Theoretical study of “protonated pyruvate”: A methylhydroxycarbene—carbon dioxide complex—implications for the decarboxylation of pyruvic acid

Abstract

Two conformers of protonated pyruvate, CH3C+(OH)COO, with the OH group either trans or cis to the methyl group and the carboxylate group in the CCC plane have been studied using the ab initio SCF/3‐21G method, as well as by some semiempirical AM1 calculations. Both ab initio SCF and AM1 curves for the potential energy as a function of the CCOO distance exhibit a minimum corresponding to a complex of methylhydroxycarbene, CH3COH, associated with carbon dioxide, but only the AM1 curves predict an inner minimum corresponding to a covalently bonded protonated pyruvate molecule with a CCOO distance of 1.6–1.7 Å. The two models also disagree on the dissociation pathway for pyruvic acid, with the AM1 calculations predicting formation of acetyl and HOCO radicals while the ab initio method predicts dissociation into methylhydroxycarbene and carbon dioxide following an initial intramolecular proton transfer. The weakly bound complexes of methylhydroxycarbene and carbon dioxide have been studied in some detail using ab initio SCF and MP2 methods in conjunction with 6‐311G** basis sets, obtaining equilibrium geometries and vibrational frequencies. In addition, the lactone‐type isomer of protonated pyruvate, which contains a CCO ring, was also studied. The conclusions of these calculations are consistent with those from earlier work using the smaller 3‐21G basis set. The most stable complex is predicted to occur between trans‐methylhydroxycarbene and carbon dioxide where substantial stabilization is provided by an OH ⃛ OC hydrogen bond. © 1993 John Wiley & Sons, Inc.

Hydroxyoxiranone: an ab initio MO investigation of the structure and stability of a model for a possible α-lactone intermediate in hydrolysis of sialyl glycosides

“…To our surprise, preliminary studies on the sialyl zwitterion 2 in vacuo with the AM1 method (using the Spartan 3.1 program and using the molecule Builder facility) 16 indicated a spontaneous elongation of the C α ᎐C bond to yield a complex of sialyl carbene with carbon dioxide. Similar behaviour has been reported in a theoretical study 17 (using similar methods to those employed here) of 'protonated pyruvate', CH 3 C ϩ (O)CO 2 Ϫ , which dissociates to a complex of methylhydroxycarbene and carbon dioxide. For the model compounds studied here, the relative energies in vacuo of the zwitterion and carbene ϩ carbon dioxide vary with the level of theory used.…”

Decomposition of Pyruvic Acid on the Ground-State Potential Energy Surface

“…The transition state for lactone formation involves initial proton transfer from the carboxyl group to the α-keto group (as in TS3 and TS4 ), accompanied by ring closing at the other carboxyl O atom. The lactone isomer of pyruvic acid was proposed in a theoretical study by Norris et al and was also considered by Kakkar et al, although no prior works appear to have examined the barrier for isomerization back to the acid form. The ability for hydroxy-α-lactones to readily isomerize with their more-stable carboxylic acid forms is of interest, however, as these compounds have been suggested as possible intermediates in the enzymatic hydrolysis of keto acids. , …”

Conformer-Specific Photolysis of Pyruvic Acid and the Effect of Water

“…Previous photolysis studies investigated pyruvic acid reactivity in the gas and aqueous phases, which o c c u r w i t h c o m p l e t e l y d i ff e r e n t m e c h anisms. 11,22,[25][26][27][28][29][30][31][32][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]54,57,58,66,106 To understand the role of water in the gas-phase photolysis of pyruvic acid, we experimentally measured the dark conformer population and the conformer-specific photolysis rate constants. With complementary computational work, we also studied the inter-and intramolecular hydrogen bonds of conformers with water.…”

“…Gas-phase pyruvic acid can photodissociate via excitation to S 1 following two different pathways: (1) a concerted reaction through a five-membered cyclic transition state by H-transfer and C–C cleavage or (2) a Norrish Type 1 C–C cleavage . The first pathway leads to the formation of CO 2 and a reactive intermediate, methylhydroxycarbene (MHC), which quickly isomerizes to acetaldehyde. ,,,− Although direct experimental detection of MHC during the photolysis of gas-phase pyruvic acid has yet to be explored, theoretical studies argue that the major photodecomposition pathway of gas-phase pyruvic acid is through the first pathway, involving MHC, as it is the most energetically favored mechanism. ,,, Conversely, the photolysis of aqueous pyruvic acid, from T 1 , generates organic radicals that can lead to a variety of oxidized products, including the formation of complex oligomers. − ,− ,,,, Here, we attempt to provide an explanation of the role of water in the gas-phase pyruvic acid conformer distribution and, by extension, the chemistry in aqueous atmospheric reaction environments.…”