Arsinous acid as a thiol binding group: Potential cysteine peptide tagging functionality that binds a single thiol

A simple aryl arsinous acid (ArAs(CH₃)OH) was prepared by ortho mercuration of p-cresol followed by Pd-catalyzed reaction with methylarsenic dibromide, purification as the mercaptoethanol adduct, and deprotection using a silver salt. Isothermal titration calorimetry experiments showed reaction with mercaptoethanol with an association constant of 6 × 10⁴ M ^-1. Rapid exchange of thiols at the arsenic atom was demonstrated by ¹H-NMR.