Nucleophile isotope effects ester hydrolysis

In a review of nucleophile isotope effects, results of studies of the hydrolysis of esters and amides by carboxypeptidase were discussed.10... 

Enzyme action, discovery of the mechanisms of, 1947-1963, 21, 1 Equilibrating systems, isotope effects on nmr spectra of, 23, 63 Equilibrium constants, N.M.R. measurements of, as a function of temperature, 3,187 Ester hydrolysis, general base and nucleophilic catalysis, 5, 237 Exchange reactions, hydrogen isotope, of organic compounds in liquid ammonia, 1, 156... 

The normal primary deuterium isotope effect in the first part shows that an OH bond is being broken in tile rate-detertnining step. Imidazole is loo weak a base to remove the OH proton completely so its role must be as general base catalyst. Attack on the carbonyl is the slow step with (aster breakdown of the tetrahedral intermediate arid hydrolysis of the lactone. Lactones arc hydroly,sed faster than esters because they lack anomeric stabilization (p. 1134). [ he role ttf the OH group is intramolecular nucleophilic catalyst. 

Solvolyses of Phosphoric Acid Derivatives.—The solvolysis of organic phosphates has been reviewed. A significant 0 isotope effect was observed in the solvolysis of the dianion of 2,4-dinitrophenyl phosphate, and since no such isotope effect is observed in the alkaline solvolysis of the dibenzyl ester this has been adduced as evidence for a monomeric metaphosphate elimination in the former case. The nucleophilic attack of hydroxide ion on bis-(2,4-dinitrophenyl) phosphate is inhibited by micelles of non-anionic detergents and this is attributed to binding of the substrate. Hydrolysis of 3,4-dimethoxyphenyl phosphate proceeds by way of the monoanion, the neutral molecule, and the conjugate acid, and is thus in accord with earlier results on other methoxyphenyl phosphates. ... 

In the first one, the termolecular problem (i.e. the fact that in GAC and GBC three molecules have to come together in the transition state) is avoided by making a reaction intramolecular. Normally, ester formation and hydrolysis are specrfic-acid-catalysed only, but here there is catalysis by a weak acid acetic acid. A normal kinetic isotope effect k(HOAc)/k(DOAc) = 2.3 shows that proton transfer occurs in the rate-determining step and there is a large negative AS = -156 J mol" K". This is GAC of nucleophilic attack on a carbonyl group, admittedly in a rather special molecule. 

The role of the metal ion in ester hydrolysis catalysed by CPA has been examined with both Zn +- and Co +-substituted enzymes. When the terminal carboxyl of the substrate is electrostatically linked to argenine-145 and the aromatic side-chain lies in a hydrophobic pocket, the only residues close enough to the substrate to enter catalysis are glutamate-270, tyrosine-248, the metal ion, and its associated water. Low-temperature studies aid the elucidation of the mechanism. Between - 25 and - 45 °C in ethylene glycol-water mixtures two kinetically discrete processes are detected, the slower of which corresponds to the catalytic rate constant. The faster reaction is interpreted as deacylation of a mixed anhydride acyl-enzyme intermediate formed by nucleophilic attack by glutamate-270 on the substrate (Scheme 6). Differences in the acidity dependences of the catalytic rate constant with the metal ions Zn + (p STa 6.1) and Co +-(pATa 4.9) suggest that ionization of the metal-bound water molecule occurs and is involved in the decay of the anhydride. The catalytic rate constant shows an isotope effect in DgO. 

As formaldehyde hydrazones are particularly stable towards hydrolysis or polymerization, they can be used in water, a solvent that enhances the reactivity of the azomethine carbon for nucleophilic addition to a-keto esters, to give highly functionalized tertiary alcohols. A significant solvent isotope effect points to the active participation of water as an acid catalyst. ... 

Brensted p for bases whose conjugate acids have > 7 is 0.95 and represents nucleophilic catalysed hydrolysis (Proctor and Page, 1979). Evidence for an intermediate ester formed during the reaction has been obtained with alkoxide ions and phosphate dianion (Proctor and Page, 1979 Bundgaard and Hansen, 1981). Weakly basic catalysts probably act as general base catalysts. The reaction with acetate exhibits a solvent isotope effect of 2.1, as expected for general base catalysis but not for a... 

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