Thermodynamic quantities for the protonation of amino acid amino groups from 323.15 to 398.15 K

Flow claorimetry has been used to study the interaction of protons with glycine, DL--alanine, -alanine, DL-2-aminobutyric acid, 4-aminobutyric acid, and 6-aminocaproic acid in aqueous solutions at temperatures from 323.15 to 398.15 K. By combining the measured heats for amino acid solutions titrated with NaOH solutions with the heat of ionization for water, the log K, H^o, S^o, and C_p ^o values for the protonation of the amino groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The H^o and S^o values increase while log K values decrease as temperacture increases. The trends for log K, H^o, S^o, and C_p ^o are discussed in terms of changes in long-range and short-range solvent effects. The trend in H^o, S^o, and C_p ^o values with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction between the two oppositely charged groups within the molecule.