The introduction of a PEG linker to a molecule provides many benefits for pharmaceutical and biotech R&D due to its water solubility, lower toxicity and non-immunogenicity.
The introduction of different functional groups to the end of a PEG linker allows for more site-specific reactions. For example, proteins have various amino acid residue that may be involved in chemical reactions with amine, sulfhydryl, carboxyl and carbonyl groups being more likely to be targeted for bioconjugation. By altering the end group of the PEG linkers, it is easier to target these amino acid residues or specific functional groups. Table 1 lists reactive groups and available PEG linkers that can be used to target these reactive groups.
Table 1: Reactive groups and corresponding activated PEG linkers
PEG NHS ester
PEG PFP ester
Carboxyl and Active Ester
Copper Free Click Chemistry
Modified PEG linkers can be used for different applications. In some instances, reaction conditions must be altered so that proteins are able to properly react with the PEG reagent. Below are some common PEGylation chemistry reactions.
PEG aldehyde can be used in bioconjugation due to reactions between aminooxy or hydrazide moiety.
PEG Amines can react with acids, succinimidyl-active esters or pentaflurophenyl esters for labelling, chemical modification and surface or particle modification.
PEG Thiols are reactive with maleimides, disulfides, haloacteamides and other types of thiols. They can also be involved in metal surface binding.
Click chemistry describes a wide variety of reactions that occur between two reactive functional groups that can attach to one another under milder, aqueous conditions. Click chemistry tools have improved over the years. First generation click chemistry tools involved copper-catalyzed reactions of terminal alkyne and azide groups. Second generation click chemistry tools made use of strain-promoted alkyne azide reactions without being copper-catalyzed. Third generation Click Chemistry involves the reaction between tetrazine and alkenes such as trans-cyclooctene.
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4. “Click Chemistry” Sigma-Aldrich, 6 July 2018.