Use of fluorescein mercuric acetate as a probe in studies of thiol-containing proteins.

The reaction of fluorescein mercuric acetate (FMA) with thiol groups is inhibited by barbital (Veronal). However, barbital has no effect on the reaction of 5,5'-dithiobis(2-nitrobenzoic acid) with thiol groups. Complex formation between FMA and barbital was shown by an increase in the fluorescence polarization of FMA in the presence of barbital. These findings indicate that the inhibitory effect of barbital is due to its interaction with FMA. The extent of this inhibition, which depends on the nature of the thiol-containing substance, presumably reflects the structure surrounding the thiol groups. Furthermore, the absorption and fluorescence properties of FMA conjugated with thiol groups are sensitive to the environment. Because of these properties, FMA can be used to investigate the environment of thiol groups.