The enthalpy of formation of methionine revisited

Abstract

Methionine (2-amino-4-(methylthio)butanoic acid) is a sulfur-containing essential α-amino acid. Despite its importance, its enthalpy of formation is contentious. From the analysis of its polymorphism confirms that the previously reported calorimetric experiments were made in the temperature interval in which a phase transitions occurs. A new value for the enthalpy of formation is suggested based on these results, as well as G4 quantum chemical calculations and thermochemical estimates. From the analysis of experimental enthalpy of formation measurements, the group increment temperature adjusted enthalpy of sublimation, quantum chemical calculations, and difference quantity derived estimates, our recommended value for the gas phase enthalpy of formation of methionine is –(430 ± 10) kJ·mol−1.