Juan Llor

Rates and equilibria of aldimine formation between pyridoxal 5′-phosphate and N-hexylamine

The rate and equilibrium constants of the reaction between n-hexylamine and pyridoxal 5′-phosphate have been studied in aqueous solution as a function of pH at 25 ± 0.1 °C and at an ionic strength of 0.1. The pH profiles of the observed constants have been explained by protonation of the pyridoxal 5′-phosphate and its Schiffs base. This study shows that there is an intramolecular general acid catalysis of the Schiffs base formation, which can explain why the aldimine is formed at a reasonably high rate at neutral pH, in spite of the low amount of unprotonated reacting amine at this pH.

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

Thermodynamic values of the ionization constants and tautomeric equilibrium constants of pyridoxine and 3-hydroxypyridine were determined in dioxane–water at 25 °C. The Born model is unable to explain the effect of the solvent on these constants. The logarithms of the equilibrium constants calculated change linearly according to the logarithm of the molar water content in the mixture, up to 40% dioxane in the case of pyridine ionizations, and up to 70% dioxane (maximum content investigated) for the phenol ionizations. The effect of protonation of either the phenolate or pyridinium nitrogen groups on the pK of the other group in these substances increases when the water content in the medium is decreased.

Thermodynamics of the solution equilibria of 3-hydroxypyridine and pyridoxine in water-dioxane mixtures

The thermodynamics of tautomeric and microscopic ionization equilibrium constants of 3-hydroxypyridine and pyridoxine have been determined in waterdioxane mixtures (0–70% weight fraction in dioxane) ranging from 10°C to 50°C. Generally, for both types of processes, the entropic contributions to the Gibbs energy predominate and this tendency increase concomitantly with the dioxane content in the media. The thermodynamic parameters are consistent with the corresponding values obtained from macroscopic ionization processes. An equation for the equilibrium constant K as a function of temperature and a solvent characteristic parameter is proposed to predict the K values throughout the whole range of the temperature and solvent compositions studied.

Chemistry of amidines. Part 1. Determination of the site of initial protonation in N′-pyridylformamidines

A series of substituted N,N-dimethyl-N′-pyridylformamidines has been synthesised and the pKa values of the conjugate acids have been measured in water at 25 °C. Consideration of the pka values shows that initial protonation is predominantly on the imino nitrogen of the amidine system rather than on the pyridyl nitrogen. The effect of the protonation on the 1H and 13C NMR spectra of the amidines is discussed.

A Correlation Between Solvatochromic Solvent Polarity Parameters and the Ionization Constants of Various Phenols in 1,4-Dioxane–Water Mixtures

Precise thermodynamic ionization constants K for 3-nitrophenol, 3,4-dichlorophenol, and 4-cyanophenol have been obtained in 1,4-dioxane-water mixtures (0–70% volume fraction in dioxane) at 25°C using a potentiometric method. The same information for another twelve cationic, neutral, and anionic phenols were taken from the literature. Three different methods were used to study the effects of the solvents on the ionization constants: one involves a single polarity parameter, ET(30); the next involves the Kamlet–Taft multiparametric method; and the last involves the preferential solvation model. The pK values follow the preferential solvation model, but the parameters obtained are highly correlated. Using the data for the phenol molecule as reference, a linear correlation between ΔpK and ET(30) has been used to develop a new method of obtaining pK values for any binary solvent composition, with only the pK in water known. The pK(s) values correlate with the molecular parameters for the dipolarity/polarizability of the solvent π* and its hydrogen-bond donor ability α. The preferential solvation parameter, f12/1, correlates with the parameter for the hydrogen-bond donor ability of the solvent. All the phenols follow Hammetts equation and the reaction constants have been calculated for the different water–dioxane mixtures.

Complete Resolution of the Ionization Equilibria of 5-Deoxypyridoxal in Water-Dioxane Mixtures

We have used a potentiometric method to determine the thermodynamic equilibrium constants for the macroscopic ionization processes of 5-deoxypyridoxal (DPL) in water-dioxane mixtures (0-70% weight fraction in dioxane) at temperatures ranging from 10°C to 50°C. These data, together with previously published equilibrium constants for the tautomerism and hydration processes, have allowed us to resolve the complete microconstant system. We have also calculated the microscopic ionization equilibrium constants under all the experimental conditions. The changes of standard thermodynamic function for the macroscopic and microscopic ionization processes were obtained in various water-dioxane mixtures at 25°C. The values of a given microscopic pK with different solvents and temperatures fit very well to an equation which relates this magnitude with the thermodynamic parameters, the solvation of the components of the reaction, and a solvent parameter. We have obtained an interesting linear correlation between the thermodynamic parameters corresponding to all the microscopic ionizations of DPL and the net change of the solvation during the process: enthalpies correlate linearly for all the microscopic ionizations, while entropies do so for the phenols and pyridinium ions separately.

Thermodynamics of proton dissociation of acridinium ion in aqueous solution

Abstract Standard thermodynamic function changes (ΔGθ°, ΔHθo, ΔSθo and ΔCpθo) are reported for proton dissociation of acridinium cation in aqueous solution. These values have been determined from a series of precise and accurate acid equilibrium constants, Ka. The equilibrium constants have been obtained spectrophotometrically over a range of temperatures from 2 to 66°C at several ionic strengths (I = 0.005, 0.007, 0.01, 0.05 and 0.25 M). Although the ionic strength does not affect in general the Gibbs free-energy values, the results suggest a possible influence on entropy and enthalpy changes.

Thermodynamic equilibrium constants for pyridoxal and pyridoxal 5′-phosphate in dioxane–water mixtures

The electronic absorption spectra of pyridoxal and pyridoxal 5′-phosphate have been obtained in water–dioxane mixtures (0–70% v/v) at 25 °C, and resolved using log–normal curves. The thermodynamic values of the hydration, hemiacetal, and tautomeric equilibrium constants together with the microscopic pK values for these compounds in all the mixtures studied could then be obtained. The variation of these pK values according to solvent polarity fits Marshalls model satisfactorily.

197 - Polarography of pyridoxal 5′-phosphate in aqueous and nonaqueous solvents

Abstract The redox behaviour of pyridoxal 5′-phosphate has been studied in aqueous and non-aqueous solvents (dimethylsulfoxide, and water-N,N′-5′-dimethylformamide mixtures). The polarographic curves for pyridoxal 5′-phosphate can be ascribed to the reduction of the aldehyde group in the overall experimental conditions used. The total wave-height corresponds to a two-electron process in all experimental conditions. Depending on these conditions, one, two or three waves were observed. Proton transfer occurred before the rate-limiting step. These transfers occur in a heterogeneous reaction on the surface of the D.M.E. Evidence of autoprotonation in non-aqueous solvents has been obtained.

Thermodynamics of the Tautomeric, Hydration and Hemiacetalization of 5-Deoxypyridoxal and Pyridoxal in Water-Dioxane Mixtures

The resolution of electronic spectra using log-normal curves has permitted a quantitative description of the equilibria of pyridoxal, one of the vitamin B6 molecules, and of the related 5-deoxypyridoxal in solution. Tautomeric, hydration and hemiacetalization equilibrium constants for water-dioxane mixtures of these compounds are reported (0–70% weight fraction in dioxane) at temperatures ranging from 10°C to 50°C. These results provide the standard thermodynamic function changes for the indicated processes in the different mixtures studied. The aldehyde content in the two compounds increases concomitantly with the dioxane content of the solution and also when it changes from the cationic to the anionic species. 5-deoxypyridoxal becomes significantly more hydrated in its more protonated form, Kh(+a)/Kh(±a)≈7–8, as do other aldehydes. Hemiacetalization in pyridoxal follow a similar pattern, Khe (±a)/Khe (−a)≈20–30, due principally to differences in the enthalpy contribution. Hemiacetalization is favored over hydration by ∼20.6 kJ-mol−1 in water solution.