An Equilibrium Study of the Conversion of L-Phenylalanine to trans-Cinnamic Acid and Ammonia

Article abstract of DOI:10.1021/j100288a028

The thermodynamics of the enzymatic conversion (L-phenylalanine ammonia-lyase) of aqueous L-phenylalanine to trans-cinnamic acid and ammonia has been investigated by using high-performance liquid chromatography (HPLC).The reaction was carried out in 0.1 M Tris/HCl buffer containing ammonium chloride over the pH range 7.0-7.7, at ionic strength from 1.0 to 2.1 mol kg^-1, and over the temperature range 285-316 K.Analysis of the HPLC data using an estimated heat capacity change of 50 J mol^-1 K^-1 and an ion-size parameter of 1.6 mol^-1/2 kg^1/2 leads to an equilibrium constant of 1.16 +/- 0.3 mol kg^-1 and an enthalpy change of 24.8 +/- 2.0 kJ mol^-1 at 298.15 K for the process L-phenylalanine^+/-(aq) = trans-cinnamic acid^-(aq) + NH4⁺(aq).The use of these thermodynamic parameters in an equilibrium model for this system allows for the prediction of values of the apparent equilibrium constant as a function of pH, temperature, and composition and also of the effect of these parameters on the optimal product yield of L-phenylalanine during its manufacture from trans-cinnamic acid and ammonia.The available thermochemical data for this generic type of reaction can be rationalized in terms of a scheme which views the entropy changes for related processes to be comparable and then attributes differences in Gibbs energy changes to differences in enthalpy changes which can be influenced by effects such as resonance stabilization of the double bonds which are formed.