Fast Protein Modification in the Nanomolar Concentration Range Using an Oxalyl Amide as Latent Thioester**

We show that latent oxalyl thioester surrogates are a powerful means to modify peptides and proteins in highly dilute conditions purified aqueous media or mixtures as complex cell lysates. Designed be shelf-stable reagents, they can activated on demand enable ligation reactions withpeptide concentrations low few hundred nM at rates approaching 30 M-1 s-1.