Thermodynamic constants for tautomerism, hydration, and ionization of vitamin B6 compounds in water/dioxane

The macroscopic deprotonation constants of phenol, pyridine, p-nitrophenol, salicylaldehyde, 4-pyridinaldehyde, pyridoxine, 3-hydroxypyridine, 5-deoxypyridoxal, pyridoxal, and pyridoxal 5'-phosphate have been determined at 25 degrees C in water/dioxane mixtures. Many of the hydration and tautomeric constants and microscopic pK values of these compounds have also been measured under the same conditions. These values are discussed with reference to Hammett's and Marshall's equations and a general equation that predicts these equilibrium constants in the media under discussion has been formulated. The significance of these findings on the chemistry of vitamin B6 and its importance in the study of the catalytic pathways of vitamin B6-dependent enzymes are also discussed.