Resonance thioesters

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

The enhanced reactivity of thioesters results from two major differences. First, the resonance stabilization of a thioester is less than that of an ester. In the thioester, the second resonance form involves overlap between a 2p orbital on carbon and a 3p orbital on sulfur (Figure 21-12). These orbitals are different sizes and are located at different distances from the nuclei. The overlap is weak and relatively ineffective, leaving the C — S bond of a thioester weaker than the C—O bond of an ester. 

The resonance overlap in a thioester is not as effective as that in an ester. 

The utility of thiol esters and carbonates as protecting groups is limited by their vulnerability to hydrolysis, The poor overlap between the non-bonded electrons on the sulfur atom (3p) and the n-system of the carbonyl (2p) precludes or diminishes resonance stabilisation of the type enjoyed by normal esters thereby raising their ground state energy, The carbonyl group of the thioester is more electrophilic than a normal ester and hence more reactive. Thiocarbonate derivatives are marginally more stable. The 5-benzoyl derivative of cysteine is 95% hydrolysed in 30 minutes with 2 M ammonia whereas the 5-benzyloxycarbonyl derivative is only 20% hydrolysed in 30 minutes under the same conditions.54... 

The hydrolysis of a thioester is thermodynamically more favorable than that of an oxygen ester because the electrons of the C=0 bond cannot form resonance structures with the C—S bond that are as stable as those that they can form with the C—O bond. Consequently, acetyl CoA has a high acetyl potential (acetyl group-Pansfer potential) because Pansfer of the acetyl group is exergonic. Acetyl CoA carries an activated acetyl group, just as ATP carries an activated phosphoryl group. 

Mechanism I A drawing of another resonance form of 3-9-1, 3-9-2, shows that the oxygen of the thioester has negative character and could act as an intramolecular nucleophile ... 

On the other hand, infra red spectroscopic studies indicate that this resonance stabilization does not occur with thioesters. 

Oxygen esters are stabilized by resonance structures not available to thioesters. 

Realistically, we did not have the option of incorporating a second thio substituent to enhance the acidity of the substrate. Rather, we expected the twin effects of the thioester s electronic influence and the resonance stabilization of the Michael/ retro-Michael tandem to enable the formation of the enolate under mildly basic conditions. The electronic effect of the thioester was supported by a calculation, indicating that the pKa of the a-proton of the thioester would lower (by 1.5 p/C, units) in comparison with 9 [30]. It was not clear if this would be enough to permit racemization under reaction conditions that would support sufficient enzyme activity but relatively little work would be required to test the hypothesis. 

The energy-rich nature of thioesters, as compared with ordinary esters, is related primarily to resonance stabilization (Figure 14.9). Most esters can resonate between two forms (Figure 14,9). Stabilization involves Pi-electron overlap, giving partial double-bond character to the C-0 link. In thioesters, the larger atomic size of S (as compared with O) reduces the Pi-electron overlap between C and S, so that the C-S structure does not contribute significantly to resonance stabilization. Thus, the thioester is destabilized relative to an ester, so that its iG of hydrolysis is increased. 

The first enzyme in the system catalyzes the reaction of TPP with pyruvate to form the same resonance-stabilized carbanion formed by pyruvate decarboxylase and by the enzyme in Problems 8 and 9. The second enzyme of the system (E2) requires lipoate, a coenzyme that becomes attached to its enzyme by forming an amide with the amino group of a lysine side chain. The disulfide linkage of lipoate is cleaved when it undergoes nucleophilic attack by the carbanion. In the next step, the TPP carbanion is eliminated from the tetrahedral intermediate. Coenzyme A (CoASH) reacts with the thioester in a transthioesterification reaction (one thioester is converted into another), substituting coenzyme A for dihydrolipoate. At this point, the final reaction product (acetyl-CoA) has been formed. However, before another catalytic cycle can occur, dihydrolipoate must be oxidized back to lipoate. This is done by the third enzyme (E3), an FAD-requiring enzyme (Section 25.3). Oxidation... 

This step is a Claisen-type condensation with CH3CSC6A being attacked. Malonyl coenzyme A is a B-keto thioester, therefore the a-hydrogen is acidic, with the resultant anion being highly stabilized by resonance ... 

Thioesters are high-energy compounds. The possible dissociation of the products after hydrolysis and resonance structures of the products facilitate reaction. 

Since the NMR resonances of the carbonyl of thioesters are shifted (A 20 to 30 ppm), NMR spectroscopy should allow the direct monitoring of the formation and decay of a thioacyl intermediate. Using = O] N-benzoylimidazole (5 = 168.7 ppm), we were able to observe directly a thioacyl intermediate at 8 = 195.9 ppm in the presence of papain under the cryoenzymological conditions of -t C in 25 percent aqueous dimethyl sulfoxide. Moreover, the thioacyl species is clearly a productive intermediate since the decrease in its signal intensity was accompanied by an increase in the product resonances and by release of free enzyme (half-life, 96 minutes) determined by titration of its thiol group. The line width of the resonance at 195.9 ppm was 25 Hz [5,6]. 

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