Enthalpies of sublimation of l-methionine and dl-methionine: Knudsen’s effusion mass spectrometric study

Abstract

We report sublimation enthalpies of sulfur-containing amino acids, l- and dl-methionine, investigated by Knudsen’s effusion method with mass spectrometric vapor composition control. The fragmentation of molecules under electron ionization (EI) was discussed. For these compounds, the molar sublimation enthalpies were derived from the Clausius-Clapeyron equation. Using the isobaric heat capacities in the crystal phases of the compounds the standard molar enthalpies of sublimation at T = 298.15 K, were obtained. In addition, conformational analysis of methionine (Met) was performed by hybrid DFT and MP2 calculations. Analysis of natural bond orbitals (NBO) carried out for the three low-energetic conformers of Met revealed intramolecular hydrogen bonds in one of these. The average interaction energies between a lone electron pair (donor) of nitrogen and “antibonding” orbital of O–H bond (acceptor) were obtained and used in the interpretation of the structure.