PEGylation is the covalent attachment of one or more polyethylene glycol chains to a peptide, usually at the N-terminal amine, a lysine side-chain amine, or an engineered cysteine thiol. The polymer carries a large shell of ordered water, so a 3 kDa peptide with a 5 kDa PEG behaves hydrodynamically like a protein several times its true molecular weight. That raises it above the glomerular filtration threshold and sterically shields cleavage sites from proteases.
Why PEGylation matters in peptide research
The practical consequence is a longer circulating half-life. PEG-MGF is the familiar catalogue example: the unmodified mechano-growth-factor fragment is reported to clear within minutes in rodent work, while the PEGylated construct persists far longer, which is why the two are studied for different questions rather than as substitutes.
PEGylation also breaks a naive mass assumption. Labelled milligrams of a PEGylated product include the polymer, so the peptide content per vial is lower than the same nominal mass of unmodified material; any molar calculation must use the conjugate’s mass, not the parent sequence’s. PEG is polydisperse too, so mass spectra show a distribution of peaks rather than one sharp ion, and purity figures on a certificate of analysis should be read with that in mind. See the PEG-MGF overview and why DAC, PEG and acetylation exist.