Abstract
The solid phase thermal decompositions of L-cystine and its three stereoisomers have been investigated in vacuo between 150 and 230°C. The decomposition processes have been examined by measuring the total pressure and the partial pressures of the four gaseous products, NH3, H2O, H2S, and CO2, as well as by the semiquantitative determination of sulfur polymers. The rate equation for the thermal decomposition of L-cystine can be approximated as zeroth order, with an induction period at the initial stage of the decomposition. A remarkable change in the reaction rate was found at around 170-180°C, above which evaporated sulfur polymers can be detected. At lower temperature the dehydration reaction is predominant, while the rates of formation of NH3, H2S, and CO2 become much larger than that of H2O as the temperature increases. The observed induction periods at T<200°C are different for each of the product gases, and depend strongly on the decomposition temperature and the history of the starting materials. The reactions that occur during this period are discussed. The different behavior observed in the thermal reactions of other sulfur-containing amino acids, L-cysteine and L-methionine, and some non sulfur-containing ones are discussed in relation to the different strengths of the hydrogen bond networks in the crystalline states.
