Selective Recognition of Protein Tetraserine Motifs with a Cell-Permeable, Pro-fluorescent Bis-boronic Acid

There is considerable interest in novel biomolecule imaging tools that avoid the use of fluorescent proteins.1-3 One widely used class of such reagents are “pro-fluorescent ” biarsenical dyes such as FlAsH,4 ReAsH,5 CrAsH,6 and Cy3As.7 These cell-permeable molecules selectively label recombinant proteins containing a linear4 or split8 tetracysteine motif via thiol-arsenic exchange reactions that convert the nonfluorescent 1,2-ethanedithiol (EDT)-bound forms of these dyes into highly fluorescent protein-bound com-plexes. Despite their utility, however, biarsenicals are plagued by high background signals and cytotoxicity9,10 and can be challenging to apply in oxidizing cellular locales.11,12 Nontoxic, redox-insensitive alternatives that combine the convenience and selectivity of a biarsenical with the brightness of a fluorescent protein would be a valuable addition to the cell biologist’s “fluorescent toolbox”.2 Here we report that [(3-oxospiro[isobenzofuran-1(3H),9′-[9H]xan