Synthesis and spectral characterization of sulfhydryl-reactive fluorescent probes

We synthesized two sulfhydryl-reactive fluorescent probes, Br-ANT (2-amino-benzoic acid, 2-(bromoacetyl)hydrazide) and Br-MANT (N-[2-[(bromoacetyl)amino]ethyl]-2-(methylamino)benzamide). Br-ANT and Br-MANT contain an anthraniloyl and N-methyl anthraniloyl group, respectively, linked to the sulfhydryl-reactive bromoacetyl moiety. The cysteine adducts have absorption maxima at 323 and 326 nm, with molar extinction coefficients of 2100 and 2900 M-1 cm-1, for Br-ANT and Br-MANT, respectively, making these probes excellent acceptors for tryptophan. The absorption spectra and quantum yields were constant at pH levels useful for protein studies (pH 5-8). Quantum yields of Br-ANT and Br-MANT were 0.16 and 0.42, emission maxima were 432 and 440 nm, and fluorescence lifetimes were 1.3 and 7.8 ns, respectively. The emission of Br-ANT-Cys and Br-MANT-Cys shifted to shorter wavelengths with decreasing solvent polarity. Polarization values were maximal between 330 and 375 nm. Both probes reacted selectively and stoichiometrically with the single cysteine residue of a model protein. The labeled protein exhibited relatively long lifetimes (9-10 ns), suggesting that these probes will be generally useful for rotational studies.