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| - Electronic Factors Influencing the Decarboxylation of β-KetoAcids. A Model Enzyme Study
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| - A theoretical study of the mechanism of decarboxylation ofβ-keto acids is described. A cyclictransition structure was found with essentially complete protontransfer from the carboxylic acidto the β-carbonyl group. The activation barrier fordecarboxylation of formylacetic acid is predictedto be 28.6 kcal/mol (MP4SDTQ/6-31G*//MP2/6-31G*) while loss ofCO2 from its anion exhibits abarrier of only 20.6 kcal/mol (MP4SDTQ/6-31+G*//MP2/6-31+G*).Barrier heights of decarboxylation of malonic acid and α,α-dimethylacetoacetic acid arepredicted to be 33.2 and 26.7 kcal/mol,respectively. Model enzyme studies using a thio methyl ester ofmalonate anion suggests that therole of malonyl-CoA is to afford a polarizable sulfur atom to stabilizethe developing enolate anionin the transition structure for decarboxylation. Adjacentpositively charged ammonium ions arealso observed to stabilize the loss of CO2 from acarboxylate anion by through-bond Coulombicstabilization of the transition structure.
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