Fig. 1. (A) Design of reagents for perfluoroarene macrocyclization by cross-linking two Cys-residues in a protein in aqueous conditions. (B) Mechanism of S_NAr reaction. (C) Rate constants (k) of the reaction between substituted fAr and βME, as measured by ¹⁹F NMR, increase with the increasing electronegativity of R group. As a measure of electronegativity, we used pK_a in a series of phenylketones for which many values are known23 or can be calculated (ESI Fig. S4†). (D) To find the S_NAr reagents for rapid modification in water, we plotted molar concentration of fAr necessary to reach 50% conversion in 30 minutes (calculated as ln(2)/(1800 × k), where k is the rate constant) and % of organic co-solvent necessary to dissolve the fAr at this concentration. Most fAr are poorly reactive and/or too insoluble in aqueous solvents; only DFS and perfluoropyridine (fPy) can be used in conditions that require low amount of organic co-solvent.