Liposomes as carriers in electrokinetic capillary chromatography

Effect of liposome composition on β-blocker interactions studied by capillary electrokinetic chromatography

Liposome capillary electrokinetic chromatography was used to investigate the interactions between three β-blockers of different hydrophobicity and various liposome solutions. The studied β-blockers comprised alprenolol, propranolol, and carvedilol. The composition of the liposome solutions, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1,2-dioleoyl-sn-glycero-3-phos-phoethanolamine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine, and cholesterol in various molar ratios, was designed by a response surface methodology-central composite design approach. Subsequently, after conducting the liposome capillary electrokinetic chromatography experiments and determining the retention factors from the electrophoretic mobilities of the compounds, and further calculating the distribution coefficients, an analysis of variance was performed. After extracting the statistical models, optimal operational conditions were obtained based on the developed models. To further investigate the interactions between the β-blockers and the liposomes, nanoplasmonic sensing experiments were carried out on two different liposome systems. The overall results demonstrate the strong influence of cholesterol and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine on the distribution coefficients.

Determination of the Main Phase Transition Temperature of Phospholipids by Nanoplasmonic Sensing

Our study demonstrates that nanoplasmonic sensing (NPS) can be utilized for the determination of the phase transition temperature (T_m) of phospholipids. During the phase transition, the lipid bilayer undergoes a conformational change. Therefore, it is presumed that the T_m of phospholipids can be determined by detecting conformational changes in liposomes. The studied lipids included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Liposomes in gel phase are immobilized onto silicon dioxide sensors and the sensor cell temperature is increased until passing the T_m of the lipid. The results show that, when the system temperature approaches the T_m, a drop of the NPS signal is observed. The breakpoints in the temperatures are 22.5 °C, 41.0 °C, and 55.5 °C for DMPC, DPPC, and DSPC, respectively. These values are very close to the theoretical T_m values, i.e., 24 °C, 41.4 °C, and 55 °C for DMPC, DPPC, and DSPC, respectively. Our studies prove that the NPS methodology is a simple and valuable tool for the determination of the T_m of phospholipids.

Determination of lipid bilayer affinities and solvation characteristics by electrokinetic chromatography using polymer-bound lipid bilayer nanodiscs

Styrene-maleic acid polymer-bound lipid bilayer nanodiscs have been investigated and characterized by electrokinetic chromatography. Linear solvation energy relationship analysis was employed to characterize the changes in solvation environment of nanodiscs of varied belt to lipid ratio, belt polymer chemistry and molecular weight, and lipid composition. Increases in the lipid to belt polymer ratio resulted in smaller, more cohesive nanodiscs with greater electrophoretic mobility. Nanodisc structures with belt polymers of different chemistry and molecular weight were compared and showed only minor changes in solvent characteristics and selectivity consistent with changes in structure of the lipid bilayer. Seven phospholipid and sphingomyelin nanodiscs of different lipid composition were characterized. Changes in lipid head group structure had a significant effect on bilayer-solute interactions. In most cases, changes in alkyl tail structure had no discernible effect on solvation environment aside from those explained by changes in the gel-liquid transition temperature. Comparison to vesicles of similar lipid composition show only minor differences in solvation environment, likely due to differences in lipid composition and bilayer curvature. Together these results provide evidence that the dominant solute-nanodisc interactions are with the lipid bilayer and that head group chemistry has a greater impact on bilayer-solute interactions than alkyl tail or belt polymer structure. Nanodisc electrokinetic chromatography is demonstrated to allow characterization of solute interactions with lipid bilayers of varied composition.

Phospholipid bilayer affinities and solvation characteristics by electrokinetic chromatography with a nanodisc pseudostationary phase

Phospholipid bilayer nanodiscs composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and synthetic maleic acid-styrene copolymer belts have been introduced as a pseudostationary phase (PSP) in electrokinetic chromatography and demonstrated good performance. The nanodiscs provide a suitable migration range and high theoretical plate counts. Using this nanodisc pseudostationary phase, the affinity of the bilayer structure for probe solutes was determined and characterized. Good correlation is observed between retention factors and octanol water partition coefficients for particular categories of solutes, but the general correlation is weak primarily because the nanodiscs show stronger affinity than octanol for hydrogen bond donors. This suggests that a more appropriate application of this technology is to measure and characterize interactions between solutes and lipid bilayers directly. Linear solvation energy relationship analysis of the nanodisc-solute interactions in this study demonstrates that the nanodiscs provide a solvation environment with low cohesivity and weak hydrogen bond donating ability, and provide relatively strong hydrogen bond acceptor strength.

Lipophilicity and its relationship with passive drug permeation

In this review, we first summarize the structure and properties of biological membranes and the routes of passive drug transfer through physiological barriers. Lipophilicity is then introduced in terms of the intermolecular interactions it encodes. Finally, lipophilicity indices from isotropic solvent systems and from anisotropic membrane-like systems are discussed for their capacity to predict passive drug permeation across biological membranes such as the intestinal epithelium, the blood-brain barrier (BBB) or the skin. The broad evidence presented here shows that beyond the predictive power of lipophilicity parameters, the various intermolecular forces they encode allow a mechanistic interpretation of passive drug permeation.

Experimental modelling of drug membrane permeability by capillary electrophoresis using liposomes, micelles and microemulsions

Capillary electrophoresis (CE) was evaluated as an in-vitro format for experimental modelling of membrane permeability using only nanogram quantities of drug compounds. The rationale for the CE technique emanates from emulation of a lipid-like pseudo-stationary phase that governs separations mainly as a result of differences in molecular size, lipophilicity, hydrogen bonding and charge, all of which also have a strong influence on in-vivo drug absorption. By means of micellar, microemulsion and liposome electrolytes, the migration behaviour was studied at 37°C for 22 model drug compounds. The generated CE retention factor data were then compared with membrane permeability reference data. Both simple log D and more common Caco-2 cell parameters were evaluated. In addition, permeation through intestinal segments of rat ileum and rat colon was included. An improved correlation was obtained in the order: micellar<microemulsion<liposome systems. Although the correlation for the best liposome CE system was only R2 = 0.77, the evaluation results for all emphasized the strength and flexibility of CE for assessing specific drug-membrane interaction through tailor-made lipophilic media.

A review on applications of liposomes in textile processing

This review discusses the properties of liposomes and their role in the textile process, including textile preparation and dyeing. Liposomes have a surface activity effect due to a hydrophilic head group and hydrophobic hydrocarbon tail. Its preparations do not tend to foam, which advantageously distinguishes them from other textile auxiliaries. According to the carrier role of liposomes, they can be used in several textile processes such as textile finishing and dyeing with several types of dyes and fibers. Each application is discussed in this review paper. Several types of dyes are encapsulated by liposomes in the dyeing process and their presence indicates that they have retardant and leveling effects according to their gradual release of dyes. In addition, the presence of liposomes in the textile process can improve the mechanical properties of textile products, resulting in better wash fastness properties and leveling effect and handle properties. The best character of liposomes is a reduction in temperature of process resulting to save energy and they are environment degradable materials.

Cationic surfactants for micellar electrokinetic chromatography: 1. Characterization of selectivity using the linear solvation energy relationships model

MEKC and the linear solvation energy relationship (LSER) model have been applied to two series of cationic surfactants. The synthetic flexibility of the quaternary ammonium group is exploited to generate the two series, one consisting of linear substitutions and the other incorporating the ammonium into ring structures of varying size. The effects of the head group structure on the CMC, aggregation number, and electrophoretic properties of the surfactants were determined. These surfactants were also characterized with the LSER model, which allowed the contributions of five chemical factors to the interactions between solutes and the micelles to be evaluated. Trends were observed in the cohesivity and polarity of the linear surfactant series, with both increasing with the size of the head group. No trends in the LSER parameters were observed in the cyclic series, but the LSER results do show that the surfactants with cyclic head groups provide a significantly different solvation environment from the linear series. Additional trends were observed in the aggregation behavior and chromatographic properties of the surfactants. These included changes in the CMCs, aggregation numbers, EOF, and electrophoretic mobility of the micelles that correlate to changes in head group size.

Identification of PCR products using PNA amphiphiles in micellar electrokinetic chromatography

We present a method to identify single-stranded PCR products of varying lengths by hybridization of n-alkylated peptide nucleic acids (PNA amphiphiles) to the products, followed by separation with micellar electrokinetic chromatography (MEKC). These end-attached PNA amphiphiles (PNAA) partition to nonionic micelles in the running buffer (Triton X-100), linking the tagged DNA to the micellar drag-tag. This linkage shifts the electrophoretic mobility of a tagged component away from both untagged DNA and tagged DNA of different lengths. The mobility of the tagged DNA is established by its extent of partitioning to the micelle phase as well as its size relative to the attached micelle. A model is presented that can be used to determine the length of an unknown oligomer given an experimentally obtained mobility. We find that the collective action of micelles that transiently attach to the tagged DNA impart about the same hydrodynamic drag as covalently bound "drag-tags" of a similar size. With the use of the PNAA-MEKC method, PCR products of 88, 134, 216, and 447 bases are clearly resolved in less than 5 min. To our knowledge, this work represents the first use of surfactant micelles as drag-tags to separate DNA in capillary electrophoresis. Furthermore, the PNAA tag only attaches to DNA containing a target sequence, helping ensure that only the desired PCR products are analyzed.

Anionic phospholipid coatings in capillary electrochromatography

Anionic phospholipids phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylinositol (PI), and phosphatidylserine (PS) were examined for their effect on 1-palmitoyl-2-oleyl-sn-glycero-3-phosphatidylcholine (POPC)-containing liposomes used as coating material in capillary electrochromatography. Liposome solvent was N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES) buffer at pH 7.4 with and without 3 mM of CaCl2. The background electrolyte solution was HEPES buffer at pH 7.4. The net charge, size, and short-term stability of the liposomes were measured with a Zetasizer. Results showed that calcium interacts with all liposomes but most strongly with POPC/PA. The relative migration times, retention factors, and resolution of the model analytes (one cationic, three uncharged ions, and one anionic) were studied. All liposomes successfully coated the silica capillary. Without calcium the strongest interaction and best separation of the analytes were with the POPC/PI and POPC/PS coatings, while interactions with the POPC/PA coating were weak. Calcium enhanced the interactions of the model analytes with all coatings, and the interactions were then strongest with the POPC/PA coating. In the presence of calcium there appears to be a slight reorganization of the coating with increasing number of runs. Our results indicate strong interactions between calcium and the phosphate groups in phospholipids and demonstrate the significant role of the phospholipid polar head group in phospholipid coatings on silica surfaces.

Evaluation of bilayer disks as plant cell membrane models in partition studies

We have studied the partitioning of a set of phenolic compounds used as lignin precursor models into lipid bilayer disks and liposomes. The bilayer disks are open bilayer structures stabilized by polyethylene glycol-conjugated lipids. Our results indicate that disks generate more accurate partition data than do liposomes. Furthermore, we show that the partitioning into the membrane phase is reduced slightly if disks composed of 1,2-distearoyl-sn-glycero-3-phosphocholine and cholesterol are exchanged for disks with a lipid composition mimicking that of the root tissue of Zea mays L.

Electrokinetic chromatographic characterization of novel pseudo-phases based onN-alkyl-N-methylpyrrolidinium ionic liquid type surfactants

MEKC and linear solvation free energy relationships (LSFERs) have been used to characterize the solute distribution between water and self-assemblies formed from ionic liquid type mono-chain cationic surfactants containing a cyclic pyrrolidinium head group. Several features of the solvation environment afforded by these micellar solutions were found to be quite different from that of CTAB, a structurally analogous cationic surfactant with a conventional, acyclic quaternary ammonium head group. None of the LSFER coefficients were found to vary in any systematic way with increasing alkyl chain length for these unique surfactants. Overall, however, these surfactants display different behavior than do all known cationic detergents such as CTAB. In chemical terms, pseudo-phases formed by these N-alkyl-N-methylpyrrolidinium bromides interact more strongly with polar compounds and less strongly with compounds having nonbonding or pi-electrons and are more cohesive compared to the well-studied CTAB.

Cholesterol-rich membrane coatings for interaction studies in capillary electrophoresis: Application to red blood cell lipid extracts

The purpose was to develop a stable biological membrane coating for CE useful for membrane interaction studies. The effect of cholesterol (chol) on the stability of dipalmitoylphosphatidylcholine (DPPC) and sphingomyelin (SM) coatings was studied. In addition, a fused-silica capillary for CE was coated with human red blood cell (RBC) ghost lipids. Liposomes prepared of DPPC/SM with and without chol or RBC ghost lipids were flushed through the capillary and the stability of the coating was measured electrophoretically. Similar mixtures of DPPC/SM with and without chol were further studied by differential scanning calorimetry. The presence of phosphatidylcholine as a basic component in the coating solution of DPPC/SM/chol was found to be essential to achieve a good and stable coating. The results also confirmed the stability of coatings obtained with solutions of DPPC with 0-30 mol% of chol and SM in different ratios, which more closely resemble natural membranes. Finally, the electrophoretic measurements revealed that a stable coating is formed when capillaries are coated with liposomes of RBC ghost lipids.

Capillary electrophoresis of liposomes functionalized for protein binding

CE enabled assessing the attachment of hexa-histidine-tagged proteins to functionalized phospholipid liposomes. The liposomes were made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, phosphatidyl-ethanolamine, cholesterol and distearoyl-glycero-3-phosphoethanolamine-N-methoxy(polyethylene glycol) in a molar ratio of 29:26:40:5. The unilamellar vesicles, which had an average diameter of 170 nm, were labelled by inclusion of FITC-dextran for fluorescence detection. CE was carried out in poly(vinyl alcohol) (PVA)-coated capillaries at 25 degrees C with a BGE consisting of Tris-HCl (50 mM, pH 8.0). For conjugation of the liposomes with the proteins (soluble synthetic receptor fragments with molecular mass of 60 and 70 kDa, respectively), Ni(2+) was implanted into the vesicle surface by an anchor lipid containing a nitrilotriacetate acid (NTA) group as complexation agent for the metal ions. The difference in surface charge enabled the separation of the different species of interest by CE: plain vesicles, vesicles functionalised with Ni-NTA, vesicle-protein complexes and the species formed upon removal of the Ni-ions by complexation with EDTA. Loss of the Ni-ions resulted in the release of the proteins and the reappearance of the plain Ni-free NTA-liposome species in the electropherograms.

Cationic lipid vesicles as coating precursors in capillary electrochromatography: Separation of basic proteins and neutral steroids

1,2-Dioleyl-3-trymethylammoniumpropane (DOTAP) lipid vesicles were employed as coating precursors to obtain a semipermanent cationic lipid bilayer in silica capillary. The coating procedure was relatively fast and simple. Reliable results for the separation of four basic proteins (alpha-chymotrypsinogen A, ribonuclease A, cytochrome C, lysozyme) were obtained by using an acetate buffer under acidic conditions. The RSDs of the migration times were not higher than 0.5% run-to-run and about 1% day-to-day (3 days), while the RSDs of the peak areas were within 7% day-to-day (3 days). The day-to-day RSD of the EOF mobility of about 1%, confirmed that the DOTAP coating was stable for the separation of basic proteins, under acidic buffers. In addition to basic proteins the DOTAP coating was found suitable under acidic conditions for the repeatable separation of neutral steroids. The potential of DOTAP as a carrier in background electrolyte solution was studied.

Analysis of liposomes by capillary electrophoresis and their use as carrier in electrokinetic chromatography

This contribution reviews work about liposomes in the context of electrically driven separation methods in the capillary format. The discussion covers four topics. The one broaches the application of liposomes as pseudo-stationary phases or carriers in vesicle or liposome electrokinetic chromatography (EKC) in the way as microemulsions and micelles are used; it includes the chromatographic use of liposomal bilayers as stationary phases attached to the wall for capillary electrochromatography (CEC). The second topic is the characterization and separation of liposomes as analytes by capillary electrophoresis (CE). Then the determination of distribution coefficients and binding constants between liposomes and ligands is discussed, and finally work dealing with peptides and proteins are reviewed with lipid bilayers as constituents of the electrically driven separation system.

Interactions of drugs and an oligonucleotide with charged membranes analyzed by immobilized liposome chromatography

We studied the effect of charged lipids or detergent on the retention of drugs and an oligonucleotide by immobilized liposome chromatography to characterize solute-membrane interactions. This is a novel approach in analysis of oligonucleotide-liposome interactions. The charged lipids (phosphatidylserine or distearoyltrimethylammoniumpropane) or detergent (sodium dodecylsulfate) interacted electrostatically in a concentration-dependent matter with the solutes. The oligonucleotide ions presumably bound to the liposomes by multipoint interactions, which was saturable. Sodium dodecylsulfate seemed to affect the drug-membrane interactions more strongly than phosphatidylserine did, probably due to different positioning in the bilayer.

Dynamic process of phospholipid–protein interaction studied by capillary isoelectric focusing with whole-column imaging detection

Liposomes are vesicles formed by the aggregation of amphiphilic phospholipid molecules, which can mimic natural cell membranes. Interaction between liposome and protein is important for the structure and function of natural cell membranes. In this study, a CIEF method with whole-column imaging detection was developed for monitoring the dynamic process of phospholipid-protein interactions. The CIEF profiles at successive interaction times clearly displayed the formation of the different conjugates between phospholipid and protein at different stages. Due to the diversity of the chemical and physical properties of targeted proteins in this study (trypsin inhibitor, beta-lactoglobulin B, phosphorylase b, and trypsinogen), different dynamic processes of phospholipid-protein interactions were exhibited. The type of phospholipids played an important role in the dynamic process of phospholipid-protein interaction, as noted by the use of zwitterionic phospholipid (phosphatidylcholine) and acidic phospholipid (phosphatidylserine). Mechanisms involved in these interactions were discussed by monitoring the dynamic processes in this study.

Effect of surfactant counterion and organic modifier on the properties of surfactant vesicles in electrokinetic chromatography

Counterion and organic modifier are two parameters in EKC that can be varied in order to obtain improved solubility, selectivity, and efficiency. The effect of changing surfactant counterion and/or organic modifier on the chromatographic and electrophoretic properties of cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfate (SOS) vesicles is examined in EKC. The vesicles are prepared in a 1:3.66 cationic/ anionic mole ratio for a total surfactant concentration of 69 mM. The cationic CTAB is replaced by cetyltrimethylammonium chloride (CTAC) and the first use of CTAC/SOS vesicles is reported. The mean diameter of the CTAC/SOS vesicles is 96 nm while that of the CTAB/SOS vesicles is 85 nm. A class I modifier (2-amino-1-butanol) and a class II modifier (acetonitrile) have similar effects on the EOF, elution range, methylene selectivity, and the efficiency of the CTAB/SOS vesicles and the CTAC/SOS vesicles. Upon addition of 10% ACN, there is roughly a 10-fold increase in the efficiency of heptanophenone, a model hydrophobic compound, compared to the efficiency using unmodified vesicles. Linear free energy relationship (LFER) analysis using the Abraham solvation model is employed to characterize solute-vesicle interactions. The results suggest that organic modifier-vesicle interactions depend somewhat on the counterion.

Small diamines as modifiers for phosphatidylcholine/phosphatidylserine coatings in capillary electrochromatography

Greater stability of liposome coatings and improved resolution of model steroids in capillary electrochromatography (CEC) were sought by adding small diamines (ethylenediamine, diaminopropane, bis-tris-propane, or N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid, HEPES)) to the liposome solution before coating of fused silica capillaries. The phospholipid coatings consisted of 1 mM of 8:2 mol% phosphatidylcholine (PC)/phosphatidylserine (PS) and 5 mM of modifier in buffer solutions (acetate, phosphate, or Tris) at pH 4.0-7.4. The coating was based on a published procedure, and five steroids were used as neutral model analytes in evaluation of the coating. The results showed that under optimal conditions, the small linear diamines increased the packing density of anionic phospholipids, leading to improved separations. In addition, the choice of buffer for the liposome coating and separation appeared to influence the performance of the coatings. While buffers with amino groups take part in the phospholipid bilayer formation, buffers like phosphate may even have negative effect on coating formation. The factors affecting phospholipid coatings with diamines as modifiers are clarified.

Capillary electrophoresis-based analysis of phospholipid and glycosaminoglycan binding by human beta2-glycoprotein I

Human beta2-glycoprotein I (beta2gpI) is a phospholipid and heparin binding plasma glycoprotein involved in autoimmune diseases characterized by blood clotting disturbances (thrombosis) together with the occurrence of autoantibodies against beta2gpI. With the final goal of assessing autoantibody influence on binding interactions of beta2gpI we have studied the development of capillary electrophoresis (CE)-based assays for interactions of negatively charged ligands with beta2gpI. In the development of suitable conditions for analysis at neutral pH of this basic protein (pI about 8) we found the pH hysteresis behavior of fused silica surfaces useful since the protonated surface after an acid pre-wash counteracted protein adsorption efficiently in contrast to more laborious procedures including acrylamide/dimethylacrylamide coatings that did not permit analysis of this particular protein. This simple approach made estimates of heparin-beta2gpI interactions possible and the principle was shown also to work for detection of betagpI binding to anionic phospholipids. Utilizing the pH hysteresis effect may be a simple solution to the adsorption problems often encountered in analyses of proteins by CE.

Development and initial evaluation of PEG-stabilized bilayer disks as novel model membranes

We show in this study that stable dispersions dominated by flat bilayer disks may be prepared from a carefully optimized mixture of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), cholesterol, and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-5000] [PEG-DSPE(5000)]. By varying the content of the latter component, the average diameter of the disks can be changed in the interval from about 15 to 60 nm. The disks show excellent long-term stability, and their size and structure remain unaltered in the temperature range between 25 and 37 degrees C. The utility of the disks as artificial model membranes was confirmed and compared to uni- and multilamellar liposomes in a series of drug partition studies. Data obtained by isothermal titration calorimetry and drug partition chromatography (also referred to as immobilized liposome chromatography) indicate that the bilayer disks may serve as an attractive and sometimes superior alternative to liposomes in studies aiming at the investigation of drug-membrane interactions. The disks may, in addition, hold great potential for structure/function studies of membrane-bound proteins. Furthermore, we suggest that the sterically stabilized bilayer disks may prove interesting as carriers for in vivo delivery of protein/peptide, as well as conventional amphiphilic and/or hydrophobic, drugs.

Lipid vesicles in capillary electrophoretic techniques: characterization of structural properties and associated membrane-molecule interactions

This paper reviews the use of lipid vesicles as model membranes in capillary electrophoresis (CE). The history and utility of CE in the characterization of microparticles is summarized, focusing on the application of colloidal electromigration theories to lipid vesicles. For instance, CE experiments have been used to characterize the size, surface properties, enclosed volumes, and electrophoretic mobilities of lipid vesicles and of lipoprotein particles. Several techniques involving small molecules or macromolecules separated in the presence of lipid vesicles are discussed. Interactions between the analytes and the lipid vesicles - acting as a pseudostationary phase or coated stationary phase in electrokinetic chromatography (EKC) - can be used to obtain additional information on the characteristics of the vesicles and analytes, and to study the biophysical properties of membrane-molecule interactions in lipid vesicles and lipoproteins. Different methods of determining binding constants by EKC are reviewed, along with the relevant binding constant calculations and a discussion of the application and limitations of these techniques as they apply to lipid vesicle systems.

Influence of pH on formation and stability of phosphatidylcholine/phosphatidylserine coatings in fused-silica capillaries

The effect of pH on the formation and stability of phospholipid coatings in fused-silica capillaries in electrophoresis was investigated. A liposome solution consisting of 3 mM of 80:20 mol% phosphatidylcholine/phosphatidylserine (PC/PS) in N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES) buffer was used as coating material. The coating was prepared by a method described earlier and five steroids were used as neutral model analytes. First, the effect of pH of the coating solution on the formation and stability of phospholipid coatings was studied at pH 6.5-8.5. The pH of the background electrolyte (BGE) solution (HEPES) was either kept constant at pH 7.4 or made similar to the pH of the liposome coating solution. Results showed that attachment of the coating on the fused-silica wall mostly depends on the protonation of amines of the phospholipids and HEPES. The ability of the phospholipid coating to withstand changes in pH was then investigated by coating at pH 7.5 and separating steroids with acetic acid, 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS), HEPES, or glycine BGE, adjusted to pH between 4.5 and 10.8. The results showed that with use of BGE solution at pH 10.8, the separation of steroids was not successful and the electroosmotic flow was high because of leakage of the phospholipid coating during preconditioning of the capillary with BGE solution. There was no phospholipid leakage with a BGE solution of pH 4.5, indicating that the protonated form of the functional groups of PS and HEPES participating in the attachment of the phospholipid coating to the capillary play an essential role in the success of the coating.

Influence of cetyltrimethylammonium bromide on phosphatidylcholine-coated capillaries

Large unilamellar vesicles of egg-phosphatidylcholine (eggPC), a naturally occurring phospholipid, were used in capillary electrophoresis (CE) for semi-permanent coating of fused silica capillaries. The stability of the phospholipid coating was tested at different cetyltrimethylammonium bromide (CTAB) concentrations with and without CaCl(2) present in the coating solution. The effect of physical factors influencing the coating stability (e.g. duration of the coating time, storage temperature of the coating solution) were also studied. Standing overnight in background electrolyte (BGE) solution did not alter the eggPC phospholipid coating noticeably. The performance of the coating was tested with a mixture of basic proteins (lysozyme, ribonuclease A and alpha-chymotrypsinogen A). Highest efficiencies (over 200,000 plates m(-1)) were achieved when the capillary was filled for 15 h with a liposome solution containing both CTAB and CaCl(2).

Simple coating of capillaries with anionic liposomes in capillary electrophoresis

A new and relatively simple method was developed for coating of capillaries in electrophoresis with liposomes. The liposomes, with a diameter of about 100 nm, are large unilamellar vesicles prepared by extrusion. The liposomes contained 1-palmitoyl-2-oleyl-sn-glycero-3-phosphatidylcholine (POPC) or POPC with different proportions of bovine brain phosphatidylserine (PS) and cholesterol. They formed a bilayer structure on the silica surface enabling the separation of neutral compounds. The effectiveness of the coating in separation was evaluated with use of uncharged steroids as model compounds. The coating was also studied by measuring the electroosmotic flow. The best results, taking into consideration both separation and stability, were achieved with anionic 80:20 mol% POPC/PS liposomes. In addition, the effect of coating conditions on the results was investigated. Among the buffers studied [N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES), phosphate, tris(hydroxymethyl)aminomethane (Tris) and N-tris(hydroxymethyl)methylglycine (Tricine)], HEPES seemed to have a significant effect on the success of the coating. Successful separation of steroids was achieved only when HEPES buffer was used in the coating procedure and in the background electrolyte solution for the separation. With all other buffers the peaks of the model compounds overlapped.

Application of capillaries with minimized electroosmotic flow to the electrokinetic study of acidic drug-beta-oleoyl-gamma-palmitoyl-L-alpha-phosphatidyl choline liposome interactions

Interaction of a model set of common drugs varying widely in their polarity as well as in their chemical structure (salicylic acid, acetylsalicylic acid, ketoprofen, phenytoin and propranolol) with beta-oleoyl-gamma-palmitoyl-L-alpha-phosphatidyl choline (POPC) liposomes was investigated by means of capillary electrophoresis. Two phosphate buffers differing in their pH (50 mM, pH 7.5 and 9.2) were used both for liposome reconstitution and as background electrolytes for capillary electrophoresis using capillaries with minimised electroosmotic flow (EOF). The liposomes showed practically no electrophoretic mobility and formed a stable plug in the capillary. At alkaline pH (9.2), the polyimide coated capillary exhibited residual endoosmotic flow (the EOF marker appeared before the detection window around 40 min as compared to 2.2 min in the untreated capillary; attempts to reveal endoosmotic flow at pH 7.5 were unsuccessful). The concentration of the mixture of the test compounds was 50 microg/ml (except for ketoprofen concentration of which was 5 microg/ml due to the lower solubility of the drug), i.e. large enough to exceed the binding capacity of the injected liposome plug at least at the neutral pH (7.5) which consequently resulted in two regions in the electropherogram, namely that which contained the unbound species and that corresponding to the liposome (lipid)-bound fraction. On the other hand in runs done at high pH of the background electrolyte (9.2) the whole amount injected interacted with the liposomes. Acidic drugs and phenytoin were run with negative polarity at the injection site. It was documented that both at pH 7.5 and 9.2 the investigated solutes interacted with POPC liposomes, though at pH 7.5 the equilibrium between the bound and unbound drugs was in favor of the unbound species. On the contrary, at pH 9.2 binding was considerably stronger and only the liposome bound fraction was seen upon electrophoresis. The well-known instability of phenytoin at room temperature resulted in the formation of an acidic hydrolytic product which was strongly bound to liposomes at the higher pH value. While no binding of phenytoin could be established at pH 7.5, at pH 9.2 this compound was degraded (hydrolyzed) and its degradation product was clearly bound to liposomes. It has to be emphasized that binding experiments must be done separately for acidic/neutral and basic drugs; binding of acidic/neutral drugs must be done at reversed polarity, while in order to reveal binding of basic drugs, positive polarity at the injection site must be used.

Unilamellar liposomes covalently coupled on silica gel for liquid chromatography

Silica gel was used as a support for the covalent coupling of liposomes, which could overcome drawbacks of soft gel beads in column efficiency and separation speed. The influences of the concentration of added dimethylaminopyridine and reaction time on the chloroformate activation reaction of silica gel were investigated. Temperature and pH for covalent coupling of liposomes on the activated silica gel were also optimized. Experimental results indicated that the stability of the covalently coupled liposome columns was obviously superior to that of the noncovalently coated liposome columns but the selectivity of both columns was basically identical. Separation and analysis of a crude extract of a traditional Chinese medicine Ligusticum Wallichii and a mixture of small peptides on both columns further support this conclusion.

Rapid determination of liposome-water partition coefficients (Klw) using liposome electrokinetic chromatography (LEKC)

Liposome Electrokinetic Chromatography (LEKC) provides a simple and facile approach for determining liposome-water partition coefficients, Klw. LEKC is a Capillary Electrophoresis (CE) technique where liposomes are incorporated into a buffer solution and act as a pseudostationary phase providing sites of interaction for solutes. The retention factors of solutes in LEKC are directly proportional to Klw. This article describes how LEKC can be used to determine Klw for both neutral and charged solutes. The Klw values for a group of neutral aromatic compounds, beta-blockers, and other drugs are reported. In addition, the usefulness of two quantitative structure partition relationships (QSPR) for estimation of Klw is demonstrated. One is the logarithmic linear relationship between liposome water and octanol-water partition coefficients (Pow). The other is the Linear Solvation Energy Relationship (LSER). The calculated Klw values from the two QSPR agree nicely with the observed values and with one another.

Study on liposomes by capillary electrophoresis

Liposomes made of mixtures of zwitterionic and anionic lipids were investigated by means of capillary electrophoresis and dynamic light scattering. The influence of the molar lipid ratio and of the buffers, used in the running electrolyte solution, on the physical characteristics of the liposomes were investigated. Data on effective electrophoretic mobilities, total charges as well as sizes of the liposomes are given. In addition, examples on the use of liposomes as carriers in electrokinetic capillary electrophoresis for the separation of benzene derivatives, steroids, and phenols are shown.