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In this work, comprehensive investigations of l- and dl-methionine have been conducted by the methods of differential scanning calorimetry (DSC), Knudsen effusion mass spectrometry (KEMS), and quantum chemistry. Heat capacities of crystalline l- and dl-methionine were measured by DSC in the temperature range of 209–473 K and their thermodynamic functions (TFs) have been determined. The structures and molecular parameters of 29 gaseous conformers were computed in the framework of DFT theory (B3LYP/cc-pVTZ) and TFs of methionine in the ideal gas state have been obtained. Sublimation of l- and dl-methionine was investigated by KEMS in the temperature range 369–431 K. Both compounds were found to sublime congruently in the form of monomers. The saturated vapor pressure in the studied temperature range has been obtained as ln (p/Pa) = −(16905 ± 166)/(T/K) + (38.145 ± 0.411) (l-methionine) and ln­(p/Pa) = −(16969 ± 140)/(T/K) + (38.473 ± 0.350) (dl-methionine). Sublimation enthalpies Δsub H°(298.15 K) equal to 147.9 ± 1.2 kJ·mol–1 (l-methionine) and 148.7 ± 2.6 kJ·mol–1 (dl-methionine) have been determined by the second and third law techniques.

The molecular structure and conformational properties of tryptophan have been investigated by gas-phase electron diffraction and theoretical methods. Quantum chemical calculations realized by program Gaussian 03 (B3LYP/cc-pVTZ) have been predicted the existence of six conformers at the temperature of experiment (T = 495 K). The ability of gas-phase electron diffraction method to distinguish the structure of conformers was estimated theoretically. Conformers have different orientations of carboxylic and amine group, backbone and indole fragment to each other. These conformers can be divided on two groups: distinguishable parameters (with different torsion angle C(OOH)-C(HNH2)-C(H2)-C(ind)) and weekly distinguishable ones (with different torsion angles H-N-C-C and H-O-C-C) by gas-phase electron diffraction. The molecular parameters of the conformers were determined. The conformers have intramolecular hydrogen bonding of the H2N···HO. The analysis of the gas-phase electron diffraction data have been carried out assuming the saturated vapor of tryptophan at T = 495 K consists of mixture at least of two conformers with lowest energy. It was shown that optimal ratio between conformers I : II was 50 : 50, respectively. The geometrical parameters of amino acids molecules (glycine, alanine, tryptophan) obtained by gas-phase electron diffraction were compared. The influence of the intramolecular hydrogen bond was established onto the structural parameters of the backbone of the above amino acids molecules.

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