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.
This paper reports an experimental and theoretical study of the standard (p(degrees) = 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(degrees) = 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(degrees) = 0.1 MPa) enthalpy of formation of l-cysteine in the gaseous phase, DeltafH(degrees)m(g) = -382.6 +/- 1.8 kJ x 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 x 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.
Roux
Maria Victoria
MV
Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain. victoriaroux@iqfr.csic.es
Foces-Foces
Concepción
C
Notario
Rafael
R
da Silva
Manuel A V Ribeiro
MA
da Silva
Maria das Dores M C Ribeiro
Md
Santos
Ana Filipa L O M
AF
Juaristi
Eusebio
E
D000596:Amino Acids / Q000737:chemistry*; D003198:Computer Simulation; D003545:Cysteine / Q000737:chemistry*; D003553:Cystine / Q000737:chemistry*; D006801:Humans; D008968:Molecular Conformation; D015394:Molecular Structure; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*; D013816:Thermodynamics
D000596:Amino Acids; D013438:Sulfhydryl Compounds; D003553:Cystine; D013455:Sulfur; D003545:Cysteine