Spectroscopic properties of 4-pyridoxic acid as a function of pH and solvent

被引:13
作者
Bueno, C [1 ]
Guerrero, J [1 ]
Encinas, MV [1 ]
机构
[1] Univ Santiago Chile, Fac Quim & Biol, Santiago, Chile
关键词
D O I
10.1002/hlca.200490087
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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 'pK(a)' of H-0 = -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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页码:940 / 948
页数:9
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