The Fluorescein Arsenical Hairpin Binder (FlAsH) method is presented for fluorescence labeling of proteins in living cells. Labeling proteins with the membrane-permeant fluorogenic biarsenical dye fluorescein arsenical hairpin binder-ethanedithiol (FlAsH-EDT2). The membrane-permeant fluorogenic biarsenicals FlAsH-EDT2 and ReAsH-EDT2 can be prepared in good yields by a straightforward two-step procedure from the inexpensive precursor dyes fluorescein and resorufin.
