Transition State Analysis of Acid-Catalyzed Hydrolysis of an Enol Ether, Enolpyruvylshikimate 3-Phosphate (EPSP)

Abstract

Proton transfer to carbon represents a significant catalytic challenge because of the large intrinsic energetic barrier and the frequently unfavorable thermodynamics. Multiple kinetic isotope effects (KIEs) were measured for acid-catalyzed hydrolysis of the enol ether functionality of enolpyruvylshikimate 3-phosphate (EPSP) as a nonenzymatic analog of the EPSP synthase (AroA) reaction. The large solvent deuterium KIE demonstrated that protonating C3 was the rate-limiting step, and the lack of solvent hydron exchange into EPSP demonstrated that protonation was irreversible. The reaction mechanism was stepwise, with C3, the methylene carbon, being protonated to form a discrete oxacarbenium ion intermediate before water attack at the cationic center, that is, an AH(‡)*AN (or AH(‡) + AN) mechanism. The calculated 3-(14)C and 3,3-(2)H2 KIEs varied as a function of the extent of proton transfer at the transition state, as reflected in the C3-H(+) bond order, nC3-H+. The calculated 3-(14)C KIE was a function primarily of C3 coupling with the movement of the transferring proton, as reflected in the reaction coordinate contribution ((light)ν(‡)/(heavy)ν(‡)), rather than of changes in bonding. Coupling was strongest in early and late transition states, where the reaction coordinate frequency was lower. The other calculated (14)C and (18)O KIEs were more sensitive to interactions with counterions and solvation in the model structures than nC3-H+. The KIEs revealed a moderately late transition state with significant oxacarbenium ion character and with a C3-H(+) bond order ≈0.6.

Mechanistic Investigations of Unsaturated Glucuronyl Hydrolase from Clostridium perfringens

“…Catalysis by the enzyme enolpyruvylshikimate-3-phosphate synthase (AroA) provides an interesting comparison with UGL. Extensive calculations of the reaction coordinate for both the nonenzymatic acid-catalyzed (29) and AroA-catalyzed (30) cases showed a change in the nature of the transition states, with experimental KIE data in good agreement. Both reactions proceed through a slow irreversible initial protonation followed by fast attack of the nucleophile on the carbocation, but the enzymatic case exhibited a much earlier transition state, with the C-H bond order of 0.24 versus 0.6 in the nonenzymatic case.…”

On the mechanism of phosphoenolpyruvate synthetase (PEPs) and its inhibition by sodium fluoride: potential magnesium and aluminum fluoride complexes of phosphoryl transfer

“…campestris) resulted in a significant reduction in virulence (Tang et al 2005). PEPs is also of interest as a catalyst for the production of isotopically enriched PEP analogues (Lou et al 2012;Jakeman and Evans 1998).…”

Cyclisation Reactions Involving Alkyl Enol Ethers

“…[10b] On the other hand, transition state analysis of the acid-catalysed hydrolysis of a biologically relevant enol ether revealed an irreversible and rate-determining nature of the proton transfer to the β-carbon with the formation of an oxocarbenium ion intermediate. [14] The reactivity of enol ethers under acidic conditions is often delicate and difficult to control due to competitive polymerisation, in particular in the presence of Lewis acids. [15] However, both Brønsted and Lewis acid-catalysed cyclisations involving enol ethers have been described.…”