Mechanism of Decarboxylation of Pyruvic Acid in the Presence of Hydrogen Peroxide

Abstract

The purpose of this work was to probe the rate and mechanism of rapid decarboxylation of pyruvic acid in the presence of hydrogen peroxide (H2O2) to acetic acid and carbon dioxide over the pH range 2 – 9 at 25°C, utilizing UV spectrophotometry, high performance liquid chromatography (HPLC), and proton and carbon nuclear magnetic resonance spectrometry (1H, 13C-NMR). Changes in UV absorbance at 220 nm were used to determine the kinetics since the reaction was too fast to follow by HPLC or NMR in much of the pH range. The rate constants for the reaction were determined in the presence of molar excess of H2O2 resulting in pseudo first order kinetics. No buffer catalysis was observed. The calculated second order rate constants for the reaction followed a sigmoidal shape with pH independent regions below pH 3 and above pH 7 but increased between pH 4 and 6. Between pH 4 and 9, the results were in agreement with a change from rate determining nucleophilic attack of the deprotonated peroxide species, HOO−, on the α-carbonyl group followed by rapid decarboxylation at pH values below 6 to rate-determining decarboxylation above pH 7. The addition of H2O2 to ethyl pyruvate was also characterized.

Effect of Molecular Structure on the Relative Hydrogen Peroxide Scavenging Ability of Some α-Keto Carboxylic Acids

“…1 For the analysis of 2-4 a Shimadzu SIL 10-A system including a SIL-10A autoinjector, a SPD-10A UV-VIS detector, a SCL-10A system controller, LC-10AT pumps, and Class-VP version 4.10 software by Shimadzu Scientific Instruments (Columbia, MD) was used. A Phenomenex HyperClone 5 mm dimethyloctylsilane (C8) 120 Å 150 Â 4.6 mm column thermostated at 30 C was used.…”

“…1 Briefly, the fast kinetics were monitored by using a UV spectrophotometer, Spectramax® PLUS 384 (Sunnyvale, CA).…”

“…1,[16][17][18] Based on previous studies on the equilibrium of hydration and dissociation of these same a-keto carboxylic acids, 1-4, and the fast kinetics and mechanistic details of the addition-decarboxylation of pyruvic acid (1) in the presence of H 2 O 2 , 1,19 one could hypothesize that peroxide-driven decarboxylation of a-keto carboxylic acids 2-4 proceeded through the reversible intermediate peroxide addition to the a-carbonyl group followed by the irreversible decarboxylation step (Fig. 1).…”

“…14 Finding new, non-toxic excipients, physically and chemically compatible with the drugs, useful as potential antioxidants and peroxide scavengers is a challenge. Pyruvic acid and other a-keto carboxylic acids are H 2 O 2 scavengers 1,15 and could be useful in preventing drug/peroxide-driven oxidation.…”

“…14 Finding new, non-toxic excipients, physically and chemically compatible with the drugs, useful as potential antioxidants and peroxide scavengers is a challenge. Pyruvic acid and other a-keto carboxylic acids are H 2 O 2 scavengers 1,15 and could be useful in preventing drug/peroxide-driven oxidation.The use of several techniques, such as proton and carbon nuclear magnetic resonance (NMR) and UV-spectrophotometry, has definitively elucidated the mechanism of reaction of pyruvic acid with hydrogen peroxide. 1,16-18 Based on previous studies on the equilibrium of hydration and dissociation of these same a-keto carboxylic acids, 1-4, and the fast kinetics and mechanistic details of the addition-decarboxylation of pyruvic acid (1) in the presence of H 2 O 2 , 1,19 one could hypothesize that peroxide-driven decarboxylation of a-keto carboxylic acids 2-4 proceeded through the reversible intermediate peroxide addition to the a-carbonyl group followed by the irreversible decarboxylation step (Fig.…”