THE pH AND pD DEPENDENCE OF THE SPONTANEOUS AND MAGNESIUM-ION-CATALYZED DECARBOXYLATION OF OXALACETIC ACID

Abstract

Spectrophotoinetric studies of the spontaneous and magnesium-ion-catalyzed decarboxylation of oxalacetic acid in Hz0 and D?O have been carried out over a range of pH, pD, and magnesium ion concentrations.The rate of decarboxylation of oxalacetic acid depends on the proposed equilibrium system between the acid anion and magnesium chelate under a variety of conditions.The absorbancy indexes and the apparent equilibrium constant of the keto and en01 forms of the magnesium chelate were estimated from a combination of the kinetic and spectral data. IKTRODUCTIONOxalacetic acid has been shown t o decompose to COz and pyruvic acid spontaneously (1, 2, 3), and catalytically by the action of polyvalent cations (1, 4, 5) and enzymes (6). The catalysis by metal ions has served as a model for the enzymatic reaction (7,8). The kinetics of the spontaneous decarboxylation of oxalacetic acid in aqueous solution have been studied over a range of temperature and a t varying degrees of dissociation (9,lO). Steinberger and Westheimer (8) have proposed that the keto form of the acid decarboxylates by using the a,a-dimethyl substituted oxalacetic acid, in which only the keto form can exist. In this paper the decomposition was followed spectrophotometrically as a function of pH and pD. The results indicate that the keto form of the monoanion of oxalacetic acid is the predominant species leading t o decarboxylation.The decarboxylation of oxalacetic acid is also catalyzed by a variety of multivalent cations (11)(12)(13)(14). Magnesium ion was found to have a low catalytic activity compared t o other divalent metals, but still it involves a change of spectrum indicating the formation of a chelate. The results of the present study show a direct correlation between the rate of decarboxylation of oxalacetic acid and the nlagnesium ion concentration as a function of pH and pD. The range of optimal activity (pH 5 to 6) for the magnesium-ion-catalyzed reaction coincides with the range found for the enzj-rnatic decarboxylation (11,15,16). E X P E R I M E N T A LThe following materials were commercial preparations: oxalacetic acid (OAA) (Sigma Chemical Co.), MgC12.6H20 reagent grade, Tris(hydroxymethyl)ami~~omethane (Tris) was primary standard grade.The D2O (99.78 atom % excess D) was purchased from the Atomic Energy Commission of Canada Limited, Ottawa.Measurements of pH and p D of each reaction mixture were made a t the end of the experiment with a Beckman Model G pH meter standardized with pH 7 buffer. When DzO was used as a solvent the measurements were made with the same glass and calomel electrodes and the p D was calculated by adding 0.4 units to the observed meter reading (17). The buffer system used was a Tris-acetate mixture. T o adjust the pH, 1 M basic Tris and 1 Macetic acid were mixed in the proper proportions. For the DzO systems all the reagents were made u p in DzO. Absorption spectra were measured with a Bausch and Lomb Spectronic 505 spectrophotometer. Rates of reaction were followed either in the Beckman DU spectropho...

Spontaneous decarboxylation of oxalacetic acid

“…The formation of the intramolecular hydrogen bond should assist the charge dispersion of compound I in the activated complex to facilitate the spontaneous decarboxylation according to the mechanism in Scheme 3 (7,12).…”

“…T h e biological importance of this reaction and similarities in enzymatic and non-enzymatic (chemical) reactions have stimulated the study of the chemical decarboxylation of oxalacetate (7, l l ) . Kinetic studies over a range of pH values reveal that the predominant species for spontaneous decarboxylation is the monoanion, whereas that for-metal-catalyzed reaction is the dianion (7,8,12). In solution, oxalacetate ions exist as both en01 and keto tautomers (13) ; however, the keto form is the active structare which decarboxylates (7).…”

“…Although an analogous mechanism has been proposed for the spontaneous decarboxylation of oxalacetate (12), no conclusive evidence is available. I n this paper, experimental results which lead to the formulation of the mechanism for the spontaneous decarboxylation are presented.…”

Biosynthesis of Phosphoserine in the Methanococcales

“…The molecular ion (MH ϩ ) is shown first, if observed, followed by the base peak (in italics), and characteristic fragment ions are listed in decreasing order of intensity: 2-oxoglutarate, 4 Kinetics of oxaloacetate decarboxylation. Rates of oxaloacetate decarboxylation were measured by UV absorbance spectroscopy (22). Reaction mixtures contained 0.3 to 2.3 mM freshly prepared oxaloacetate added to 300 l of solution containing 50 mM HEPES-KOH (pH 7.5) and 80 mM KCl at 50°C.…”

Lithium Chloride Catalyzed Decarboxylation of Oxalacetic Acid in Ethanol

“…This reaction has been shown to be promoted by metal ions, but only by the di-and tri-valent metal salts (1,(5)(6)(7)(8)(9)(10). The rates are dependent on the solvent, pH, and ionic strength (11,12,13). The addition of the di-and trivalent metal salts to ethanolic OAA solution causes a shift of the absorption n~aximum to higher wavelengths and an increase in the integral absorption, indicating formation of a chelate (14).…”