Effect of temperature and pressure on the protonation of glycine.

Flow calorimetry has been used to study the interaction of glycine with protons in water at temperatures of 298.15, 323.15, and 348.15 K and pressures up to 12.50 MPa. By combining the measured heat for glycine solutions titrated with NaOH with the heat of ionization for water, the enthalpy of protonation of glycine is obtained. The reaction is exothermic at all temperatures and pressures studied. The effect of pressure on the enthalpy of reaction is very small. The experimental heat data are analyzed to yield equilibrium constant (K), enthalpy change (DeltaH), and entropy change (DeltaS) values for the protonation reaction as a function of temperature. These values are compared with those reported previously at 298.15 K. The DeltaH and DeltaS values increase (become more positive), whereas log K values decrease, as temperature increases. The trends for DeltaH and DeltaS with temperature are opposite to those reported previously for the protonation of several alkanolamines. However, log K values for proton interaction with both glycine and the alkanolamines decrease with increasing temperature. The effect of the nitrogen atom substituent on log K for protonation of glycine and alkanolamines is discussed in terms of changes in long-range and short-range solvent effects. These effects are used to explain the difference in DeltaH and DeltaS trends between glycine protonation and those found earlier for alkanolamine protonation.