A Calorimetric and Equilibrium Investigation of the Reaction {Methyl Ferulate(aq) + H2O(l) = Methanol(aq) + Ferulic Acid(aq)}

Microcalorimetry and high-performance liquid chromatography (HPLC) have been used to conduct a thermodynamic investigation of the reaction: {methyl ferulate(aq) + H₂O(l) = methanol(aq) + ferulic acid(aq)}, as catalyzed by feruloyl esterase. Values of the apparent equilibrium constant K′ = (29.6 ± 0.7) (T = 298.15 K, citrate buffer at pH 4.98, ionic strength I = 0.39 mol · kg^-1) and of the calorimetrically determined enthalpy of reaction Δ_rH(cal) = (4.0 ± 0.9) kJ · mol^-1 (T = 298.15 K and citrate buffer at pH 4.81, I = 0.36 mol · kg^-1) were measured. A chemical equilibrium model, together with pKs and standard enthalpies of reaction ΔrH for the H ⁺(aq) binding reactions of the reactants and products, was then used to calculate the values K = (1.89 ± 0.06) · 10^-4, Δ_rH° = (7.3 ± 1.7) kJ · mol^-1, Δ_rG° = (21.25 ± 0.07) kJ · mol^-1, and Δ_rS° = - (46.8 ± 5.7) J · K^-1 · mol^-1 for the chemical reference reaction {methyl ferulate(aq) + H₂O(l) = methanol(aq) + ferulic acid^-(aq) + H⁺(aq)}. These values of K and Δ_rH° are similar in magnitude to the corresponding values reported for the reaction {propyl gallate(aq) + H₂O(l) = 3,4,5-trihydroxybenzoic acid^-(aq) + 1-propanol(aq) + H⁺(aq)}. The results obtained in this study can be used in a chemical equilibrium model to calculate how K′ and other standard transformed properties such as the standard transformed enthalpy Δ_rH′°, standard transformed Gibbs free energy Δ_rG′°, and the change in binding of H ⁺(aq), Δ_rN(H⁺), vary with the independent variables T, pH, and I.