Kooperative Imidazol-SH-Katalyse als enzymatische Modellreaktion

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Abstract
The concerted catalysis by imidazole and SH-groups was studied. The following compounds were synthesized: 4-mercaptomethylimidazole, 4-(2-mercaptoethyl) imidazole, cyclo-(cysteinyl-histidyl), together with their corresponding disulfides, S-benzyl ethers and S-acetyl compounds. The pK values of these compounds were determined by potentiometration. The values for the SH groups were also determined by photometric titration and compared with those of the corresponding imidazolyl alcohols. The problem of correctly allocating pH-values to ionizations in the molecules is discussed. The catalysis of the hydrolysis of p-nitrophenyl-acetate by the above compounds was compared with the enzymic hydrolysis. 4-Mercaptomethylimidazole and 4-(2-mercaptoethyl)imi-dazole show one quarter to one fifth of the esterase activity of chymo-trypsin with p-nitrophenylacetate as the substrate. The mechanism of the reaction is discussed. The rate of alkaline hydrolysis of the S-acetyl compounds was measured and compared with that of other thiol esters.