Thermodynamic constants for tautomerism and ionization of pyridoxine and 3-hydroxypyridine in water–dioxane

Sánchez‐Ruiz, José M.; Llor, Juan; Cortijo, Manuel

doi:10.1039/p29840002047

Cited by 39 publications

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“…Studies on the solvent-dependent absorption spectra of pyridoxyl compounds have revealed that the existence of different tautomeric forms depends on the polarity of the solvent [24]. These tautomers are due to intramolecular H transfer from the protonated pyridinium HN group to the 3-O À -atom, a process that is favored in protic media [8].…”

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confidence: 98%

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

Bueno

Guerrero

Encinas

2004

Helvetica Chimica Acta

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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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confidence: 98%

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

Bueno

Guerrero

Encinas

2004

Helvetica Chimica Acta

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“…From the change of the absorption spectrum, the pK values of the different protonated forms of the ground state have been determined [3 ± 5]. Also, strong changes of the absorption characteristics of these compounds with the solvent polarity have been detected [6] [7]. These changes have been interpreted in terms of the different tautomeric forms present in the different media.…”

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Photophysical and Photochemical Studies of Pyridoxamine

Bueno

Encinas

2003

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The absorption and fluorescence emission of pyridoxamine were studied as function of pH and solvent properties. In the ground state, pyridoxamine exhibits different protonated forms in the range of pH 1.5 ± 12. Fluorescence studies showed that the same species exist at the lowest singlet excited state but at different pH ranges. The phenol group is by ca. 8 units more acidic in the excited state than in the ground state. On the other hand, the pyridine N-atom is slightly more basic in the lowest excited state than in the ground state. Excitation spectra and emission decays in the pH range of 8 ± 10 indicate the protonation of the pyridine N-atom by proton transfer from the amine group, in the ground and singlet excited states. Spectroscopic studies in different solvents showed that pyridoxamine in the ground or excited states exhibits intramolecular proton transfer from the pyridine N-atom to the phenol group, which is more favorable in solvents of low hydrogen-bonding capacity. The cationic form with the protonated phenolic group, which emits at shorter wavelength, is the dominant species in nonprotic solvents, but, in strong proton-donor solvents, both forms exist. The fluorescence spectra of these species exhibit blue shift in protic solvents. These shifts are well-correlated with the polarity and the H-donor ability of the solvent.

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“…Thus the above hypotheses (a) and (6) would explain the general behaviour of the thermodynamic magnitudes for the fust and second macroscopic protonation (Fi s. [1][2][3][4] and also E the differences between the values of AH, and -0As: for both protonations ( Figs. 1 and 2; Figs.…”

Section: Resultsmentioning

confidence: 99%

“…We have reported in recent papers on the solution equilibria of compounds belonging to the vitamin B6 group and other analogous compounds in water-dioxane, with special emphasis on the ionization processes (1)(2)(3)(4)(5)(6). T o c a n y out these studies we designed an experimental method that allows us to calculate the macroscopic ionization equilibrium constants corrected to zero ionic strength, that is to say, the thermodynamic ionization equilibrium constants (1)(2)(3), with great saving in time and materials.…”

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confidence: 99%

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The thermodynamics of the ionization of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures

Asensio¹,

López-Cantarero²,

Llor³

1992

Can. J. Chem.

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A potentiometric method has been used to determine the thermodynamic equilibrium constants for the macroscopic ionization processes of 3-hydroxypyridine and pyridoxine in water-dioxane mixtures (0-70% weight fraction in dioxane) at temperatures from 10 to 50°C. From these data the reaction standard thermodynamic function changes were obr tained for the processes in various water-dioxane mixtures at 25'C. It was found, in both molecules, that during the fxst ionization process the contributions of entropy and enthalpy to free energy are similar but that during the second ionization the main contribution comes from entropy, especially in mixtures with a high dioxane content. The effect of the solvent on the tautomeric equilibrium in the net zero charge specie could explain this behaviour. SALVADOR B. ASENSIO, ENRIQUE LOPEZ-CANTARERO et JUAN LLOR. Can. J. Chem. 70, 1635 (1992). On a fait appel a une methode potentiomktnque pour determiner les constantes thermodynamiques d'equilibre des processus macroscopiques d'ionisation de la 3-hydroxypyridine et de la pyridoxine, dans des melanges eau-dioxane (0-70% en poids de dioxane), 2 des temperatures allant de 10 2 50°C. A partir de ces donnCes, on a CvaluC les changements dans la fonction thermodynamique standard de la reaction pour les processus qui se produisent dans des melanges eaudioxane, B 25°C. On a trouvC que, pour les deux molCcules, les contributions de l'entropie et de I'enthalpie B 1'Cnergie libre du premier processus d'ionisation sont semblables, mais que dans la deuxikme ionisation la contribution principale provient de l'entropie, s@cialement dans des melanges contenant une grande quantitC de dioxane. Ce comportement peut &tre explique par l'effet de solvant sur I'equilibre tautomerique sur la charge nette zero.[Traduit par la rkdaction]

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Thermodynamic constants for tautomerism and ionization of pyridoxine and 3-hydroxypyridine in water–dioxane

Cited by 39 publications

References 4 publications

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

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

Photophysical and Photochemical Studies of Pyridoxamine

The thermodynamics of the ionization of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures

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