Ionization behavior of aqueous short-chain carboxylic acids: a carbon-13 NMR study

The 13C chemical shift of each carbon of aqueous acetic, propionic, and butyric acids has been measured as a function of pH or of added equivalents of base. A plot of chemical shifts for the carboxyl, alpha, and beta carbons as a function of pH is sigmoidal and yields pKa values that agree closely with values obtained by potentiometric titration. In contrast, a plot of chemical shift as a function of added equivalents of base is linear and has a sharp break at the equivalence point. Based on this result, we propose that the local (microscopic) ionization state of the carboxyl group can be determined directly by NMR without need for pH or pK determinations. In addition to titration curves, the effects of concentration, ionic strength, and temperature upon fatty acid chemical shifts are reported. For aqueous acids, changes in ionic strength and temperature have no effect on chemical shifts. However, changes in concentration do affect chemical shifts, probably as a result of changes in the relative degree of acid-acid and acid-water hydrogen bonding. Our results provide necessary background data for 13C NMR studies of higher fatty acids in lipid-lipid and lipid-protein systems.