Thermodynamic equilibrium constants for pyridoxal and pyridoxal 5′-phosphate in dioxane–water mixtures

The electronic absorption spectra of pyridoxal and pyridoxal 5′-phosphate have been obtained in water–dioxane mixtures (0–70% v/v) at 25 °C, and resolved using log–normal curves. The thermodynamic values of the hydration, hemiacetal, and tautomeric equilibrium constants together with the microscopic pK values for these compounds in all the mixtures studied could then be obtained. The variation of these pK values according to solvent polarity fits Marshall's model satisfactorily.