Experimental and computational thermochemical study of sulfur-containing amino acids: L -cysteine, l -cystine, and l -cysteine-derived radicals. S-S, S-H, and C-S bond dissociation enthalpies

Abstract

This paper reports an experimental and theoretical study of the standard (p° = 0.1 MPa) molar enthalpies of formation at T = 298.15 K of the sulfur-containing amino acids l-cysteine [CAS 52-90-4] and l-cystine [CAS 56-89-3]. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline l-cysteine and l-cystine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g) and H2SO4·115H2O, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of l-cysteine were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpy of formation of l-cysteine in the gaseous phase, ΔfH°m(g) = −382.6 ± 1.8 kJ·mol−1. Due to the low vapor pressures of l-cystine and since this compound decomposes at the temperature range required for a possible sublimation, it was not possible to determine its enthalpy of sublimation. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP and/or G3 levels were performed. Enthalpies of formation, using atomization and isodesmic reactions, were calculated and compared with experimental data. A value of −755 ± 10 kJ·mol−1 was estimated for the enthalpy of formation of cystine. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) of S−H, S−S, and C−S bonds, and enthalpies of formation of l-cysteine-derived radicals, were also computed.