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 x 10(4) M-1. Rapid exchange of thiols at the arsenic atom was demonstrated by H-1-NMR.