Thermodynamics and Kinetics of the Reaction between Pyridoxal-5-Phosphate and Hydrazides of 2-Methylfuran-3-Carboxylic and Thiophene-3-Carboxylic Acids in an Aqueous Solution

The stability constants of pyridoxal-5-phosphate hydrazones formed with 2-methylfuran-3-carbohydrazide and thiophene-3-carbohydrazide in an aqueous solution at pH 1.9, 6.6, 7.0, and 7.4 are determined via spectrophotometry. The kinetics of the processes of formation and hydrolysis of the Schiff bases are studied, and the constant of the direct and reverse reactions are calculated from the electronic absorption spectra. The stability constants of the Schiff bases are calculated from their ratio. The thermodynamic parameters of the reaction of formation (log K, ΔH, and TΔS) of both hydrazones at pH 6.6 are determined via calorimetry. The reasons for the differences between the equilibrium constants calculated from the data of spectrophotometric and kinetic experiments are discussed, and the reliability of the obtained results is analyzed.