Immobilized artificial membrane liquid chromatography: proposed guidelines for technical optimization of retention measurements

The objectives of this study were to establish guidelines for the proper measurement of capacity factors (log k(IAMw) on immobilized artificial membrane (IAM) stationary phases. In this context, some aspects related to the extrapolation of log(kIAMw) values, the stability and properties of IAM.PC.DD2 stationary phases and the column-to-column variability are discussed. No significant difference was observed when using either acetonitrile or methanol for the linear extrapolation of log k(IAM) values. However, methanol seems more appropriate when working with ionized compounds. Plotting isocratic capacity factors against the percentage (v/v) of co-solvent instead of the mole fraction leads to more reliable log k(AMW) values. Furthermore, our results with a YMC ODS-AQ and an IAM.PC.DD2 HPLC column indicate that only small differences arise between extrapolated capacity factors when using the (w(w))pH or the (s(w))pH operational scale and correcting or not the ionic strength for dilution caused by the co-solvent. The use of the (s(w))pH scale is recommended when working with ionized compounds in order to avoid parabolic relationships during linear extrapolation. The pH-dependent retention of three ionizable drugs on an IAM.PC.DD2 phase showed that secondary interactions with the charged moieties of the chromatographic surface affect the retention of ionized compounds around physiological pH. Finally, it was shown that column ageing occurs also with IAM.PC.DD2 stationary phases and that it depends on the column as well as on the investigated analyte. The intra-batch variability for IAM.PC.DD2 phases was small, whereas a marked and solute-dependent batch-to-batch variability was apparent.     

Quantitative structure-retention relationship studies using immobilized artificial membrane chromatography I: amended linear solvation energy relationships with the introduction of a molecular electronic factor

Linear solvation energy relationships (LSERs) amended by the introduction of a molecular electronic factor were employed to establish quantitative structure-retention relationships using immobilized artificial membrane (IAM) chromatography, in particular ionizable solutes. The chromatographic indices, log k(IAM), were determined by HPLC on an IAM.PC.DD2 column for 53 structurally diverse compounds, including neutral, acidic and basic compounds. Unlike neutral compounds, the IAM chromatographic retention of ionizable compounds was affected by their molecular charge state. When the mean net charge per molecule (delta) was introduced into the amended LSER as the sixth variable, the LSER regression coefficient was significantly improved for the test set including ionizable solutes. The delta coefficients of acidic and basic compounds were quite different indicating that the molecular electronic factor had a markedly different impact on the retention of acidic and basic compounds on IAM column. Ionization of acidic compounds containing a carboxylic group tended to impair their retention on IAM, while the ionization of basic compounds did not have such a marked effect. In addition, the extra-interaction with the polar head of phospholipids might cause a certain change in the retention of basic compounds. A comparison of calculated and experimental retention indices suggested that the semi-empirical LSER amended by the addition of a molecular electronic factor was able to reproduce adequately the experimental retention factors of the structurally diverse solutes investigated.     

Retention characteristics of an immobilized artificial membrane column in reversed-phase liquid chromatography

Retention for a varied group of compounds on an immobilized artificial membrane column (IAM PC DD2) with a methanol-water mobile phase is shown to fit a second-order model for the retention factor (log k) as a function of the volume fraction of organic solvent. The numerical value of the intercept obtained by linear extrapolation to zero organic solvent (log k(w)) is shown to depend on the range of mobile phase composition used for the extrapolation. Each series of intercepts so obtained represents a different hypothetical distribution system as identified by the system constants of the solvation parameter model. Although a linear model is a poor fit for isocratic retention data, the linear solvent strength gradient model provides a reasonable estimate of isocratic retention factor values that are (slightly) larger than experimental values, but provide the same chemical information for the system. These preliminary results suggest that gradient elution may prove to be a rapid and useful method for creating system maps for column characterization and method development. In this work a system map is provided for methanol-water compositions from 0 to 60% (v/v) methanol and additional system constants for acetonitrile-water compositions containing 20 and 30% (v/v) acetonitrile. It is shown that the main factors contributing to retention on the IAM PC DD2 column are favorable cavity formation and dispersion interactions, electron lone pair interactions and the hydrogen-bond basicity of the sorbent. The latter feature more than any other distinguishes the IAM column from conventional chemically bonded phases. Interactions of a dipole-type (weakly) and inability to compete with the mobile phase as a hydrogen-bond acid reduce retention. A comparison of system constant ratios is used to demonstrate that the retention properties of the IAM column are not easily duplicated by conventional chemically bonded phases. The retention characteristics of the IAM column, however, are strongly correlated with the retention properties of pseudostationary phases used for micellar electrokinetic chromatography, which provide a suitable alternative to IAM columns for physical property estimations. By the same comparative method it is shown that retention on the IAM column possesses some similarity to biomembrane absorption processes, allowing suitable correlation models to be developed for the estimation of certain biopartitioning properties.     

Electrostatic interactions and ionization effect in immobilized artificial membrane retention. A comparative study with octanol-water partitioning

The present study is a continuation of our efforts to investigate the effect of electrostatic interactions and ionization on immobilized artificial membrane (IAM) retention. The previous set of neutral and basic drugs was extended to include acids and ampholytes and analogous buffer conditions in the mobile phase were used, namely morpholinepropanesulfonic acid and phosphate buffer saline, adjusted at pH 7.4. The important contribution of electrostatic forces in IAM retention of positively charged species was further justified by the results of the present study, while analogous electrostatic interactions for ionized acidic drugs were not found to affect significantly the affinity for the IAM stationary phase. The critical role of shielding or exposure of the charged centers on the IAM surface, as a result of the effect of the aqueous component of the mobile phase, was evaluated by the use of water instead of buffer for a number of drugs. Measurements at pH 5.0 demonstrated the effect of ionization in IAM retention despite the partial compensation by electrostatic interactions in the case of protonated basic drugs. Silanophilic interactions were also found to play a potential role as secondary interactions in IAM retention. IAM chromatographic indices were compared to octanol-water distribution coefficients and the corresponding relationships established. Finally, solvation analysis was applied in the aim to gain insight in the balance of forces between IAM retention and octanol-water partitioning. The results showed that apart from electrostatic interactions, there is no significant differentiation between the two systems.     

Retention characteristics of porous graphitic carbon in subcritical fluid chromatography with carbon dioxide-methanol mobile phases

Numerous relationships usually used in high-performance liquid chromatography (HPLC) for describing the retention on porous graphitic carbon (PGC) have been applied in subcritical fluid chromatography, with CO2-methanol mobile phases. As reported in HPLC, octanol-water partition coefficient failed to fit the retention, whereas satisfactory results were obtained with the sum of partial negative charges. A better fit was reached by using the solvation parameter model, allowing a better understanding of the interactions developed between the solute, the stationary and the mobile phases. Results show that the dominant contribution to retention was given by the polarizability (E) and the volume (V), while the hydrogen-bond basicity (B) was not selected in the retention model, whatever the methanol content. The increase in methanol percentage favours the retention decrease, mainly through the volume for hydrophobic compounds, and through the hydrogen-bond acidity for polar compounds.     

Selectivity of stationary phases in reversed-phase liquid chromatography based on the dispersion interactions

Selectivity of 15 stationary phases was examined, either commercially available or synthesized in-house. The highest selectivity factors were observed for solute molecules having different polarizability on the 3-(pentabromobenzyloxy)propyl phase (PBB), followed by the 2-(1-pyrenyl)ethyl phase (PYE). Selectivity of fluoroalkane 4,4-di(trifluoromethyl)-5,5,6,6,7,7,7-heptafluoroheptyl (F13C9) phase is lowest among all phases for all compounds except for fluorinated ones. Aliphatic octyl (C8) and octadecyl (C18) phases demonstrated considerable selectivity, especially for alkyl compounds. While PBB showed much greater preference for compounds with high polarizability containing heavy atoms than C18 phase, F13C9 phase showed the exactly opposite tendency. These three stationary phases can offer widely different selectivity that can be utilized when one stationary phase fails to provide separation for certain mixtures. The retention and selectivity of solutes in reversed-phase liquid chromatography is related to the mobile phase and the stationary phase effects. The mobile phase effect, related to the hydrophobic cavity formation around non-polar solutes, is assumed to have a dominant effect on retention upon aliphatic stationary phases such as C8, C18. In a common mobile phase significant stationary phase effect can be attributed to dispersion interaction. Highly dispersive stationary phases such as PBB and PYE retain solutes to a significant extent by (attractive) dispersion interaction with the stationary phase ligands, especially for highly dispersive solutes containing aromatic functionality and/or heavy atoms. The contribution of dispersion interaction is shown to be much less on C18 or C8 phases and was even disadvantageous on F13C9 phase. Structural properties of stationary phases are analyzed and confirmed by means of quantitative structure-chromatographic retention (QSRR) study.     

Salt effects on the interaction of an amphiphilic model molecule with immobilized phosphatidylcholine monolayers

The retention of hydrocortisone (used as an amphiphilic model solute) on an immobilized artificial membrane (IAM) column was investigated in relation to the mobile phase concentration of three sodium salts (representing different rankings in the Hofmeister series, i.e. perchlorate, chloride and sulfate) in order to provide insight into the nature of the solute interactions with phospholipid monolayers. The influence of the salt series on solute retention was found to follow the Hofmeister series, emphasizing the role of hydrophobic effect in the solute retention mechanism on phospholipid monolayers. Retention models based on the extended Wyman relations (preferential interaction theory) were developed to analyze more quantitatively the salt effects on the hydrocortisone retention factor. This analysis as well as additional thermodynamic study suggested that the hydrocortisone binding to IAM involved both an insertion into the hydrophobic inside governed by hydrophobic effects and contacts with the interfacial region implying interactions such as van der Waals interactions/hydrogen bonds between the solute hydroxyl groups and the polar headgroups of phospholipidmonolayers.     

Comparison of the retention behavior of β-blockers using immobilized artificial membrane chromatography and lysophospholipid micellar electrokinetic chromatography

We have demonstrated that significant differences exist between the retention of eight β-blockers analyzed with immobilized artificial membrane (IAM) and lysophospholipid micellar electrokinetic capillary (LMEKC) chromatographic methods. The general retention trends are maintained with highly hydrophilic compounds such as atenolol eluting first and more hydrophobic compounds such as propranolol eluting last. The retention order, however, is different and would result in major ranking differences. LMEKC demonstrates a better correlation with liposomal partitioning (R²=0.95) than does IAM chromatography (R²=0.60). LMEKC, with its higher efficiency, can allow a more specific evaluation of lipophilicity than IAM chromatography and is useful in the analysis of pharmaceutical candidates, particularly for ranking purposes.     

Combined supercritical fluid chromatographic methods for the characterization of octadecylsiloxane-bonded stationary phases

In this paper, we present a combination of a key-solute test based on retention and separation factors of large probe solutes (carotenoid pigments) and a quantitative structure-retention relationship analysis based on the retention factors of small probe solutes (aromatic compounds) to investigate the different chromatographic behavior of octadecylsiloxane-bonded stationary phases of all sorts: classical, protected against silanophilic interactions or not, containing polar groups (endcapping groups or embedded groups). Varied chemometric methods are used to enlighten the differences between the 27 phases tested. The results indicate that the two approaches chosen (carotenoid test and solvation parameter model) are complementary and provide precise information on the chromatographic behavior of ODS phases.     

Combined supercritical fluid chromatographic tests to improve the classification of numerous stationary phases used in reversed-phase liquid chromatography

In this paper, we present a combination of a key-solute test based on retention and separation factors of large probe solutes (carotenoid pigments) and a quantitative structure-retention relationship analysis based on the retention factors of small probe solutes (aromatic compounds), both performed in supercritical fluid chromatography, to investigate the different chromatographic behaviour of octadecylsiloxane-bonded stationary phases of all sorts: classical, protected against silanophilic interactions or not, containing polar groups (endcapping groups or embedded groups). The results indicate that the two approaches chosen (carotenoid test and solvation parameter model) are complementary and provide precise information on the chromatographic behaviour of ODS phases. The applicability of the classification to the selection of stationary phases is evidenced with some examples of separations.     

Biomimetic chromatographic analysis of selenium species: Application for the estimation of their pharmacokinetic properties

The retention behavior of selenites, selenates, seleno-dl-methionine, selenocystine, selenocystamine, selenourea, dimethyl selenide, and dimethyl diselenide was investigated by means of biomimetic liquid chromatography. For this purpose, two immobilized artificial membrane (IAM) columns, namely, IAM.PC.DD2 and IAM.PC.MG, and two immobilized plasma protein columns, human serum albumin (HSA) and α₁-acid glycoprotein (AGP) columns, were employed using different mobile phase conditions in respect to pH and buffer composition. In general, satisfactory interrelations between retention factors obtained with the two IAM stationary phases and HSA/AGP columns were obtained. Large differences were observed between biomimetic retention factors and octanol–water logD values, since the latter fail to describe electrostatic interactions. In contrast, despite the column diversity, the net retention outcome on all four biomimetic columns was quite similar, especially in the presence of phosphate-buffered saline, which by its effective shielding alleviates the differences between the stationary phases. Of the two IAM columns, IAM.PC.DD2 showed better performance when compared with HSA and AGP columns as well as to octanol–water partitioning. Biomimetic chromatographic indices were further used to estimate the percentage of human oral absorption and plasma protein binding of the eight selenium species investigated, according to equations previously reported in the literature. The estimated values of human oral absorption imply moderate absorption only for dimethyl diselenide, which also may exhibit considerable plasma protein binding. Moderate affinity for plasma proteins should also be expected for dimethyl selenide and selenocystamine.

Evaluation of the immobilized artificial membrane phosphatidylcholine: Drug discovery column for high-performance liquid chromatographic screening of drug–membrane interactions

Chromatographic retention factors (k′) of a series of eight β-adrenoceptor antagonist compounds (β-adrenolytic drugs) were determined employing an immobilized artificial membrane column (IAM.PC.DD). The influence of mobile phase pH, ionic strength, and organic modifier composition was studied in order to examine column performance. After the IAM.PC.DD columns were exposed to approximately 7000 column volumes of a 0.01 M PBS mobile phase, five out of six columns tested showed significant peak broadening and decreased k′ values indicative of premature column failure. The data suggested that the immobilized phospholipids stationary phase was removed by the 0.01 M PBS mobile phase. The β-adrenolytic drug's log k′_IAM values obtained with an IAM.PC.DD column were compared to an ^esterIAM.PC.MG column for predicting drug membrane interactions. For the linear regression analysis between log k′_IAM and the logarithm of the n-octanol–water partition coefficients (r_IAM.PC.DD=0.8710 vs. r_IAM.PC.MG=0.9538), the C₁₈ HPLC retention factors (r_IAM.PC.DD=0.8408 vs. r_IAM.PC.MG=0.9380), the liposome partition coefficients (r_IAM.PC.DD=0.8887 vs. r_IAM.PC.MG=0.9187), and various pharmacokinetic parameters, significantly better correlations were obtained with the ^esterIAM.PC.MG column than the IAM.PC.DD column.     

Linear free energy relationship as a tool for characterization of three teicoplanin-based chiral stationary phases under various mobile phase compositions

Teicoplanin, teicoplanin aglycon, and methylated teicoplanin aglycon chiral stationary phases (CSPs) have been compared on the basis of the regression coefficients calculated from the linear free energy relationship (LFER) equation. The parameters have been obtained from the measurements of a set of 34 structurally diverse solutes. Influence of mobile phase composition - variation of methanol (MeOH) content - on the participation of different interactions types in the retention mechanism has been evaluated. Retention of the various interaction forces in analytes differs with both the CSP and the mobile phase composition. Hydrophobic interactions play a major role in mobile phases for high buffer contents. The more hydrophobic the CSP, the more important are they in the retention mechanism. With increase of MeOH contents in the mobile phase the major role in the interaction mechanism is shifted to more polar forces in which basicity and dipolarity/polarizability dominate. Although the LFER model does not address chiral aspects, we have attempted to explore the importance of the individual interactions in chiral discrimination of amino acids and their N-tert-butyloxycarbonyl derivatives.     

Modelling of the association mechanism of a series of rodenticide molecules with lipid membrane investigated by computational chemistry and biochromatography

The increase of pest rodents population in urban and rural areas is tackled by dissemination of baits poisoned with anticoagulant compounds. In order to modelize the cell membrane transport of these rodenticides, which have toxic effect on human keratynocytes and break the vitamin K cycle, a new general model based on the perturbation method was developed to describe the association process between these rodenticide and an immobilized artificial membrane (IAM). The thermodynamic functions of the rodenticide transfer from the bulk solvent to the IAM surface were also determined. The variation plots of the solute transfer data versus the salt concentration (x) in the bulk solvent allow to demonstrate that the rodenticides—IAM surface association mechanism was governed by both the hydrophobic effect and the van der Waals interactions/hydrogen bonds between the rodenticide polar groups with the polar headgroups of phospholipid monolayers (polar retention effect). This result was also corroborated by a comparison of the number of water molecules surrounded the rodenticide in the medium (obtained by computational chemistry) and the number of water molecule release at the IAM—rodenticide interface (obtained thanks to the Tanford's equation).     

Analysis and critical comparison of the reversed-phase and ion-exchange contributions to retention on polybutadiene coated zirconia and octadecyl silane bonded silica phases

The two major modes of retention of basic compounds in reversed-phase liquid chromatography on both octadecyl silane bonded silica-based (ODS) and polybutadiene coated zirconia (PBD-ZrO2) materials are hydrophobic and ion-exchange (Coulombic) interactions. Although the influence of reversed-phase and Coulombic interactions on the chromatography of organic cations is qualitatively well recognized, the quantitative relationship between hydrophobic and ion-exchange interactions remains unclear. In this work, the retention mechanisms on both of the above types of phases were probed by studying the retention of a homologous series of p-alkylbenzylamines as a function of the ammonium concentration in the eluent. The various columns tested were studied in terms of plots of retention factor vs. the inverse of the displacingcation concentration. The analysis of such plots as well as plots of log k' vs. number of methylene groups in the solutes and plots of log k' vs. log[NH4+] clearly shows that at least two types of sites--a pure reversed-phase site and a "hydrophobically-assisted ion-exchange site" similar to the type of site described by Neue [J. Chromatogr. A 925 (2001) 49] are needed to explain the observations. In addition, we have found a quantitative measure of the relative amount of reversed-phase and ion-exchange interaction on a given solute has on a given stationary phase which allows unambiguous classification of columns. It is now clear that ion-exchange contributions to retention on PBD-ZrO2, sometimes exceeding 90%, are even more important than previously thought and relative to hydrophobic interaction much more significant on PBD-ZrO2 than on ODS type-B silicas.