4,4'-Dinitrodiphenyldisulfides of different charge type as probes for the electrostatic environment of sulfhydryl groups.
5,5'-Dithiobis-(2-nitro-N-trimethylbenzyl ammonium iodide) (I) and 5,5'-dithiobis-(2-nitro-N-2'-hydroxyethyl benzamide) (II) were synthesized as positively charged and neutral analogs of Ellman's reagent (5,5'-dithiobis-(2-nitrobenzoic acid) (III). Their reaction rates with a variety of thiols with different charge showed that sulfhydryl groups with no charge react about 25 times more rapidly with I than with III. A positive charge removed three single bonds from the sulfhydryl group decreases this ratio to about 3.5 to 1 while a negative charge within three single bonds increases it to 120 to 1. The reactivity of II was much higher than that of III but smaller than that of I. Comparison of the rates at 15 and 25 degrees C gave activation enthalpies (10.6-17.4 kcal/mol) that did not depend in a clear-cut way on the charge of the reacting species. Measurements at different salt concentrations showed an enhancement of the differences between I, II, and III at low ionic strength and a leveling effect of added salt.