Breakdown of the tetrahedral intermediate

Even alkyl benzoate esters give only a small amount of exchange imder basic hydrolysis conditions. This means that reversal of the hydroxide addition must be slow relative to the forward breakdown of the tetrahedral intermediate. ... 

Aminolysis of esters often reveals general base catalysis and, in particular, a contribution to the reaction rate fi om terms that are second-order in the amine. The general base is believed to function by deprotonating the zwitterionic tetrahedral intermediate. Deprotonation of the nitrogen facilitates breakdown of the tetrahedral intermediate, since the increased electron density at nitrogen favors expulsion of an anion ... 

The principal difference hes in the poorer ability of amide ions to act as leaving groups, compared to alkoxides. As a result, protonation at nitrogen is required for breakdown of the tetrahedral intermediate. Also, exchange between the carbonyl oxygen and water is extensive because reversal of the tetrahedral intermediate to reactants is faster than its decomposition to products. 

In some amide hydrolyses, the breakdown of the tetrahedral intermediate in the forward direction may require formation of a dianion ... 

Another possibility is that breakdown of the tetrahedral intermediate is rate determining, the transition state being very early, as in 2. 

Equations (7-66) and (7-67), or related versions, have been used by Hupes and Jencks and by Castro and co-workers to account for curvature. The quantity p/(S defines the center of eurvature of the plot and is expected to occur when the Y>Ka of the nucleophile is equal to the p/( of the leaving group." For weaker nucleophiles pKa < p/(S), breakdown of the tetrahedral intermediate will be rate determining, because the leaving group X is a stronger nucleophile than is N , so 2 < -li if, however, p/( > p/(S, the nucleophilic attack is rate determining. 

Figure 7-6. Mechanism for catalysis by an aspartic protease such as HIV protease. Curved arrows Indicate directions of electron movement. Aspartate X acts as a base to activate a water molecule by abstracting a proton. The activated water molecule attacks the peptide bond, forming a transient tetrahedral Intermediate. Aspartate Y acts as an acid to facilitate breakdown of the tetrahedral intermediate and release of the split products by donating a proton to the newly formed amino group. Subsequent shuttling of the proton on Asp X to Asp Y restores the protease to its initial state.

Few relevant data are available. Both equilibrium and rate constants have been measured for very few reaction series in solution, but comparisons are possible for lactone and thiolactone formation, and for a few anhydrideforming reactions (Tables 4 and 5). For lactone formation (Table 4) the EM for the rate process is of the same order of magnitude as that derived from the equilibrium constant data, and in some cases actually exceeds it (though only in one case by an amount clearly greater than the estimated uncertainty which is nominally a factor of 4 for these ratios). Lactonization generally involves rate-limiting breakdown of the tetrahedral intermediate, and the transition state is expected to be late and thus close in structure to the conjugate acid of the lactone. 

Fig. 3.5. Two mechanisms of breakdown of the tetrahedral intermediate, i.e., elimination of the amino group (Pathway a) or of the serine OH group (Pathway b)... 

In ester hydrolysis, rate-limiting formation of the tetrahedral intermediate usually apphes (Sec. 6.3.1) since the alkoxide group is easily expelled. In contrast, amide hydrolysis at neutral pH involves rate-limiting breakdown of the tetrtihedral intermediate, because RNH is a poor leaving group. The catalytic effect of metal ions on amide hydrolysis has been ascribed to accelerated breakdown of the tetrahedral intermediate. 

However, in these reactions it is likely that breakdown of the tetrahedral intermediate is at least partly rate determining so the general acid catalysis is probably associated with this step rather than with attack of the nucleophile on the carbonyl group. The only example of general acid catalysis of ester hydrolysis where the rate limiting step is probably nucleophilic attack on the carbonyl group appears to be the hydrolysis of tetra-O-methyl-D-glucono-... 

Transition state analogues are essentially stable molecules which resemble, in geometry and in charge distribution, metastable intermediates of the enzymic reaction. The actual transition state of the reaction will be close in structure to the metastable intermediate, and will quite likely vary slightly between different substrates accepted by the same enzyme. There will not be a unique transition state for all transformations catalysed by one particular enzyme, neither of course will there be a unique transition state for different enzymes catalysing the hydrolysis of peptide links in a protein. There will nevertheless be some similarities in mechanism, and so structures containing a tetrahedral centre have been designed to inhibit a variety of proteinases, where a tetrahedral intermediate is always presumed. Differences exist in the pathway to, and breakdown of, the tetrahedral intermediate, and its stabilization, between thiol and serine proteinases, zinc proteinases, and aspartic proteinases. 

Base-Catalyzed Hydrolysis. Let us now look at the reaction of a carboxylic ester with OH", that is, the base-catalyzed hydrolysis. The reaction scheme for the most common reaction mechanism is given in Fig. 13.11. As indicated in reaction step 2, in contrast to the acid-catalyzed reaction (Fig. 13.10), the breakdown of the tetrahedral intermediate, I, may be kinetically important. Thus we write for the overall reaction rate ... 

The acid-catalyzed hydrolysis of orthoesters is very much faster than that of esters. The second-order rate coefficient for the hydrolysis of ethyl acetate is of the order of 10"4 1-mole-1-sec1 at 25°C, whereas that for the hydrolysis of ethyl orthoacetate103 is of the order of 104 l-mole-1-sec 1, and that for the breakdown of a monoalkyl orthoester must be faster still. If the breakdown of the tetrahedral intermediate is partially rate-determining in acid-catalyzed ester hydrolysis, therefore, its concentration must be very small that is, the equilibrium for its formation must be highly unfavourable. This... 

As the leaving group is varied from ethanol to phenol to o-nitrophenol. k3 will increase, relative to k2, and a neutral, water-catalyzed elimination of the leaving group may become important. This will result in the breakdown of the tetrahedral intermediate becoming relatively faster than its formation, and thus also in a decrease in the effectiveness of acid catalysis, since the formation of the tetrahedral intermediate is not significantly acid-catalyzed. 

Finally, different mechanisms must obtain in those cases where the breakdown of the tetrahedral intermediate is the slow step of the reaction. Two kinetically equivalent mechanisms are possible in this case also. Here, too, the catalyst may be involved as a general base, in which circumstances it would catalyze the elimination of the leaving group by the E-2 mechanism, viz-... 

For alkyl esters the reaction is close to symmetrical, and it is likely that the breakdown of the tetrahedral intermediate is partially rate-determining, and will therefore be general acid-catalyzed. For aryl esters breakdown to products will be faster, and the formation of the tetrahedral intermediate should determine the rate. This might account for the more favourable entropies of activation found by Moffat and Hunt199 for the hydrolysis of aryl trifluoroacetates if the single transition state (of the addition step) occurs early, the loss of degrees of freedom compared with the initial state will also be less complete. 

The step written as k3 is complex, but the final proton transfers are assuredly very rapid, so that only the breakdown of the tetrahedral intermediate is kinetically important. Using the steady-state assumption for the tetrahedral intermediate, the rate coefficient for the overall reaction can readily be shown to be... 

From the available data it is thus reasonable to conclude that only in exceptional cases will the breakdown of the tetrahedral intermediate be a... 

Fig. 3. Proposed reaction cycle for urease. For urea, R = —NH2. Step 1 urea is activated toward nucleophilic attack by O coordination to a nickel ion the =N+H2 is stabilized by interaction with a protein carboxylate. Step 2 nucleophilic attack by a hydroxide ion, coordinated to the second nickel, to form a tetrahedral intermediate. Step 3 breakdown of the tetrahedral intermediate to form a coordinated carbamate ion. Step 4 hydrolysis releases carbamate ion, the initial product of urease on urea. Reproduced, with permission, from Ref. 34.

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