Interrelations Between the Enthalpies of Formation of the Sulfur-Containing Amino Acids L-Cystine, L-Cysteine and L-Methionine

L-Cystine, L-cysteine, and L-methionine are among the twenty-two L-α-amino acids from which all proteins are composed. With their unique sulphur atoms, Lcystine, L-cysteine, and L-methionine have been particularly problematic with regard to both qualitative and quantitative understanding of their energetics, most notably their enthalpies of formation. Using thermochemical quantities from calorimetric experiments (enthalpies of combustion, hydrogenolysis, vaporization, and sublimation), high level quantum chemical calculations, and idealized chemical reactions, these enthalpies of formation are derived and interrelated. In a brief epilogue, the conceptual trichotomy of “convenience, anthropocentrism, and folksonomy” is employed to enhance our thermochemical understanding of these species.