Rapid, stable, chemoselective labeling of thiols with Julia-Kocieński-like reagents: a serum-stable alternative to maleimide-based protein conjugation.

Cysteine-maleimide chemistry is widely used for peptide and protein modification. However, the formed succinimide linkage is readily hydrolyzed and is susceptible to an exchange reaction in vivo. We demonstrate that methylsulfonyl phenyloxadiazole compounds react specifically with cysteine under various buffer conditions and found that the resulting protein conjugates had superior stability to cysteine-maleimide conjugates in human plasma. This Thiol-Click chemistry promises a new approach to stable protein conjugates and pegylated proteins.