Reaction of thiols with acetylimidazole. Evidence for independent reaction pathways

Abstract

The reactions of acetylimidazole with weakly acidic thiols undergo a change in rate-determining step with increasing imidazole buffer concentration. The imidazole-catalyzed step is assigned to the breakdown of a tetrahedral addition intermediate and the other step to attack of thiol anion on free acetylimidazole. However, the concurrent, uncatalyzed reaction of acetylimidazolium ion with thiol anion does not undergo this change in ratedetermining step. The phosphate-catalyzed reaction also does not undergo a change in rate-determining step and phosphate can bring about a rate increase under conditions in which imidazole brings about no further rate increase. It is suggested that these results may be explained in terms of bimolecular reactions in aqueous solution that proceed by independent, concurrent pathways; i.e., the intermediates or transition states need not be at equilibrium with respect to transport processes.

Proton Equilibria in the Binding of Zn2+ and of Methylmercuric Iodide to Papain

“…The resulting values were 9.58, 9.56, and 9.59, respectively, in close correspondence to the value of pK 9.61 in 1.0 M KCI reported in the literature [11].…”

Mechanisms for enforced general acid catalysis of the addition of thiol anions to acetaldehyde

“…The change in Aiuc with increasing basicity of the nucleophile is described, within experimental error, by a coefficient py = d/3nuc/ -dpAfnuc = 0.089, as shown by the dashed line in Figure 13. This dashed line, for hydrogen bonding catalysis by water, is drawn according to log k'ha -2.75 + 0.64pAfRSH -0.3pATha + 0.026p£haPAÍrsh ~0.089pA'2rsh/2 (29) in which the coefficient of the last term is d/3nuc/-dp^RSH• The coefficient of the fourth term represents the Cordes coefficient, 1 /c2, and the coefficients of the other terms are defined by the intercepts of the Cordes plots in Figure 12. Equation 29 also predicts curvature of the lines for catalysis by other acids in Figure 13, but the data are not of sufficient accuracy to determine the presence or absence of such curvature.…”

General acid-base catalysis of complex reactions in water

“…= Off i -jo) (38) ni -ri2 = C3(pi -p2) (39) to substituent effects on the central carbon atom; they will not be described in detail here. In most cases it is clear from inspection of the appropriate diagram whether an unam-biguous substituent effect may be predicted and, if it is unambiguous, what its direction will be for the different components of the transition state.…”