The equilibria of 5-deoxypyridoxal in water–dioxane mixtures

Abstract

The electronic absorption spectra of 5-deoxypyridoxal have been obtained together with its macroscopic pK values in water-dioxane solution at 25 °C. These spectra have been resolved using log-normal curves, enabling the determination of the spectra of the formyl, hydrated, and tautomeric forms. The proportions of these forms could thus be ascertained and numerical values for the hydration and tautomeric equilibrium constants obtained. The microscopic protonation constants have been calculated from these equilibrium constants and from the macroscopic pK values, which were determined potentiometrically. The variation of these equilibrium constants according to solvent polarity fits Marshall's model satisfactorily.