Influence of the Polarity of the Medium on the Catalysis of Formation, Rate of Hydrolysis and Stability of the Schiff Bases Formed by Pyridoxal 5′-Phosphate with L-Tryptophan

The apparent rate constants of formation (k1) and hydrolysis (k2), and the equilibrium constant (KpH), of the Schiff bases formed by pyridoxal 5′-phosphate with L-tryptophan in water and different aqueous ethanol mixtures at a variable pH, 25 °C and an ionic strength of 0.1 M (1 M = 1 mol dm−3) were determined. The individual rate constants of formation and hydrolysis of the Schiff bases of the systems corresponding to the different chemical species present in the medium, as a function of its acidity, were also determined, as were the pK values for the Schiff bases. The influence of the solvent medium on the formation and hydrolysis rate constants of the Schiff bases is discussed.