5-(Alkynyl)dibenzothiophenium triflates are introduced as new reagents to prepare different protein conjugates through site-selective cysteine alkynylation. The protocol developed allows a highly efficient label of free cysteine-containing proteins with relevant biological roles, such as ubiquitin, the C2A domain of Synaptotagmin-I, or HER2 targeting nanobodies. An electrophilic bis-alkynylating reagent was also designed. The second alkynylating handle thus introduced in the desired protein enables access to protein-thiol, protein-peptide, and protein-protein conjugates, and even diubiquitin dimers can be prepared through this approach. The low excess of reagent needed, mild reaction conditions used, short reaction times, and stability of the S-C(alkyne) bonds at physiological conditions make this approach an interesting addition to the toolbox of classical, site-selective cysteine-conjugation methods
