Determination of pKa and Hydration Constants for a Series of α-Keto-Carboxylic Acids Using Nuclear Magnetic Resonance Spectrometry

Abstract

The determination of the acid-base dissociation constants, and thus the pKa values, of α-keto acids such as pyruvic acid is complex due to the existence of these acids in their hydrated and non-hydrated or oxo-state. Equilibria involved in the hydration and dehydration of the α-keto group of pyruvic acid and three other α-keto acids, 3-methyl-2-oxobutanoic acid, 4-methyl-2-oxopentanoic acid, and 2-oxo-2-phenylacetic acid, were investigated by proton and carbon nuclear magnetic resonance spectrometry, NMR, at constant ionic strength, 0.15, and 25°C. Dissociation constants for the oxo (pKaoxo) and hydrated (pKahyd) acids of each compound were estimated from the change in the degree of hydration with changes in pH and directly from the changes in chemical shifts of various hydrogen and carbons nuclei with pH. α-Keto acids showed greater hydration in their acidic forms than their carboxylate forms. The degree of hydration was sensitive to steric and electronic/resonance factors. As expected, the oxo forms of the acids were stronger acids compared with their hydrated analogs, and their dissociation constants were also sensitive to steric and electronic factors.

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

“…The physicochemical properties (hydration equilibrium and dissociation) of pyruvic acid as well as a series of other α-ketocarboxylic acids was examined in an earlier study. 1 Here, the fast kinetics and mechanism of decarboxylation of pyruvic acid in the presence of H 2 O 2 was studied at 25°C using a UV spectrophotometry technique and HPLC and NMR product analysis. A mechanism of reaction, involving the tetrahedral intermediates (2-hydroperoxy-2-hydroxypropanoate 2 ) (Scheme 1), was proposed that could explain the pH-rate profile.…”

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

“…1) and 5% or less in the hydrated forms (only the oxo form of 4 was detected by proton and carbon NMR at pH values >3). 19 Based on unpublished studies, in water solutions in the absence of added H 2 O 2 , 1-4 was found stable in the time frame of the kinetic studies presented here. NMR studies confirmed that no enol form or dimers, due to an aldol-like self-condensation, of 2-4 were observed during the peroxide reaction, kinetic analyses.…”

“…19 Water addition to the carbonyl was similar to peroxide or peroxide anion addition to the carbonyl. Equilibrium hydration of 1-4 at pH >4 is best defined by K H ion in Figure 1.…”

“…Values for K H ion from an earlier study are reproduced here in Table 1. 19 A plot of log k 1 versus log K H ion is shown in Figure 7. Although not a perfect correlation, because K H ion represented an equilibrium process and thus a ratio of the forward rate constant for the hydration step over the reverse, dehydration reaction, these results do support the idea that the kinetics of peroxide anion attack defined by k 1 were determined by steric factors (2 and 3 relative to 1) and the electrophilicity of the carbonyl carbon (mainly with 4).…”

“…Therefore, in this pH range, the reactive species involved in the formation of the tetrahedral intermediates were the deprotonated oxo forms of each a-keto carboxylic acid. 19 The impact of the buffer on the k sec was evaluated performing experiments at 2 different buffer concentrations 20 and 40 mM between pH 4 and 9 (Fig. 2).…”