Enthalpy of sublimation of hydroxyl-containing amino acids: Knudsen’s effusion mass spectrometric study

Abstract

The sublimation processes of hydroxyl amino acids, L-serine and L-threonine, were investigated by Knudsen’s effusion method with mass spectrometric vapor composition control. 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. Conformational analysis of Thr was made by using B3LYP/6-31++G(d,p). Analysis of natural bond orbitals (NBO) has been carried out for the conformers of Ser, Thr and Tyr. It was shown that the side chain of molecule had an influence on formation intramolecular H-bond.