A calorimetric study of l-, d- and dl-isomers of tryptophan

Abstract

The enthalpies of combustion and formation in solid state of tryptophan stereoisomers (l- and d-) and of their racemic mixture are reported (−406.6 ± 3.1, −407.4 ± 2.1, −410.7 ± 2.3 kJ mol−1, respectively) and compared to the literature and calculated data. The transformation (melting—decomposition) points and the associated thermal effects were determined from DSC curves in the temperature range between ambient and beyond the melting—decomposition temperatures. Samples without further purification as well as dried samples were used in the experiments. The chiral purity of the enantiomers was checked by means of polarimetry. The values of the enthalpies of formation in crystalline and ideal gas state are compared with those calculated by group additivity methods. They highlight the relative stability of tryptophan and suggest the higher stability of the racemic mixture. The melting—decomposition process revealed by DSC and thermogravimetry is discussed in relationship with the previous treatment of samples.