Thermodynamics of the tautomeric, hydration and hemiacetalization of 5-deoxypyridoxal and pyridoxal in water-dioxane mixtures

The resolution of electronic spectra using log-normal curves has permitted a quantitative description of the equilibria of pyridoxal, one of the vitamin B-6 molecules, and of the related 5-deoxypyridoxal in solution. Tautomeric, hydration and hemiacetalization equilibrium constants for water-dioxane mixtures of these compounds are reported (0-70% weight fraction in dioxane) at temperatures ranging from 10 degrees C to 50 degrees C. These results provide the standard thermodynamic function changes for the indicated processes in the different mixtures studied. The aldehyde content in the two compounds increases concomitantly with the dioxane content of the solution and also when it changes from the cationic to the anionic species. 5-deoxypyridoxal becomes significantly more hydrated in its more protonated form, K-h ((+)a)/K-h((+/-)a) approximate to 7-8, as do other aldehydes. Hemiacetalization in pyridoxal follow a similar pattern, K-he ((+/-)a)/K-he ((-)a) approximate to 20-30, due principally to differences in the enthalpy contribution. Hemiacetalization is favored over hydration by similar to 20.6 kJ-mol(-1) in water solution.