ethylene oxide units. It notes that the term polyethylene glycol, rather than polyethylene oxide, is normally used for mean molecular weights of 200 to 35,000 because the hydroxyl end groups significantly influence the properties of these molecules.
What does PEG 2000 mean?
The number after PEG is the average number of ethylene oxide units or the average molecular weight in g/mol, and the latter is the convention commonly used in the pharmaceutical industry (Padin-Gonzalez et al., 2022). So PEG 2000 is a PEG with an average molecular weight of about 2000 g/mol. The Clariant brochure states the same split: in the pharmaceutical industry the number is the mean molecular weight, whereas in the cosmetic industry it refers to the number of ethylene oxide units, and because ethylene oxide has a molecular weight of 44, average molecular weights are given as round values of n × 44. Check which convention a document is using before comparing numbers.
What is the difference between PEG 400 and PEG 6000?
Both are the same polymer at different average chain lengths. In the Clariant grade table, PEG 400 (mean molecular weight 380 to 420 g/mol) is listed as a liquid, while PEG 6000 is supplied as solid types: flakes (S), powder (P) and fine powder (PF). Per the brochure, for solid types the letter after the number shows physical form, and liquid or waxy grades carry no letter. Grades differ by supplier, so use the specification sheet for the exact lot.
| Name | Meaning of the number | Form listed in the Clariant brochure |
|---|---|---|
| PEG 400 | Mean molecular weight about 400 g/mol (380 to 420) | Liquid |
| PEG 600 | Mean molecular weight about 600 g/mol (570 to 630) | Liquid or wax |
| PEG 2000 | Mean molecular weight about 2000 g/mol | Solid type (S) |
| PEG 6000 | Mean molecular weight about 6000 g/mol | Solid types (S, P, PF) |
What does PEG do in LNP?
In lipid nanoparticles, a PEG-functionalized lipid sits on the outside of the lipid layer and acts as an external stealth moiety (Padin-Gonzalez et al., 2022). That review states that both mRNA COVID-19 vaccines it covers use PEG 2000 in this role: Pfizer/BioNTech uses ALC-0159 (2-[(polyethylene glycol)-2000]-N,N-ditetradecylacetamide) and Moderna uses PEG2000-DMG. The stealth effect is the aim of PEGylation generally (Makharadze et al., 2025).
How PEGylation is characterized
Makharadze et al. list these methods for confirming PEGylation and estimating how much PEG a particle carries:
- XPS: confirms PEGylation and estimates surface density.
- TGA: estimates PEG density from weight loss.
- NMR: the CH2 peak near 3.5 ppm gives surface density; see our NMR spectroscopy guide for the instrument.
- Fluorescence assays: dye-labelled PEG read against a standard curve.
Chromatography is covered separately; see FPLC chromatography and HIC chromatography for the techniques themselves.
Anti-PEG antibodies
PEG is not immunologically invisible. Makharadze et al. link anti-PEG antibodies to the accelerated blood clearance (ABC) phenomenon, reduced efficacy and hypersensitivity. Padin-Gonzalez et al. note pre-existing anti-PEG antibodies in healthy individuals and cite a report by Yang and Lai of 42%; treat that figure as reported there, not as a measured rate for any particular population.
Common misunderstandings
- “PEG 2000 is 2000 PEG units.” In pharmaceutical usage it is the average molecular weight; cosmetic labelling counts units.
- “A PEG is one exact molecule.” The number is a mean, so a grade is a distribution of chain lengths.
- “PEGylation only applies to proteins.” It is also applied to nanoparticles and lipids, as in LNPs.
FAQ
What does PEGylation mean?
Attaching PEG chains to a molecule or nanoparticle, typically for a hydrophilic stealth surface.
What does PEG do in LNP?
A PEG-lipid on the particle’s outer surface serves as the stealth moiety; PEG 2000 is used in both mRNA vaccine examples above.
What is the difference between PEG 400 and PEG 6000?
Average molecular weight: PEG 400 is a liquid grade and PEG 6000 a solid (flakes or powder) in the Clariant listing.
What does PEG 2000 mean?
A PEG of average molecular weight about 2000 g/mol.
Sources
- Makharadze D, et al. The Art of PEGylation: From Simple Polymer to Sophisticated Drug Delivery System. Int J Mol Sci. 2025;26(7):3102. PMC11988339 (read 2026-11-07).
- Padin-Gonzalez E, et al. Understanding the Role and Impact of PEG on Nanoparticle Formulation: Implications for COVID-19 Vaccines. Front Bioeng Biotechnol. 2022;10. doi:10.3389/fbioe.2022.882363 (read 2026-11-07).
- Clariant, Polyglykol (polyethylene glycol) APIs and excipients brochure, via CPHI Online (read 2026-11-07).
