Enthalpy of sublimation of natural aromatic amino acids determined by Knudsen’s effusion mass spectrometric method

Abstract

The sublimation process of aromatic amino acids, l-phenylalanine, l-tyrosine and l-tryptophan, were investigated by Knudsen’s effusion method with mass spectrometric vapor composition measurement. Fragmentation of molecules under electron ionization was discussed. For these compounds, the molar enthalpies of sublimation were derived from the Clausius–Clapeyron equation. Using values for the heat capacity differences between the gas and crystal phases of the compounds the molar standard enthalpies of sublimation at T=298.15K, were obtained. The molar enthalpies of solvation in water at infinite dilution (i.d.) were derived from the molar enthalpies of solution of amino acids in water at i.d. and the molar standard enthalpies of sublimation obtained. Correlations between the enthalpies of sublimation/solvation and some molecular descriptors of amino acids were discussed.