Maleimide-activated compounds readily react with thiol groups to form thioester-conjugated products. The reaction can take place at pH 7.0 in the presence of amines. Under neutral pH conditions, the maleimide group does not react with histidine or methionine.
Maleimide reactions are specific for thiols in the pH range of 6.5 to 7.5. At pH 7.0, the reaction of the maleimide with sulhydryls proceeds at a rate 1000 times greater than its reaction with amines. For proteins, the conjugation reaction is usually carried out in phosphate-buffered saline (PBS) at pH 7.0 to 7.4. At higher pH values, > 7.5 , some cross-reactivity with amino groups can take place (Brewer & Riehm 1967, Hermanson 2008).
Figure 1: The general reaction scheme of maleimides with thiol group used for bioconjugation and labeling of biomolecules. Also read Maleimide labeling of thiolated biomolecules.
Shipping and Storing
Shipping chemical products activated with maleimide is done as a dry lyophilized powder.
Please note: BSI recommends storing dry aliquots as a freeze-dried powder at -20 or (better) -80°C in the presence of a desiccant. The compounds should remain active for up to 6 months when stored as directed.
Reconstitution
We recommend reconstituting molecules activated with maleimide just before conjugation.
For reconstitution: Dissolve maleimide-modified oligos in a neutral or slightly acidic buffer (pH 5.5 to 7.0). DO NOT use basic buffers, and DO NOT use buffers that contain chemicals with amino or thiol groups (e.g., Tris, DTT).
The presence of water will slowly deactivate the maleimide group. Hence, we recommend not storing maleimide-activated molecules such as maleimide-siRNA in buffers or water.
However, siRNA not conjugated to maleimide is stable in buffers and water and can be stored as solution aliquots, preferably at -20 or -80 °C.
Reference
Brewer, C.F. and Riehmn, J.P. (1967). Evidence for possible nonspecific reactions between N-ethymaleimide and proteins. Anal Biochem 18:248-55.
Hermanson, G.T., Bioconjugate Techniques. 2nd Edition, Elsevier 2008
Maleimide-labeling-of-thiolated-biomolecules
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