MAL-dPEG®12-NHS ester, product number 10284, is one of our top crosslinking reagents. This product joins a sulfhydryl to a free amine through a hydrophilic bridge. The sulfhydryl groups react with a maleimide group via a Michael addition reaction. The amines form amide bonds with the crosslinker by nucleophilic substitution of the N-hydroxysuccinimidyl (NHS) ester of a carboxylic acid group. The maleimide and NHS functional groups on the crosslinking compound sit at either end of a long, discrete-length polyethylene glycol chain (dPEG®).
Among the most popular, most useful crosslinking reactions in bioconjugate chemistry are reactions that join free amines with free thiols. These reactions require heterobifunctional reagents that bridge the two groups. Traditional crosslinkers are hydrophobic molecules. Quanta BioDesign’s dPEG® crosslinking products are water-soluble, amphiphilic, single molecular weight PEG compounds with discrete chain lengths.
The conjugation of conventional hydrophobic crosslinking reagents to biomolecules almost inevitably triggers problems such as aggregation and precipitation of the conjugates. These problems do not occur with our water-soluble,non-immunogenic dPEG® crosslinkers. For more information about our dPEG® products, please see our “What is dPEG®?” page. Also, click here for answers to our most frequently asked questions.
How to Use MAL-dPEG®12-NHS ester
Because NHS esters hydrolyze readily in water or aqueous buffer, the NHS ester end of the molecule must conjugate to a target molecule before conjugating the maleimide end of the molecule. At pH 7.0 – 7.5, NHS esters react optimally with free amines. However, NHS esters can react with free amines with pH as low as 6.0. As the pH increases, the hydrolysis rate of the NHS ester increases. Indeed, at pH 7 and 0°C, the NHS ester half-life is four (4) to five (5) hours in aqueous buffer, while at pH 8 and 25°C, the half-life of an NHS ester is one hour. Thus, we strongly discourage storing MAL-dPEG®12-NHS ester, product number 10284, in water or aqueous buffer. Instead, we recommend that customers make new solutions of the product as needed, use them immediately, and discard unused solutions after use.
The reaction of the maleimide end of MAL-dPEG®12-NHS ester, product number 10284, with a sulfhydryl proceeds best at pH 6.5 – 7.5. Conduct the conjugation at the lowest reasonable pH within this range. Above pH 7.5, free amines compete with free thiols at the maleimide reaction site, which can cause confusing results. Moreover, at higher pH values, the maleimide ring may open to form unreactive maleamic acid. For details about maleimide-thiol reaction chemistry, please click here.
Uses of MAL-dPEG®12-NHS ester
The use of MAL-dPEG®12-NHS ester, product number 10284, has been published in numerous scientific paper and patents. The following list highlights some of the more important uses of this product:
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 Hermanson, G. T. Chapter 6, Heterobifunctional Crosslinkers. In Bioconjugate Techniques, 3rd ed.; Academic Press: New York, NY, 2013; pp 299–340, specifically page 300. Many scientists engaged in bioconjugation work consider Greg Hermanson’s book to be the definitive reference on the subject. Click here now to read a review of Greg’s book and to purchase it.
 Hermanson, G. T. Chapter 18, PEGylation and Synthetic Polymer Modification. In Bioconjugate Techniques, 3rd ed.; Academic Press: New York, NY, 2013; pp 787–838, specifically page 794.
 Hermanson, G. T. Chapter 3, The Reactions of Bioconjugation. In Bioconjugate Techniques; Academic Press: New York, NY, 2013; pp 229–258, specifically page 234.
 Hermanson, G. T. Chapter 6, op. cit., page 304.
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