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 | |
Reactivity Class Advertisement #3      | PEG Linkers |
Amine | Bis-PEG-acid Bis-PEG-NHS PEG Acid PEG Aldehyde PEG NHS ester PEG PFP ester |
Carbonyl | Aminooxy PEG |
Carboxyl and Active Ester | Amino PEG |
Click Chemistry | Alkyne PEG PEG azide |
Copper Free Click Chemistry | BCN-PEG DBCO PEG TCO PEG Tetrazine PEG |
Thiol Reactive | Bromo PEG m-PEG |
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.
Aldehyde Conjugation
PEG aldehyde can be used in bioconjugation due to reactions between aminooxy or hydrazide moiety.
Figure 1: Reaction scheme of PEG aldehyde chemistry.Amine Conjugation
PEG Amines can react with acids, succinimidyl-active esters or pentaflurophenyl esters for labelling, chemical modification and surface or particle modification.
Figure 2: Reaction scheme of amino PEG chemistry.Thiol Conjugation
PEG Thiols are reactive with maleimides, disulfides, haloacteamides and other types of thiols. They can also be involved in metal surface binding.
Click Chemistry
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.
Figure 4: First generation Click Chemistry of a Cu (I) catalyzed reaction between an azide and alkyne.
Figure 5: Second generation Click Chemistry involves strain-promoted reactions between azides and alkynes with no Cu(I) catalyst.
Figure 6: Third generation Click Chemistry between tetrazine and an alkene to promote strain-induced reactions. This reaction occurs very quickly and works without a Cu(I) catalyst.References
1. Pasut, Gianfranco, Veronese, Francesco M., Drug Discovery 10, 21 (2005).
2. “Chemistry of Crosslinking” ThermoFischer Scientific.
3. Journal of Pharmaceutical Sciences 105 (2016) 460-475.
4. “Click Chemistry” Sigma-Aldrich, 6 July 2018.
