Thermodynamics of protonation of amino acid carboxylate groups from 50 to 125°C

Article abstract of DOI:10.1007/BF00972523

Flow calorimetry has been used to study the interaction of glycine, DL-α-alanine, DL-2-aminobutyric acid, β-alanine, 4-aminobutyric acid, and 6-aminocaproic acid with protons in aqueous solutions from 323.15 K to 398.15 K and at 1.52 MPa. LogK, ΔH°, ΔS°, and ΔC°_p for the protonation of the carboxylate groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The protonation reactions are exothermic at lower temperatures and become endothermic as temperature increases. The logK, ΔH°, and ΔS° values are close together over the temperature range studied for the protonation of α-amino acids, i.e., glycine, DL-α-a/anine, and 2-aminobutyric acid. At each temperature, the magnitudes of these thermodynamic quantities increase as the number of methylene groups between the amino group and the carboxylate group increases. The ΔC°_p value for the protonation of the carboxyl group is found to lie between those of an isocoulombic reaction and a charge reduction reaction. At 323.15 K, the protonation reactions of the carboxylate groups have larger ΔC°_p values which approach those associated with charge reduction reactions. As the temperature increases, ΔC°_p decreases and approaches those found for isocoulombic reactions. This result is explained by considering long-range and short-range solvent effects. The trend in ΔH° and ΔS° with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction of the two oppositely charged groups within the molecule.