Abstract

The photophysical properties and acid/base equilibria of 4-pyridoxic acid (= 3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carboxylic acid), the final product of the catabolism of vitamin B6, have been studied in aqueous solutions. The ground state of 4-pyridoxic acid exhibits the different protonated forms A-D in the range of H0 = -6 to pH 11.5. HMQC- and HMBC-NMR Studies allowed the pH-dependent assignment of the different C-atoms, and the evaluation of the deprotonation sequence. The 3-OH group in the ground state has a 'pKa' of H0 = -0.64, which is much lower than that found for other vitamin B6 related compounds. The pK a value of the 4-COOH group is 5.4. Fluorescence studies showed that the same species exist at the lowest excited singlet state, but in different pH ranges. The 3-OH group is four pH units more acidic in the lowest excited singlet state than in the ground state. Excitation spectra and emission decays in the pH range of 8 to 11.5 indicate that the pyridine N-atom is more basic in the excited singlet state than in the ground state. The emission spectra are red-shifted in protic solvents, in agreement with an intramolecular H-bond between the ionized 3-OH group and the nonionized 4-COOH group.