N -Vinyl Acrylamides: Versatile Heterobifunctional Electrophiles for Thiol-Thiol Bioconjugations.

Herein we report the first examples of thiol-selective heterobifunctional electrophiles, N -vinyl acrylamides, that enable efficient highly selective thiol-thiol bioconjugations and cysteine modification of peptides. We demonstrate that these new classes of thiol-selective scaffolds can readily undergo a thia-Michael addition and an orthogonal radical induced thiol-ene "click" reaction under biocompatible conditions. Furthermore, the formation of an unexpected Markovnikov N , S -acetal hydrothiolation was explained using computational studies. We also reveal that N -methylation of the N -vinyl acrylamide scaffold changes the regioselectivity of the reaction. We demonstrate that use of N -vinyl acrylamides shows promise as an efficient, mild, and exquisite cysteine-selective protocol for facile construction of fluorophore-labeled peptides and proteins and that the resultant conjugates are resistant to degradation and thiol exchange, thus significantly improving their biophysical properties.