Helmut Wenck

Kinetik der Reaktion von Imidazol-SH-Verbindungen mit Ellman Reagenz [5,5′-Dithio-bis-(-2-nitrobenzoesäure)]

The rate of reaction of some derivatives and peptides of cysteine and a number of imidazole-SH-compounds with 5,5′-Dithio-bis-(2-nitrobenzoic acid) (DTNB) as well as the dependence of the reaction rate upon pH were investigated; for several compounds an equation is given, by which the rate constants and half reaction times can be estimated for various pH values. For some of the imidazole-SH-compounds studied thepK of the SH-group is no criterion of their reactivity with DTNB.

Kinetik der Reaktion von 2.4-Dinitrofluorbenzol mit Imidazol-, SH- und Imidazol-SH-Verbindungen / Kinetics of the Reaction of 2,4-dinitro-1-fluorobenzene with Imidazole-, SH- and Imidazole-SH-compounds

First order rate constants of the reaction of a series of SH-, imidazole- and imidazole/SH-compounds with FDNB as well as their pH- and temperature dependence were determined. Some of the tested imidazole/SH-compounds exhibit a higher nucleophilic reactivity as is expected on the basis of their pKSH-values. This enhanced reactivity is caused by an activation of the SH-groups by a neighbouring imidazole residue. The pH-independent rate constants were calculated using the Lindley equation. The kinetics of DNP-transfer from DNP-imidazole to SH-compounds were investigated. The pH-dependence of the reaction displays a maximum curve. Donor in this reaction is the DNP-imidazolecation and acceptor the thiolate anion. The reaction rate of FDNB with imidazole derivatives is two to three orders of magnitude slower than with SH-compounds. No inter- or intra-molecular transfer of the DNP-residue from sulfure to imidazole takes place.