Rates of thiol-disulfide interchange reactions between mono- and dithiols and Ellman's reagent

The rate constants for thiol-disulfide interchange between 21 mono- and dithiols and Ellmans reagent correlate with the pKus of the thiol groups with a Bronsted coefficient of p = 0.36. The maximum rates ofreduction are ob-served for thiols having pK, values close to the pH of the solution in which the reactions were carried out. In the di-lute solutions examined (10-4-10-6 M in each reagent), the rate of the second, intramolecular interchange step in reactions of dithiols was faster than that of the first, intermolecular interchange, regardless of the size of the cyclic disulfide formed. A convenient synthesis of a mixture of diastereomers of 1,4-dimercapto-2,3-butanediol (i.e., of a mixture of dithiothreitol, DTT, and dithioerythritol, DTE) has been developed from 1,2:,4-diepoxybutane and thiolacetic acid. Oxidation of cysteine sulfhydryl groups during isolation, storage, and use of proteins is often an important contributor to their deactivation.2 Although the rate of oxidation can be decreased by limiting access of oxygen to the enzyme, it is usually impractical to exclude oxygen completely, particularly in practical synthetic and analytical applications. The most effective and widely used reagents for protecting the cysteine