Refined Data on the Sublimation Enthalpy and Thermodynamic Functions of l- and dl-Methionine

Abstract

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.

Saturated Vapor Pressure and Sublimation Enthalpy of l-serine and l-cysteine

“…This work continues studies , and reports the data for l -serine and l -cysteine. These two compounds are thermally less stable than other proteinogenic amino acids.…”

“…The experimental setup employed in this work, the effusion cell parameters, and the methodological aspects of the amino acid investigation were described in our previous papers. 1,2 The sublimation of L-serine and L-cysteine was studied in the temperature ranges of 387−442 and 361−429 K, respectively. The heating and temperature stabilization at the first measurement point took about 2 h, after which temperature was decreased or increased in steps of 5−10 K. The establishment of equilibrium at each step took 15−30 min, which was judged by the immutability of ion currents in time.…”

“…This highly sensitive method allows measuring the vapor pressure in the range of 10 −5 to 10 Pa 3 and therefore does not require very high temperatures. The new data 1,2 made it possible to eliminate contradictions in the literature data and to recommend the most reliable values of the sublimation enthalpy based on independent approaches to data processing (through the methods of the second and third laws of thermodynamics). This work continues studies 1,2 and reports the data for Lserine and L-cysteine.…”

“…The new data 1,2 made it possible to eliminate contradictions in the literature data and to recommend the most reliable values of the sublimation enthalpy based on independent approaches to data processing (through the methods of the second and third laws of thermodynamics). This work continues studies 1,2 and reports the data for Lserine and L-cysteine. These two compounds are thermally less stable than other proteinogenic amino acids.…”

“…We recently reported the saturated vapor pressure and the thermodynamic parameters of sublimation for l -tryptophane and l - and dl -methionine obtained by Knudsen effusion mass spectrometry (KEMS). This highly sensitive method allows measuring the vapor pressure in the range of 10 –5 to 10 Pa and therefore does not require very high temperatures.…”