title: 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 because of the existence of these acids in their hydrated and nonhydrated 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, at constant ionic strength, 0.15, and 25 °C. Dissociation constants for the oxo (pKa(oxo)) and hydrated (pKa(hyd)) 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.
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047.
authors.forename: Antonio
authors.initials: A
authors.lastname: Lopalco
authors.affiliation: Nuclear Magnetic Resonance Laboratory, Lawrence, Kansas 66045.
authors.forename: Justin
authors.initials: J
authors.lastname: Douglas
authors.affiliation: Department of Pharmacy-Drug Sciences, University of Bari Aldo Moro, Bari 70100, Italy.
authors.forename: Nunzio
authors.initials: N
authors.lastname: Denora
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047. Electronic address: stella@ku.edu.
authors.forename: Valentino J
authors.initials: VJ
authors.lastname: Stella
mesh_terms: D002264:Carboxylic Acids / Q000032:analysis / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007651:Keto Acids / Q000032:analysis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D014867:Water / Q000032:analysis / Q000737:chemistry*
chemical_list: D002264:Carboxylic Acids; D007651:Keto Acids; D014867:Water
keywords: NMR spectroscopy; dissociation constants; hydration; non-linear regression; pyruvic acid; structure property relationship; α-keto carboxylic acids