Protolytic catalysis

Schowen, R.L. (1988). Structural and energetic aspects of protolytic catalysis by enzymes charge-relay catalysis in the function of serine proteases. In Mechanistic Principles of Enzyme Activity, Liebman, J.P. and Greenberg, A. (eds), pp. 119-168. VCH Publishers, New York... 

Rate-limiting protolytic catalysis of this process is suggested by the observation of a solvent isotope effect of 1.6 on for peptide hydrolysis by stromelysin [71]. 

The synthesis of most often used glycol derivatives is carried out on long-term reflux with removal of the released aliphatic alcohol by distillation [20]. The resulting silicon glycolates are quite sensitive to moisture and are hydrolyzed by neutral water without any need of protolytic catalysis. 

General base and nucleophilic catalysis of ester hydrolysis and related reactions, 5, 237 H2O-D2O Mixtures, protolytic processes in, 7, 259... 

It can be shown that the mechanism expressed in equation (41) always appears as a first-order reaction catalyzed by bases (proton acceptors) in general, when the hydrogen ion concentration is so great that [RH] [ ]. If 1 takes up protons to form mainly SH, we measure the velocity of the protolytic reaction between RH and B, i.e., the velocity of dissociation of the very weak acid RH. If I takes up protons to form RH, the apparent basic catalysis is a disguised acid catalysis of the ion I. If neither of these possibilities is especially favored, we get an apparent basic catalysis which is the result of both a protolytic reaction between RH and the bases and between the acids and I. From the kinetic experiments, it is impossible to decide which of the three possible cases we have in a given reaction. 

These early views envisaged reactions which could take place in the absence of a catalyst, but which were facilitated by its presence. Evidence gradually accumulated to show that many of the reactions subject to acid-base catalysis could not take place at all in the complete absence of catalysts, apparently spontaneous reactions being often due to catalysis by acidic or basic solvent molecules, or by some adventitious acidic or basic impurity. This seemed to indicate that the catalyst took a fundamental part in the reaction, possibly in a chemical sense. It was soon realized that the essential property of acids and bases was their power respectively to lose and to add on a proton, and enquiry also showed that substrates involved in acid catalysis could always be supposed to have some basic properties, while those in base-catalyzed reactions could always in principle act as acids, though the acid-base properties of the substrates were often so weak as to elude detection by ordinary means. This led to the suggestion that acid-base catalysis always involves an acid-base reaction between the catalyst and the substrate. Such a reaction is also often termed a protolytic reaction, since it involves the transfer of a proton between the two reacting species. 

Acid-base catalysis appears to be an important factor in virtually all enzymatic reactions. The rates of protolytic reactions have been discussed... 

Salomaa P, Kankaanpena A, Lajunen M (1966) Protolytic cleavage of vinyl ethers. General acid catalysis, structural effects, and deuterium solvent isotope effects. Acta Chem Scand 20 1790-1801... 

Evidence for the protiocatalytic nature of the diacetoxylation of alkenes using PhI(OAc)2 as oxidant is presented. Cu(OTf)2, Pd(Otf)2 were used as catalyst and Pd + and Cu + ions interacted with the oxidant in the initiation phase of the catalytic transformation. However, 1 equiv. of triflic acid formed in the first cycle functioned as the active catalyst. On the basis of the observed proton catalysis, the infra- and inter-molecular triflic acid-catalysed dioxygenation for a range of alkene substrates is suggested. It is pointed out that Pd-catalysed reactions performed under basic conditions are not explicable by the suggested protolytic scheme. ... 

Optimization of reaction complex topology implies the positioning of charged enzyme groups such that there is an optimum interaction with (activated) reactants. Significant protonation (in protolytic enz5unes) or electron transfer (in redox catalysis) can already occiu in the adsorbed state. This is due to the unique electrostatic properties of the enzyme, which is shielded from the solvent by its hydrophobic peptide framework. 

This example, where W is a solvent molecule, corresponds to a protolytic transfer. When W is the conjugate base of the catalyst, W=A , the mechanism is called prototropic. Table 13.1 presents the four possible combinations for acid and base catalysis that correspond to the possible identities of species involved in the general catalytic mechanism. 

Polarographic data yield ki2 = 1.3 X lO W" sec, which agrees well with specific rates of similar reactions shown in Table II. The specific rate kn of the much slower dehydration reaction has been determined by both the temperature and pressure jump methods to be about 0.5 sec at pH 3 and 25 °C with some general acid-base catalysis. While the hydration-dehydration equilibrium itself involves no conductivity change, it is coupled to a protolytic reaction that does, and a pressure jump determination of 32 is therefore possible. In this particular case the measured relaxation time is about 1 sec. The pressure jump technique permits the measurement of chemical relaxation times in the range 50 sec to 50 tisec, and thus complements the temperature jump method on the long end of the relaxation time scale. 

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