Application of perfluorinated acids as ion-pairing reagents for reversed-phase chromatography and retention-hydrophobicity relationships studies of selected β-blockers

The addition of the homologous series of perfluorinated acids-trifluoroacetic acid (TFAA), pentafluoropropionic acid (PFPA), heptafluorobutyric acid (HFBA) to mobile phases for reversed-phase high-performance liquid chromatography (RP-HPLC) of β-blockers was tested. Acidic modifiers were responsible for acidification of mobile phase (pH 3) ensuring the protonation of the β-blockers and further ion pairs creation. The effect of the type and concentration of mobile phase additives on retention parameters, the efficiency of the peaks, their symmetry and separation selectivity of the β-blockers mixture were all studied. It appeared that at increasing acid concentration, the retention factor, for all compounds investigated, increased to varying degrees. It should be stressed that the presence of acids more significantly affected the retention of the most hydrophobic β-blockers. Differences in hydrophobicity of drugs can be maximized through variation of the hydrophobicity of additives. Thus, the relative increase in the retention depends on either concentration and hydrophobicity of the anionic mobile phase additive or hydrophobicity of analytes. According to QSRR (quantitative structure retention relationship) methodology, chromatographic lipophilicity parameters: isocratic log k and log k_w values (extrapolated retention to pure water) were correlated with the molecular (log P_o/w) and apparent (log P_app) octanol–water partition coefficients obtained experimentally by countercurrent chromatography (CCC) or predicted by Pallas software. The obtained, satisfactory retention-hydrophobicity correlations indicate that, in the case of the basic drugs examined in RP-HPLC systems modified with perfluorinated acids, the retention is mainly governed by their hydrophobicity.     

Structure-retention and mobile phase-retention relationships for reversed-phase high-performance liquid chromatography of several hydroxythioxanthone derivatives in binary acetonitrile-water mixtures

The reversed-phase high-performance liquid chromatographic (RP-HPLC) behavior of some newly synthesized hydroxythioxanthone derivatives using binary acetonitrile-water mixtures as mobile phase has been examined. First, the variation in the retention time of each molecule as a function of mobile phase properties was studied by Kamlet-Taft solvatochromic equations. Then, the influences of molecular structure of the hydroxythioxanthone derivatives on their retention time in various mobile phase mixtures were investigated by quantitative structure-property relationship (QSPR) analysis. Finally, a unified model containing both the molecular structure parameters and mobile phase properties was developed to describe the chromatographic behavior of the systems studied. Among the solvent properties, polarity/polarizability parameter (π^*) and hydrogen-bond basicity (β), and among the solute properties, the most positive local charge (MPC), the sum of positive charges on hydrogen atoms contributing in hydrogen bonding (SPCH) and lipophilicity index (log P) were identified as controlling factors in the RP-HPLC behavior of hydroxythioxanthone derivatives in actonitrile-water binary solvents.     

Investigation of the chromatographic behaviour of some selenium species—Comparison with their octanol-water partitioning

The retention behaviour of selenites (Se(IV)), selenates (Se(VI)), seleno-dl-methionine (Se-Met), selenocystine (Se-Cyst), selenocystamine (Se-CM) and selenourea (Se-U) was investigated using a Discovery end-capped reversed-phase column as stationary phase and different mobile phase conditions. Extrapolated to 100% aqueous mobile phase retention factors (log k_w) of the investigated Se species, determined using different methanol fractions (φ) as organic modifier, were compared with the corresponding actual values. The proper operation of this column even at 100% aqueous phase proved to be valuable for the accurate determination of log k_w values of Se-CM and Se-Cyst, presenting a convex curvature log k = f(φ) at low MeOH fractions, often neglected in the extrapolation procedure. The effect of the presence of n-decylamine as well as saturation of the mobile phase with n-octanol was also studied. For ampholytic Se-Met and Se-Cyst the effect of n-decylamine in retention reflected the predominance of zwitterionic nature in the case of Se-Met in contrary to the non-zwitterionic species found in the case of Se-Cyst, in accordance with our previous findings concerning partitioning experiments in the n-octanol/water system. Finally, an attempt was made to correlate log k_w values with the logarithm of n-octanol/water distribution coefficient, log D, of the investigated Se species and an indicative log D value of Se-U was derived.     

Lipophilicity data for some preservatives estimated by reversed-phase liquid chromatography and different computation methods

The chromatographic behavior of some preservatives was performed by reversed-phase high-performance liquid chromatography on C18 (LiChroCART, Purosphere RP-18e), C8 (Zorbax, Eclipse XDB-C8), CN100 (Säulentechnik, Lichrosphere) and NH₂ (Supelcosil LC-NH₂) columns. The lipophilicity estimated for the first time on the first three columns are comparable and very well correlated. The mobile phase was a mixture of methanol–water (0.1% formic acid) in different volume proportions from 40% to 60% (v/v) for RP-C18, RP-C8 and RP-CN100 column (exception for parabens on RP-C8 column—the methanol concentrations being from 55% to 65%) and from 30% to 50% (v/v) for RP-NH₂. Highly significant correlations were obtained between different experimental indices of lipophilicity (log k_w, S, φ₀, mean of k and log k, and scores of k and log k corresponding to the first principal component) and computed log P values, and C8 column seems to be more suited for estimating the lipophilicity of the investigated compounds. These direct correlations offer a very good opportunity to derive powerful predictive models via Collander-type equations. The reliability of scores values as lipophilic indices is shown by their high correlation with the log K_ow obtained using classical “shake-flask” technique, log k_w and also some of the computed log P values. In addition, the results obtained applying PCA to the retention data may be used in interpreting the molecular mechanism of interactions between eluents and stationary phases with different polarity and to explain the chromatographic behavior of compounds. Finally, the “congeneric lipophilicity chart” described by the scores corresponding to the first principal component has the effect of separating compounds from each other more effectively from congeneric ((dis)similarity) point of view. The parabens and tert-butylhydroquinone appeared to be the most lipophilic preservatives.     

Characterization of Ascentis RP-Amide column: Lipophilicity measurement and linear solvation energy relationships

This study investigates lipophilicity determination by chromatographic measurements using the polar embedded Ascentis RP-Amide stationary phase. As a new generation of amide-functionalized silica stationary phase, the Ascentis RP-Amide column is evaluated as a possible substitution to the n  -octanol/water partitioning system for lipophilicity measurements. For this evaluation, extrapolated retention factors, log k^′_w$\text{log}{k^{\prime }}_w$, of a set of diverse compounds were determined using different methanol contents in the mobile phase. The use of n-octanol enriched mobile phase enhances the relationship between the slope (S  ) of the extrapolation lines and the extrapolated log k^′_w$\text{log}{k^{\prime }}_w$ (the intercept of the extrapolation), as well as the correlation between log P   values and the extrapolated log k^′_w$\text{log}{k^{\prime }}_w$ (1:1 correlation, r² = 0.966). In addition, the use of isocratic retention factors, at 40% methanol in the mobile phase, provides a rapid tool for lipophilicity determination. The intermolecular interactions that contribute to the retention process in the Ascentis RP-Amide phase are characterized using the solvation parameter model of Abraham. The LSER system constants for the column are very similar to the LSER constants of the n-octanol/water extraction system. Tanaka radar plots are used for quick visual comparison of the system constants of the Ascentis RP-Amide column and the n-octanol/water extraction system. The results all indicate that the Ascentis RP-Amide stationary phase can provide reliable lipophilic data.     

Correlation and prediction of solute transfer to chloroalkanes from both water and the gas phase

Data have been compiled from the published literature on the partition coefficients of solutes and vapors into chloroform, carbon tetrachloride, dichloromethane and 1-chlorobutane from both water and from the gas phase. The logarithms of the water-to-chloroalkane (log P) and gas-to-chloroalkane partition coefficients (log K) are correlated with the Abraham solvation parameter model. The derived correlations describe the observed log P and log K values within standard deviations of about 0.13–0.20 log units. For chloroform and carbon tetrachloride, the derived correlations were validated using training set and test set analyses.     

Retention of structurally diverse drugs in human serum albumin chromatography and its potential to simulate plasma protein binding

The retention behavior of 39 structurally diverse neutral, basic and acidic drugs was investigated on an HSA stationary phase using PBS buffer (pH 7.0) and acetonitrile or 2-propanol as organic modifiers. Extrapolated or directly measured log k_w values as well as isocratic retention factors were correlated with plasma protein binding data taken from the literature. Retention factors determined in the presence of 10% acetonitrile led to high quality 1:1 correlation with apparent log K_HSA values. The derived reference equation was successfully validated using a secondary set of 24 drugs. Further analysis of HSA retention into more fundamental properties revealed the involvement of anionic species in solute-stationary phase interactions, expressed by the negatively charged fraction, besides the partitioning mechanism which was reflected by lipophilicity. Protonation of basic drugs, although less important, may also influence retention, leading to reduced partitioning into the HSA surface as a net effect, while it seems to have no effect on HSA binding. The above results were further confirmed by linear solvation energy relationships (LSER).     

Spectral analysis of naringenin deprotonation in aqueous ethanol solutions

The deprotonation of 5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one (naringenin) was studied in aqueous solutions of ethanol and 0.1 mol L^?1 sodium perchlorate at 25°C. The chemical species that contributed to deprotonation were evaluated together with their pure spectral characteristics and concentration profiles by some chemometric methods. The deprotonation constants assigned by pK ₁, pK ₂, and pK ₃ were determined by multivariate curve analysis of spectral data at different pcH values. The pure spectral analysis concordant with the theoretical prediction of deprotonation constants indicates that the acidity of hydroxyl groups in naringenin decreases in the order: 7-OH, 4′-OH, 5-OH. The effects of the solvent on deprotonation were analysed in terms of the linear solvation energy relationships using the model of Kamlet, Abboud, and Taft (KAT). Multiple linear regressions were aimed towards correlating the deprotonation constants with the microscopic parameters containing hydrogen-bond acidity (α), dipolarity/polarisability (π*), and hydrogen-bond basicity (β). The most significant parameter was found to be the hydrogen-bond acidity of binary mixtures.     

Production and characterization of a broad-specificity polyclonal antibody for O,O-diethyl organophosphorus pesticides and a quantitative structure-activity relationship study of antibody recognition

Polyclonal antibody (PAb) with broad-specificity for O,O-diethyl organophosphorus pesticides (OPs) against a generic hapten, 4-(diethoxyphosphorothioyloxy)benzoic acid, was produced. The obtained PAb showed high sensitivity to seven commonly used O,O-diethyl OPs in a competitive indirect enzyme-linked immunosorbent assay (ciELISA) using a heterologous coating antigen, 4-(3-(diethoxyphosphorothioyloxy)phenylamino)-4-oxobutanoic acid. The 50% inhibition value (IC₅₀) was 348 ng mL⁻¹ for parathion, 13 ng mL⁻¹ for coumaphos, 22 ng mL⁻¹ for quinalphos, 35 ng mL⁻¹ for triazophos, 751 ng mL⁻¹ for phorate, 850 ng mL⁻¹ for dichlofenthion, and 1301 ng mL⁻¹ for phoxim. The limit of detection (LOD) met the ideal detection criteria of all the seven OP residues. A quantitative structure-activity relationship (QSAR) model was constructed to study the mechanism of antibody recognition using multiple linear regression analysis. The results indicated that the frontier-orbital energies (energy of the highest occupied molecular orbital, E_HOMO, and energy of the lowest unoccupied molecular orbital, E_LUMO) and hydrophobicity (log of the octanol/water partition coefficient, log P) were mainly responsible for the antibody recognition. The linear equation was log(IC₅₀) = −63.274E_HOMO + 15.985E_LUMO + 0.556 log P − 25.015, with a determination coefficient (r²) of 0.908.     

Acidity of osmium tetroxide (OsO4) towards coordination with pyridine and its derivatives

This article reports on the Lewis acidity of osmium tetroxide (OsO₄), the most reliable reagent available for the cis-hydroxylation of alkenes. Three simple and straightforward scales, donor ability of N-donors, the Hammett reaction constant and the Electrostatic-Covalent model (ECW) were used for this purpose. Equilibrium constants (K_eq) and enthalpies for the complexation of OsO₄ with pyridine and its derivatives have been evaluated by spectrophotometeric measurements. The excellent linear relationships were observed between the equilibrium constants and either the pK_a values of the N-donors or the Hammett σ constants of the substituents on the ligands. The slope of the plot of log K_eq versus ligand’s pK_a and the Hammett ρ value derived suggest the use of these parameters as an index of the Lewis acidity strength for OsO₄. The enthalpies of complex formation of N-donors with OsO₄ also fit the ECW model to predict the values of E_A and C_A parameters for OsO₄ as 2.01 and 0.25 (kcal/mol)^1/2, respectively. The parameters can be used to make an acidity scale for OsO₄. These parameters also permit the chemists and biochemists to predict and correlate quantitatively the enthalpies of OsO₄ · Lewis base interactions.     

Solvatochromic relationship: Prediction of distribution of ionic solutes in aqueous two-phase systems

Partition ratios of several ionic compounds in 20 different polymer/polymer aqueous two-phase systems (ATPS) containing 0.15 M NaCl in 0.01 M phosphate buffer, pH 7.4, were determined. The differences between the electrostatic properties of the phases in all the ATPS were estimated from partitioning of the homologous series of dinitrophenylated-amino acids. Also the solvatochromic solvent parameters characterizing the solvent dipolarity/polarizability (π*), solvent hydrogen-bond donor acidity (α), and solvent hydrogen-bond acceptor basicity (β) of aqueous media were measured in the coexisting phases of the ATPS. The solute-specific coefficients for the compounds examined were determined by the multiple linear regression analysis using the modified linear solvation energy relationship equation. The minimal number of ATPS necessary for determination of the coefficients was established and 10 ATPS were selected as a reference ATPS set. The solute-specific coefficients values obtained with this reference set of ATPS were used to predict the partition ratios for the compounds in 10 ATPS not included in the reference set. The predicted partition ratios values were compared to those determined experimentally and found to be in good agreement. It is concluded that the presented model of solute–solvent interactions as the driving force for solute partitioning in polymer/polymer ATPS describes experimental observations with 90–95% accuracy.     

Modelling of acid-base equilibria in binary-solvent systems: A comparative study

A new approach to modelling of some binary (hydro-organic, HL/H₂O + B) acid-base systems with organic co-solvent B fully miscible with water, is suggested and applied for the determination of acidity parameters pK for some weak acids HL. The models are designed to get the pK = pK(x) relationships (x—mole fraction of B in H₂O + B) from results of pH titrations made in such systems and for the determination of pK_B = pK(1) for HL in pure B. The Redlich-Kister equation, together with its asymmetric extensions, and the Legendre functions with orthogonal polynomials, appeared to be suitable for such purposes.