Enthalpies of Combustion and Formation of Histidine Stereoisomers

Abstract

The combustion energy of histidine enantiomers (L and D) and of their racemic mixture was measured experimentally. The following values for the enthalpies of formation corresponding to the crystalline state were derived (L = −451.7, D = −448.7, DL = −451.5 kJ·mol−1), and information concerning their stability was obtained by correlating the values of the above thermochemical quantity with the structure of the molecules by using the group additivity scheme. The samples were characterized using a simultaneous thermogravimetry (TG) coupled with differential scanning calorimetry (DSC) techniques in the temperature range between ambient and beyond melting-decomposition, and the corresponding parameters were calculated. The high values of the decomposition temperatures highlight the stability of the compounds. The decomposition reactions are discussed in terms of DSC and TG data, obtained by us and other researchers.

Investigation of thermochemical features of L-alanyl-glycine and β-alanyl-L-histidine (L-carnosine)

“…The enthalpies of formation for the studied dipeptides were compared with the values of their single components, alanine, glycine 21,[23][24] and histidine. 22,24 The large negative values of the enthalpies of formation of the two dipeptides show their high stability, if the number of chemical bonds from their molecules is taken into account. The enthalpy of formation of carnosine is over 45 kJ/mol less negative than the calculated value.…”

“…In the case of carnosine, a small molecule (possibly CO) is evolved during the decomposition peak and other gaseous molecules (H2O and NH3) at higher temperatures. 22…”

“…The TG results support the statement that a small molecule (possibly CO) is evolved during the decomposition peak and other gaseous molecules (H2O and NH3) at higher temperatures. 22 There is a mutual influence of the two α-amino acids which makes the dipeptides quite different than the single component. 27 The proportional weight loss indicates the first fragment of decomposition (carboxyl group, amino group and functional groups) and the beginning of the decomposition.…”