Dissociation Constants of Some Amino Acid and Pyridinecarboxylic Acids in Ethanol–H2O Mixtures

Abstract

The dissociation constants of nicotinic, picolinic, benzoic, and o-, m-, and p-aminobenzoic acids and those of glycine, dl-alanine, β-alanine, aniline, and pyridine were determined potentiometrically and spectrophotometrically in 0, 10, 30, 50, and 72% (w/w) ethanol–water mixtures at 25 °C at various ionic strengths (0.01–0.5 mol dm−3). The pK’s of benzoic, o-, m-, and p-aminobenzoic acids increased with increase of EtOH contents while those of nicotinic acid, picolinic acid, glycine, dl-alanine, and β-alanine passed through minimum (upto 30% w/w EtOH contents). The pK’s of aniline and pyridine were almost the same as those found in literature. The variation in pK’s with the solvent composition is discussed in terms of the free energy of transfer from H2O to EtOH–H2O mixtures. The zwitterion to neutral molecule ratio of these acids is also discussed in terms of variation of composition of mixtures.

Studies on the Protonation Constants and Solvation of α-Amino Acids in Dioxan–Water Mixtures

“…1. Many studies have shown that the equilibrium constant is linearly related to the fraction of organic solvent (23)(24)(25). The results obtained in this study confirm this observation.…”

Protonation Equilibria of Biologically Active Ligands in Mixed Aqueous Organic Solvents

“…Many studies have shown that the equilibrium constant is linearly related to the fraction of organic solvent [80–83]. The results obtained in this study confirm this observation.…”

“…Many studies have shown that the equilibrium constant is linearly related to the fraction of the organic solvent [80, 81, 83]. …”

Prediction of pKa values for drug-like molecules using semiempirical quantum chemical methods

“…This hypothesis is further corroborated by the fact that introducing an additional N atom to the pyridine ring in neohetramine ( Figure 1) only significantly affects the lower pKa value (Lordi and Christian, 1956). The experimental pKa values of morphine and niacin are changed to 8.2 (Prankerd, 2007) and 4.2 (Niazi and Mollin, 1987), respectively, while the remaining experimental pKa values are taken from Eckert and Klamt (2005). When several tautomers are possible all are considered.…”