Ethynyl-1,2,3-triazolyl-phosphinate based electrophiles for cysteine labelling

Herein we report the density functional theory (DFT) guided discovery of ethynyl-triazolyl-phosphinates (PT) as a new class of electrophilic warhead for cysteine selective bioconjugation. Using DFT-calculations, we found that both the electron withdrawing character as well as the π-acidity of the P-bound triazole significantly contribute to its enhanced reactivity towards thiols. We developed a straightforward synthetic route starting from readily available organic azides and diethynyl phosphinates using CuI-catalysed azide alkyne cycloaddition in aqueous buffer to access a variety of functional electrophiles. These reagents were used to obtain fluorescent peptide-conjugates for receptor labelling on live cells and in the generation of stable and biologically active antibody-drug-conjugates. Moreover, we were able to incorporate PT-electrophiles into azide-containing proteins under native conditions and demonstrate their potential in protein-protein conjugation. Finally, we showcase the excellent cysteine selectivity of this new class of electrophile in mass spectrometry based, proteome-wide cysteine profiling.