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 permi tted a quantitative description of the equilibria of pyridoxal, one of the vitamin B-6 molecules, and of the related 5-deoxypyridoxal in sol ution. Tautomeric, hydration and hemiacetalization equilibrium constan ts for water-dioxane mixtures of these compounds are reported (0-70% w eight fraction in dioxane) at temperatures ranging from 10 degrees C t o 50 degrees C. These results provide the standard thermodynamic funct ion changes for the indicated processes in the different mixtures stud ied. The aldehyde content in the two compounds increases concomitantly with the dioxane content of the solution and also when it changes fro m the cationic to the anionic species. 5-deoxypyridoxal becomes signif icantly more hydrated in its more protonated form, K-h ((+)a)/K-h((+/- )a) approximate to 7-8, as do other aldehydes. Hemiacetalization in py ridoxal follow a similar pattern, K-he ((+/-)a)/K-he ((-)a) approximat e to 20-30, due principally to differences in the enthalpy contributio n. Hemiacetalization is favored over hydration by similar to 20.6 kJ-m ol(-1) in water solution.