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Spectroscopic Properties of 4-Pyridoxic Acid as a Function of pH and Solvent

✍ Scribed by Claudia Bueno; Juan Guerrero; María Victoria Encinas

Publisher
John Wiley and Sons
Year
2004
Tongue
German
Weight
121 KB
Volume
87
Category
Article
ISSN
0018-019X

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✦ Synopsis

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 B 6 , have been studied in aqueous solutions. The ground state of 4-pyridoxic acid exhibits the different protonated forms A ± D in the range of H 0 À 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 -pK a × of H 0 À 0.64, which is much lower than that found for other vitamin B 6 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.

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