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Secrets of an important byproduct revealed
Released By Bachem
March 4, 2022
At Bachem, technological leadership and innovative strength have been the cornerstones of our success since the very beginning of our company. We strive for technology and highest quality of our products. Investing in research in order to constantly improve our manufacturing process is one of our driving forces. In this context, we have studied and provided further optimized tools for one of the most widely used strategies for the covalent modification of peptides and proteins, the thiol-maleimide reaction. In this chemical modification reaction, however, side-reactions occur and are challenging to address. Thiazine formation during the conjugation of N-terminal cysteine peptides to maleimides is one of them, and has been revealed to be underreported in peptide literature. Thiazine occurs from a side-reaction with an unprotected N-terminal cysteine The thiol-maleimide reaction is used to add chemical labels onto biomolecules via thiol conjugation such as fluorescent dyes, PEG, radiolabels and small molecules. The advantages of this reaction are the rapid reaction speed between maleimides and thiols and the preferential chemical reaction at a neutral pH. However, there are also disadvantages like the various side reactions emerging, such as thiazine rearrangement. These side-reactions are possibly explained by the instability of the joined maleimide-cysteine. We have published a new study in the Journal of Peptide Science about thiazine rearrangement. Formation of the thiazine impurity (Scheme 1) can complicate the purification, characterization and storage of peptide conjugates leading to loss of product. This is why it is so important to investigate the mechanism. This study aimed to better understand this side-reaction and to develop tools that will prevent its formation. The thiazine side-reaction occurs during the conjugation of a maleimide with an unprotected N-terminal cysteine amino acid and leads to a thiazine derivative. For peptides that are conjugated to maleimides through an N-terminal cysteine, the resulting succinimide is susceptible to nucleophilic attack from the N-terminal amine of the cysteine. This nucleophilic attack can occur at the carbonyl at Position 1 or the carbonyl at Position 3 (Scheme 2). Transcyclization occurs through a fused bicyclic tetrahedral intermediate, which allows for the formation of a six-membered thiazine product.
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