Tautomeric Equilibrium of Pyridoxine in Water. Thermodynamic Characterization by ¹³C and ¹⁵N Nuclear Magnetic Resonance

Abstract

A remarkable temperature dependence on the 13C NMR and 15N NMR chemical shifts of pyridoxine in water (pH = 7.0) has been observed. C-3, C-6, and N-1 were the most sensitive nuclei to the temperature effect. This dependence has been explained on the basis of an equilibrium shift thermally induced between the neutral and the dipolar form of this molecule. The thermodynamic characterization of tautomeric equilibria that interconvert quickly on the NMR time scale can be carried out from the observed average 13C NMR and 15N NMR chemical shifts at different temperatures (5-90 degrees C). We have developed a new method for the estimation of the thermodynamic parameters of a given equilibrium by fitting the experimental data to a theoretical curve. This new method allows us to improve the fitting results on our previously proposed methodology. We show that there are linear correlations between the average chemical shifts obtained from different nuclei at the same temperature. This indicates that the parameters of the pure forms are related among them. We have carried out a simultaneous multiple function curve fitting of all data obtained from the most sensitive signals together using these linear correlations as restricted conditions in order to diminish the number of independent parameters to fit. To test the new methodology, we have studied the thermodynamics of the tautomeric equilibrium of pyridoxine in water. We have obtained delta H degree values ranging from -23.6 +/- 1.3 to -25.8 +/- 1.7 kJ/mol for this equilibrium depending on the used data set. This kind of methodology has, among others, the following advantages: It allows the use of a great number of experimental points from different signals in the fitting process, it yields very precise and accurate values of the tautomeric process, and it allows the resolution of the problem with only 13C NMR data in some cases saving NMR time.

Influence of aqueous dimethyl sulfoxide on pyridoxine protonation and tautomerization

“…In aqueous or aqueous-organic solution, PN exists in two forms, molecular and zwitterionic [9][10][11] (Scheme 1, K z ):…”

Towards a detailed description of pyridoxamine tautomeric species

“…NMR spectroscopy constitutes another technique widely applied to study molecular tautomerism, [12][13][14] and to derive their thermodynamic properties. 15,16 Nevertheless, it has scarcely been used to study PM tautomeric equilibrium. The determination of 13 C chemical shift values for each PM tautomeric form would help to compare and understand their different structural features, and it would enable further studies on the tautomeric equilibria.…”

“…The temperature variation of the 13 C chemical shifts was fitted to eqn (1), developed by Llor and Mun˜oz. 16…”

Solid State Structure and Solution Thermodynamics of Three-Centered Hydrogen Bonds (O∙∙∙H∙∙∙O) Using N-(2-Benzoyl-phenyl) Oxalyl Derivatives as Model Compounds

“…Furthermore, the Δδ(ΝΗ) / Δ T measurements, using DMSO- d 6 as solvent, have been used to establish the energetics associated with intramolecular hydrogen bonding [24,55]. The method is based on a nonlinear fitting (NLF) procedure, followed by van't Hoff data treatment which is applied to the equilibrium established between an intramolecular hydrogen bonding at the initial state and the association with the solvent at the final state of the NH proton, at increasing temperatures.…”