PMSF (phenyl-methyl-sulfonyl-fluoride) is used as a serine protease inhibitor because it derivatizes the catalytic serine in the active site. However it also reacts with water or (I believe at a slower rate) with ethanol, so that solutions are not stable. To test such a solution it would be nice to add a certain volume to a reagent with which it reacts rapidly and gives a color change, analogous to hydrolysis of nitro-phenyl phosphate as an assay for phosphatase, or reaction of carbonyls with phenylhydrazine. My organic qual textbook has gone missing, and anyway I don't think it lists derivatives for fluorosulfonates, but maybe some chemist could suggest a reaction to suit. Obvious answers that need not be posted are, "set up a protease assay and test inhibition" and "the stuff is cheap- just make fresh solution every time".
My thought is if it can react with any thiol containing chemical, you can do the reaction, and then just add Ellmans reagent to quantify the free sulfhydryl that are left. It is not uncommon for cysteine protease to react with PMSF. Since the fluoro group could theoretically be displaced by a free sulfhydryl group, I would look into that.
That's a good idea. Suggests two things:
Maybe the reaction with a thiol like 2-mercaptoethanol could itself be followed in the deep UV.
Since Ellman's works by liberating a thiol which ionizes and produces the color, if that thiol anion reacts with PMSF it might remove the color. So you would just add an aliquot of the questionable PMSF solution to a solution of reduced Ellman's reagent and see a decrease in absorbance.
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