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Thiol Reactive Crosslinkers for Bioconjugation

Thiol Reactive Crosslinkers for Bioconjugation

Thiol reactive crosslinkers are one of the most common classes of crosslinkers in bioconjugation (1). The popularity of conjugation to a thiol is due in part to its presence in many proteins, but they are not as prevalent as amines, which are another site for conjugation. This will allow for greater control of the conjugation. Even greater control of the conjugation process is afforded if a thiol reactive compound is combined with an amine reactive compound to create a heterobifunctional crosslinker.


Thiol reactive crosslinkers from Quanta BioDesign, Ltd.


1. Maleimide crosslinkers

Quanta BioDesign offers a variety of homo- and heterobifunctional thiol reactive crosslinkers for bioconjugation with dPEG® spacers of several different lengths. Among those we offer are maleimide crosslinkers. Near neutral pH, the double bond of the maleimide reacts preferentially and very rapidly with a thiol to form a thioether bond that is not susceptible to reduction (2). Quanta BioDesign offers homobifunctional bis-maleimides as well as a few heterobifunctional maleimide bioconjugation crosslinkers with the other end being an amine reactive active ester. One of our most popular products is Mal-dPEG®4-NHS ester, product number 10214 (shown in Figure 1). It contains a 22 atom (24.8 Å) tetraethylene glycol spacer functionalized on one end with a thiol reactive maleimidopropyl group and on the other end with an amine reactive propionic acid-N-hydroxysuccinimide (NHS) ester.


Thiol reactive PN10214, MAL-dPEG®4-NHS ester
Figure 1: Thiol reactive crosslinker PN10214, one of Quanta BioDesign’s most popular PEGylation reagents, has a thiol reactive maleimide group on one end and an amine reactive NHS ester on the other end of a tetraethylene glycol linker.


Quanta BioDesign also offers this product with dPEG®2, dPEG®6, dPEG®8, dPEG®12, dPEG®24, and longer dPEG® linkers. You can view them all on our website. They are listed below in this post.  Another version of this product is PN10551, where the NHS ester is replaced by the 2,3,5,6-tetrafluorophenyl (TFP) ester. In-house research by Quanta BioDesign, Ltd., demonstrates that the TFP ester is much less susceptible than the NHS ester to hydrolysis.



2. Pyridyl disulfide crosslinkers

Quanta BioDesign also offers pyridyl disulfide (SPDP) bioconjugation crosslinkers (see also here), and with these, the thiol reacts with the SPDP moiety to produce a new disulfide bond, as illustrated in Reaction 1. Pyridine-2-thione is generated, but it cannot react with any remaining SPDP crosslinker because it does not contain a thiol (3, 4). If desired, the newly-formed disulfide bond can be cleaved with a reducing agent. It can also be oxidized back to the disulfide bond, which provides a flexibility not available with the maleimide crosslinkers. Like the maleimides, the SPDP crosslinkers are also offered as the NHS and TFP esters.



The thiol reactive pyridyl disulfide (SPDP) group is used in bioconjugation.
Reaction Scheme for the thiol reactive pyridyl disulfide (SPDP) group in bioconjugation


Thiol reactive crosslinkers are available now from Quanta BioDesign

Both the maleimide and SPDP crosslinkers with dPEG® are available from Quanta BioDesign with a variety of PEG spacer lengths, ranging from four to twenty-four ethylene oxide units (and in some cases, even longer) Whatever the length and functionality you need for your thiol reactive crosslinking PEGylation reagent, Quanta BioDesign can provide it for you. If you do not see what you want in our catalog, contact us about a custom synthesis. We can provide you with what you are looking for.




1.  Hermanson, Greg T. Bioconjugate Techniques, 3rd Edition. Waltham, MA: Elsevier (Academic Press), copyright 2013, 1146 pages. (A copy of the 2nd edition of Greg’s phenomenal work is available from Quanta BioDesign, Ltd., for $75 plus shipping, or for free with any order of $500 or more, excluding tax and shipping. Look here for more details.)

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2.  Smyth, Derek G., Blumenfeld, O. O., and Konigsberg, W. Reactions of N-ethylmaleimide with peptides and amino acids. Biochem J. (1964), 91, 589-595.

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3.  Carlsson, J., Håkan, D., and Axén, R. Protein thiolation and reversible protein-protein conjugation. Biochem J. (1978), 173, 723-737.

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4.  Myers, D. A., Murdoch, W. J., and Villemez, C. L. Protein-peptide conjugation by a two-phase reaction, Biochem J. (1985), 227(1), 343.

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Additional Products from Quanta BioDesign, Ltd.


MAL-dPEG®2-NHS ester MAL-dPEG®4-NHS ester
MAL-dPEG®6-NHS ester MAL-dPEG®8-NHS ester
MAL-dPEG®12-NHS ester MAL-dPEG®24-NHS ester


Please call or email us and ask about our MAL-dPEG®x-TFP ester derivatives. If you want a longer length dPEG® spacer than you see here, please call or email us about that also. We will be glad to discuss them with you!

Dan Dawson, M.S. received his BS in Chemistry from the University of Indianapolis in 2006, and his M.S. in Organic Chemistry from the University of Michigan in 2008. Dan is a Process Development Chemist involved in process development and scale-up activities. You can connect with Dan on LinkedIn at

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