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Spectroscopic Properties of 4‐Pyridoxic Acid as a Function of pH and Solvent
Author(s) -
Bueno Claudia,
Guerrero Juan,
Encinas María Victoria
Publication year - 2004
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200490087
Subject(s) - chemistry , protonation , deprotonation , singlet state , ground state , excited state , pyridine , hydroxymethyl , intramolecular force , aqueous solution , photochemistry , crystallography , stereochemistry , medicinal chemistry , organic chemistry , ion , physics , quantum mechanics , nuclear physics
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 =−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 ‘p K a ’ of H 0 =−0.64, which is much lower than that found for other vitamin B 6 related compounds. The p K 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.