G. Fonrodona

Spectrophotometric, potentiometric and chromatographic pKa values of polyphenolic acids in water and acetonitrile–water media

The pKa values of nine polyphenolic acid substances in water and acetonitrile–water mixtures using potentiometric, spectrophotometric, and liquid chromatographic (LC) measurements, have been determined. Also, a new method based on the absorbance spectra at the maximum of chromatographic peaks previously obtained has been applied. This method can be applied to data obtained from LC–UV (diode array detection (DAD)) instruments and retains all the advantages of LC and spectrophotometric methods, such as the possibility of working with impure samples. Finally, the advantages of the different methods to the determination of pKa values of nine polyphenolic acids have been described in order to verify the applicability of the proposed methods in aqueous and acetonitrile–water media.

Factor analysis applied to the correlation between dissociation constants and solvatochromic parameters in water-acetonitrile mixtures: I. Solvent effects on the dissociation of carboxylic acid groups in some diuretics, quinolones, buffers and peptides

Abstract The dissociation constants in several acetonitrile-water mixtures of the carboxylic acid groups of 19 substances, included into four different sets (diuretics, quinolones, buffers and peptides) were first submitted to factor analysis (FA) in order to find how many factors affect the variations in the data sets. Then target factor analysis (TFA) was performed in order to identify these factors. The influence of solvatochromic parameters in the interactions between carboxylic acid groups and the solvents studied was identified and quantified. The model Kamlet-Taft equation for each substance was established. The general effect of the preferential solvation of electrolytes in acetonitrile-water mixtures on the values of the protonation constants in these media is also described.

Determination of phenolic acids in strawberry samples by means of fast liquid chromatography and multivariate curve resolution methods

Use of Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) is evaluated in the analysis of nine phenolic acids, both in standards mixture samples and in strawberry juice samples, by liquid chromatography with diode array detection (LC-DAD). Chromatographic coelution problems either because of unknown matrix interferences or because of the increase of organic modifier to reduce chromatographic analysis times are investigated. pH (4.25) and proportion of organic modifier in acetonitrile-water ratios (11:89, v/v) used as mobile phases have been optimized for separation of mixture of nine phenolic acids. Results obtained in the resolution and quantitation of phenolic acids in standards mixture samples and strawberry samples at two proportions of organic modifier (11:89 and 40:60 acetonitrile-water (v/v) ratios) show that the proposed MCR-ALS approach reduces analysis times and solvent expenses and improves determinations in case of strong coelution. Limits on the use of MCR-ALS are investigated in the analysis of phenolic acids in strawberry samples.

Prediction of chromatographic retention, pKa values and optimization of the separation of polyphenolic acids in strawberries.

Polyphenolic acids are a complex group of compounds that have attracted enormous attention in the last few years because of their biological properties. In this work, the proportion of organic modifier and the pH of acetonitrile-water mixtures used as mobile phases were optimized in order to separate a series of polyphenolic compounds. The linear solvation energy relationship formalism based on the single solvent polarity parameter, E(T)N was used to predict their chromatographic behavior as a function of the percentage of acetonitrile in the eluent. Moreover, the correlation established between retention and the pH of the aqueous-organic mobile phase was used to optimize the pH of the mobile phase. The optimized mobile phase is composed of acetonitrile and formic acid buffer adjusted to pH 4.25, with 12% (v/v) acetonitrile. Also, the pKa values of polyphenolic acids in acetonitrile-water mixtures were determined using chromatographic data, and in order to validate the optimized conditions, a series of polyphenolic compounds was studied in strawberries.

Solvent classification based on solvatochromic parameters : a comparison with the Snyder approach

Abstract Solvents play a major role in many chemical processes. Their effect is closely related to the nature and extent of solute/solvent interactions, developed locally in the immediate vicinity of the solutes. A new generation of empirical parameters helps to describe the solvent features in these microscopic environments. Their adequacy for the establishment of linear free energy relationships (LFER) has been widely proved. A classification scheme based on the microscopic properties of solvents is proposed, to provide a clearer insight into the solvent-space structure in terms of the similarity of solute/solvent interactions and to help chemists in the choice of a suitable solvent for each purpose. The Kamlet and Taft solvatochromic parameters α, β and π ∗ have been selected as microscopic solvent descriptors, and solvent groups have been designed according to the results obtained from the application of several multivariate clustering techniques. The present scheme is compared with Snyders triangle, an earlier solvent representation used extensively in chromatography. Analogies and differences which arise from the field of application and from the solvent descriptors used in each classification scheme are discussed.

Factor analysis applied to the correlation between dissociation constants and solvatochromic parameters in water-acetonitrile mixtures: II. Study of solvent effects on the dissociation of functional groups of diuretics, quinolones, buffers and peptides

Abstract The acid dissociation constants of series of substances with differing structural features in several acetonitrile-water mixtures were subjected to factor analysis (FA). A further target factor analysis (TFA) indicated that the Kamlet and Taft general equation was reduced in these mixtures to two or three terms: the independent term and that related to the polarity/polarizability π ∗ solvatochromic parameter, and additionally, to the hydrogen bond acceptor basicity β solvatochromic parameter. Thus, relationships between p K values and π ∗ or π ∗ and β values showing excellent correlations were established, which allowed us to calculate the p K values of substances in any acetonitrile-water mixture up to 70% (w/w). Also, the variation in the p K value with the percentage of acetonitrile in the hydro-organic mixtures could be explained in terms of the preferential solvation by water of electrolytes in the mixture studied.

Solvent effects on protonation equilibria of peptides and quinolones by factor analysis applied to the correlation between dissociation constants and solvatochromic parameters in acetonitrile-water mixtures

Abstract Dissociation constant values of series of peptides and quinolones in 5.54, 10, 16.3, 25 and 50% (w/w) acetonitrile-water mixed solvents at 298.15 K were, firstly, submitted to factor analysis in order to obtain the number of factors which affects the variation of the whole data sets and, afterwards, to target factor analysis to identify these factors. The p K values were correlated with the Kamlet and Taft solvatochromic parameters (α, β and π ∗ ) of acetonitrile-water mixtures according to the results obtained through these chemometric techniques. The equation proposed allowed the calculation of p K values of peptides in any acetonitrile-water mixture up to 50% (w/w) and, therefore, the acid-base behaviour of these compounds in widely used acetonitrile-water media is to be known. Further, an overview of the effect of preferential solvation of electrolytes in acetonitrile-water mixtures on the values of the protonation constants in these media was also obtained.

Solvent effects on mobile phases used in liquid chromatography : factor analysis applied to protonation equilibria and solvatochromic parameters

Abstract Dissociation constant values of the acid components of the standard NIST scale buffer solutions and of acetate buffer in several hydroorganic mixtures (acetonitrile–water, tetrahydrofuran–water and methanol–water) were first subjected to factor analysis in order to obtain the number of factors that affect the variation of the whole data sets, and then to target factor analysis to identify these factors. The pKa values were correlated with the Kamlet and Taft solvatochromic parameters (α, β and π*) of the solvent mixtures according to the results obtained with these chemometric techniques. Two or three factors emerged, depending on the range of percentage of organic cosolvent in the mixture. Target factor analysis results showed that Kamlet–Taft equations were reduced in these mixtures: the independent term and π* solvatochromic parameter of polarity-polarizability appear up to 50% w/w of organic solvent, as does the hydrogen bond basicity β solvatochromic parameter when data up to 70% w/w were analyzed. Further, the quasi-lattice quasi-chemical theory has been applied to quantify the preferential solvation by water of electrolytes in acetonitrile–water, tetrahydrofuran–water and methanol–water mixtures. An overview of the effect of this preferential solvation on the values of the protonation constants in these media was also obtained.

Factor analysis applied to the study of the effects of solvent composition and nature of the inert electrolyte on the protonation constants in dioxane—water mixtures

Abstract The effect of solvent properties on protonation constants was studied for water-dioxane mixtures covering a wide range of solvent compositions, from data sets on several solutes with a variety of functional groups. The establishment of the model was carried out using different approaches: the use of a classical procedure, where the best multiparametric fit to the Kamelt and Taft equation was evaluated for each substance; and the application of combined factor analysis and target factor analysis to quantify and identify the factors affecting the variation of the whole data sets, without the need to postulate any a priori hypothetical model. Further, a preliminary overview of the influence of different inert electrolytes on the values of the solute protonation constants in this solvent mixture was also obtained through factor analysis.

Autoprotolysis in aqueous organic solvent mixtures. Water/alcohol binary systems

By use of a previously derived equation, the main factors that contribute to autoprotolysis are studied for different water/ alcohol mixtures. These factors are: autoionization of water, autoionization of the alcohol, and proton transfer from water to the alcohol or from the alcohol to water. The most important factors seem to be the proton transfer from the alcohol to water, and to a much lesser degree, the autoionization of water. The autoionization of the alcohol is only significant for pure alcohol. The different factors have been correlated with the solvent properties polarity and hydrogen-bond ability. An increase in these solvent properties favours autoionization and proton transfer, and therefore decreases the observed autoprotolysis pK value. The proposed equation and the parameters calculated allow an accurate estimation of the autoprotolysis pK value for any studied water/alcohol system.