Molecular Factors Influencing Retention on Immobilized Artificial Membranes (IAM) Compared to Partitioning in Liposomes and n -Octanol

Purpose. To assess the effect of molecular factors influencing retention on immobilized artificial membrane (IAM) high-performance liquid chromatography columns compared to liposomal partitioning and traditional n-octanol/water partition coefficients. Methods. IAM capacity factors were measured at pH 7.0 on an IAM.PC.DD2 stationary phase. Liposomal partitioning at pH 7.0 and n-octanol/water partition coefficients were measured using the pH metric method. Partitioning in egg-phosphatidylcholine (PhC) liposomes was also measured by equilibrium dialysis for a series of β-blockers. Results. For the ionized β-blockers, potentiometry and equilibrium dialysis yielded consistent partitioning data. For relatively large bases, IAM retention correlated well with PhC liposome partitioning, hydrophobic forces being mainly involved. For more hydrophilic compounds and for heterogeneous solutes, in contrast, the balance between electrostatic and hydrophobic interactions was not the same in the two systems. Hydrogen bonding, an important factor in liposomes partitioning, played only a minor role in IAM retention. Conclusions. Partitioning in immobilized artificial membranes depends on size, hydrophobicity, and charge. When hydrophobic interactions dominate retention, IAM capacity factors are well correlated with liposomal partitioning. On the contary, for hydrophilic solutes, the two systems do not yield the same information and are not interchangeabl

The relative partitioning of neutral and ionised compounds in sodium dodecyl sulfate micelles measured by micellar electrokinetic capillary chromatography

The rational use of micelles in quantitative structure-activity and quantitative structure-permeation relationships implies a good knowledge of the nature of recognition forces underlying solute-micelle association. The aims of this study were to unravel the intermolecular interaction forces responsible for the association of neutral and ionised compounds with negatively charged sodium dodecyl sulfate (SDS) micelles, using micellar electrokinetic capillary chromatography (MEKC). The MEKC capacity factors (log k(MEKC)) of 36 neutral model solutes were analysed by linear solvation free-energy relationships (LSERs). The results indicate that the size and H-bond acceptor strength of solutes are mainly responsible for their MEKC retention. Compared to n-octanol, the SDS micelles are more cohesive and stronger H-bond donors. Strong attractive electrostatic interactions govern solute-micelle association for positively charged compounds and micelles of the opposite charge, whereas repulsive electrostatic interactions occur between negatively charged solutes and micelles of the same charge. The capacity factors measured for the ionised forms of the acids and bases under study (log k(MEKC)(I)) indeed lie on two distinct plateau, about -1.0 for the former and about 2.0 for the latter and depend on the solute's charge more than on its chemical structure. Thus, the derivation of a diff(log k(MEKC)(N-I)) value, defined as the difference between the log k(MEKC) values of the neutral and charged species, strongly correlates with the respective log k(MEKC)(N) value and does not afford additional information

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