A method for measuring nonelectrolyte partition coefficients between liposomes and water

Nonane and Hexanol Adsorption in the Lamellar Phase of a Nonionic Surfactant: Molecular Simulations and Comparison to Ideal Adsorbed Solution Theory

Adsorption of n-nonane/1-hexanol (C9/C6OH) mixtures into the lamellar phase formed by a 50/50 w/w triethylene glycol mono-n-decyl ether (C10E3)/water system was studied using configurational-bias Monte Carlo simulations in the osmotic Gibbs ensemble. The interactions were described by the Shinoda-Devane-Klein coarse-grained force field. Prior simulations probing single-component adsorption indicated that C9 molecules preferentially load near the center of the bilayer, increasing the bilayer thickness, whereas C6OH molecules are more likely to be found near the interface of the polar and nonpolar moieties, swelling the bilayer in the lateral dimension. Here, we extend this work to binary C9/C6OH adsorption to probe whether the difference in the spatial preferences may lead to a synergistic effect and enhanced loadings for the mixture. Comparing loading trends and the thermodynamics of binary adsorption to unary adsorption reveals that C9-C9 interactions lead to the largest enhancement, whereas C9-C6OH and C6OH-C6OH interactions are less favorable for this bilayer system. Ideal adsorbed solution theory yields satisfactory predictions of the binary loading.

Automated Coarse-Grained Mapping Algorithm for the Martini Force Field and Benchmarks for Membrane-Water Partitioning

With a view to high-throughput simulations, we present an automated system for mapping and parameterizing organic molecules for use with the coarse-grained Martini force field. The method scales to larger molecules and a broader chemical space than existing schemes. The core of the mapping process is a graph-based analysis of the molecule's bonding network, which has the advantages of being fast, general, and preserving symmetry. The parameterization process pays special attention to coarse-grained beads in aromatic rings. It also includes a method for building efficient and stable frameworks of constraints for molecules with structural rigidity. The performance of the method is tested on a diverse set of 87 neutral organic molecules and the ability of the resulting models to capture octanol-water and membrane-water partition coefficients. In the latter case, we introduce an adaptive method for extracting partition coefficients from free-energy profiles to take into account the interfacial region of the membrane. We also use the models to probe the response of membrane-water partitioning to the cholesterol content of the membrane.

Light-triggered hydrophilic drug release from liposomes through removal of a photolabile protecting group

The antibiotic penicillin G cannot be completely incorporated into hydrophobic lipid-membranes owing to its hydrophilicity. Through modification with a hydrophobic and photolabile protecting group, penicillin G was effectively incorporated into liposomes and released by photoirradiation at 365 nm.

Penicillin G as an antibiotic was released from liposomes by increase of hydrophilicity by photocleavage of a hydrophobic protecting group.

Composition Dependence of Water Permeation Across Multicomponent Gel-Phase Bilayers

The permeability of multicomponent phospholipid bilayers in the gel phase is investigated via molecular dynamics simulation. The physical role of the different molecules is probed by comparing multiple mixed-component bilayers containing distearylphosphatidylcholine (DSPC) with varying amounts of either the emollient isostearyl isostearate or long-chain alcohol (dodecanol, octadecanol, or tetracosanol) molecules. Permeability is found to depend on both the tail packing density and hydrogen bonding between lipid headgroups and water. Whereas the addition of emollient or alcohol molecules to a gel-phase DSPC bilayer can increase the tail packing density, it also disturbed the hydrogen-bonding network, which in turn can increase interfacial water dynamics. These phenomena have opposing effects on bilayer permeability, which is found to depend on the balance between enhanced tail packing and decreased hydrogen bonding.

Structural basis for the interaction of the beta-secretase with copper

The β-secretase (BACE1) features a unique sulfur rich motif (M₄₆₂xxxC₄₆₆xxxM₄₇₀xxxC₄₇₄xxxC₄₇₈) in its transmembrane helix (BACE1-TM) which is characteristic for proteins involved in copper ion storage and transport. While this motif has been shown to promote BACE1-TM trimerization and binding of copper ions in vitro, the structural basis for the interaction of copper ions with the BACE1-TM is still not well understood. Using molecular dynamics (MD) simulations, we show that membrane embedded BACE1-TMs adopt a flexible trimeric structure that binds and conducts copper ions through variable coordination. In coarse-grained (CG) MD simulations, the spontaneous assembly of BACE1-TMs trimers results in a right-handed helix packing arrangement. In subsequent atomistic MD simulations the sulfur rich motif defines characteristic copper ion coordination sites along a constricted partially solvated axial pore. Sliding and tilting of BACE1-TMs along smooth A₄₅₉xxxA_463/464xxA₄₆₇ surfaces, facilitated by a central P472 induced kink, enables copper ions to alternate between different coordination sites, including the prominent C466 and M470. We shed light into the structural arrangement of BACE1-TM trimers and propose a mechanism for copper ion conduction that might also apply to other proteins involved in metal ion transport.

Formation of lipid membrane-incorporated small π-molecules bearing hydrophilic groups

Lipid membrane-incorporated π-conjugated guest molecules (LMIGs) have been classified into four categories, including stable LMIGs, the precipitation or dissolution of some of the guest molecules from the LMIGs and the formation of small aggregates.

Capacities of membrane lipids to accumulate neutral organic chemicals

Lipids have been considered as the predominant components for bioaccumulation of organic chemicals. However, differences in accumulation properties between different types of lipid (e.g., storage and membrane lipids) have rarely been considered. Moreover, in view of toxic effects on organisms, chemical accumulation specifically in biological membranes is of particular importance. In this review article, partition coefficients of 240 neutral organic compounds between liposomes (phospholipid membrane vesicles) and water (K(lipw)), reported in the literature or measured additionally for this work, were evaluated. Values of log K(lipw) and log K(ow) (octanol-water partition coefficients) differ by 0.4 on average. Polyparameter linear free energy relationships (PP-LFERs) can describe the log K(lipw) data even better (standard deviations = 0.28-0.31) than the log K(ow) model. Recent experimental data for highly hydrophobic compounds fit well to the PP-LFERs and do not indicate the existence of a previously postulated "hydrophobicity cutoff". Predictive approaches based only on the molecular structure (KOWWIN, SPARC, COSMOthermX, COSMOmic) were also evaluated for K(lipw) prediction. The PP-LFERs revealed that partition coefficients into membrane lipids can be two log units higher than those into storage lipids for H-bond donor compounds, suggesting that distinguishing between the two lipids is necessary to account for the bioaccumulation of these compounds, and that tissues rich in membrane lipids (e.g., kidneys, liver) instead of fat tissue can be the primary phase for accumulation.

Alcohol solubility in a lipid bilayer: Efficient grand-canonical simulation of an interfacially active molecule

We derive a new density-biased Monte Carlo technique which preserves detailed balance and improves the convergence of grand-canonical simulations of a species with a strong preference for an interfacial region as compared to the bulk. This density-biasing technique is applied to the solubility of "alcohol" molecules in a mesoscopic model of the lipid bilayer, a system which has anesthetic implications but is poorly understood.

Evolutionary combinatorial chemistry, a novel tool for SAR studies on peptide transport across the blood-brain barrier. Part 2. Design, synthesis and evaluation of a first generation of peptides

The use of high-throughput methods in drug discovery allows the generation and testing of a large number of compounds, but at the price of providing redundant information. Evolutionary combinatorial chemistry combines the selection and synthesis of biologically active compounds with artificial intelligence optimization methods, such as genetic algorithms (GA). Drug candidates for the treatment of central nervous system (CNS) disorders must overcome the blood-brain barrier (BBB). This paper reports a new genetic algorithm that searches for the optimal physicochemical properties for peptide transport across the blood-brain barrier. A first generation of peptides has been generated and synthesized. Due to the high content of N-methyl amino acids present in most of these peptides, their syntheses were especially challenging due to over-incorporations, deletions and DKP formations. Distinct fragmentation patterns during peptide cleavage have been identified. The first generation of peptides has been studied by evaluation techniques such as immobilized artificial membrane chromatography (IAMC), a cell-based assay, log Poctanol/water calculations, etc. Finally, a second generation has been proposed.

Biopartitioning micellar separation methods: modelling drug absorption

The search for new pharmacologically active compounds in drug discovery programmes often neglects biopharmaceutical properties as drug absorption. As a result, poor biopharmaceutical characteristics constitute a major reason for the low success rate for candidates in clinical development. Since the cost of drug development is many times larger than the cost of drug discovery, predictive methodologies aiding the selection of bioavailable drug candidates are of profound significance. This paper has been focussed on recent developments and applications of chromatographic systems, particularly those systems based on amphiphilic structures, in the frame of alternative approaches for estimating the transport properties of new drugs. The aim of this review is to take a critical look at the separations methods proposed for describing and predicting drug passive permeability across gastrointestinal tract and the skin.

A new look at the hemolytic effect of local anesthetics, considering their real membrane/water partitioning at pH 7.4

The interaction of local anesthetics (LA) with biological and phospholipid bilayers was investigated regarding the contribution of their structure and physicochemical properties to membrane partition and to erythrocyte solubilization. We measured the partition into phospholipid vesicles-at pH 5.0 and 10.5-and the biphasic hemolytic effect on rat erythrocytes of: benzocaine, chloroprocaine, procaine, tetracaine, bupivacaine, mepivacaine, lidocaine, prilocaine, and dibucaine. At pH 7.4, the binding of uncharged and charged LA to the membranes was considered, since it results in an ionization constant (pK(app)) different from that observed for the anesthetic in the aqueous phase (pK(w)). Even though it occurred at a pH at which there is a predominance of the charged species, hemolysis was greatly influenced by the uncharged species, revealing that the disrupting effect of LA on these membranes is mainly a consequence of hydrophobic interactions. The correlation between the hemolytic activity and the LA potency shows that hemolytic experiments could be used for the prediction of activity in the development of new LA molecules.

Determination of Vesicle–Water Partition Coefficients by Electrokinetic Chromatography: Study of Temperature Effect

Vesicular electrokinetic chromatography was used to investigate solute partitioning from the aqueous phase into dihexadecyl hydrogen phosphate (DHP) vesicles. Retention factors of neutral solutes are related to their partition coefficients between the aqueous phase and vesicles (K(vw)). The K(vw) of the aromatic test solutes were readily obtained from the slopes of the linear relationships between retention factors versus DHP concentrations. The technique offers the advantages of speed, automation, and small sample size for determination of partition coefficients. The K(vw) values of 43 uncharged solutes were measured at below as well as above the phase transition temperatures. The logarithms of partition coefficients (log K(vw)) of solutes at 71 degrees C (above T(c)) were slightly higher than those at 36 degrees C (below T(c)). The solvation characteristics of DHP were also studied using linear solvation energy relationships at the two temperatures.

Binding of small alcohols to a lipid bilayer membrane: does the partitioning coefficient express the net affinity?

The total vapor pressures at 26 degreesC of binary (water-alcohol) and ternary (water-alcohol-vesicle) systems were measured for six short chain alcohols. The vesicles were unilamellar dipalmitoyl phosphatidylcholine (DMPC). The data was used to evaluate the effect of vesicles on the chemical potential of alcohols expressed as the preferential binding parameter of the alcohol-lipid interaction, gamma23. This quantity is a thermodynamic (model-free) measure of the net strength of membrane-alcohol interactions. For the smaller investigated alcohols (methanol, ethanol and 1-propanol) gamma23 was negative. This is indicative of so-called preferential hydration, a condition where the affinity of the membrane for water is higher than the affinity for the alcohol. For the longer alcohols (1-butanol, 1-pentanol, 1-hexanol) gamma23 was positive and increasing with increasing chain length. This demonstrates preferential binding, i.e. enrichment of alcohol in the membrane and a concomitant depletion of the solute in the aqueous bulk. The measured values of gamma23 were compared to the number of alcohol-membrane contacts specified by partitioning coefficients from the literature. It was found that for the small alcohols the number of alcohol-membrane contacts is much larger than the number of preferentially bound solutes. This discrepancy, which is theoretically expected in cases of very weak binding, becomes less pronounced with increasing alcohol chain length, and when the partitioning coefficient exceeds approximately 3 on the molal scale (10(2) in mole fraction units) it vanishes. Based on this, relationships between structural and thermodynamic interpretations of membrane partitioning are discussed.

Surface active drugs: self-association and interaction with membranes and surfactants. Physicochemical and biological aspects

Many pharmacologically active compounds are of amphiphilic (or hydrophobic) nature. As a result, they tend to self-associate and to interact with biological membranes. This review focuses on the self-aggregation properties of drugs, as well as on their interaction with membranes. It is seen that drug-membrane interactions are analogous to the interactions between membranes and classical detergents. Phenomena such as shape changes, vesiculation, membrane disruption, and solubilization have been observed. At the molecular level, these events seem to be modulated by lipid flip-flop and formation of non-bilayer phases. The modulation of physicochemical properties of drugs by self-association and membrane binding is discussed. Pathological consequences of drug-membrane interaction are described. The mechanisms of drug solubilization by surfactants are reviewed from the physicochemical point of view and in relation to drug carrying and absorption by the organism.

Saturable ethanol binding in rat liver mitochondria

The binding of ethanol to rat liver mitochondria is shown to be saturable at physiologically relevant ethanol concentrations. This effect is reversible and is not observed in extracted mitochondrial phospholipids. Brief exposure of the mitochondria to heat abolishes saturable ethanol binding. Previously, saturable ethanol binding was reported in rat liver microsomes. Taken together, the studies indicate that saturable ethanol binding motifs may be widespread in cellular membranes. The possibility is raised that incomplete expression of the hydrophobic effect in membrane assembly results in the expression of amphipathic packing defects which display an affinity for and a sensitivity to ethanol. The presence of saturable binding modalities is reconciled with the long-standing consensus on the biodistribution of ethanol - that ethanol's interactions with tissue are negligible - on the grounds that the affinities of ethanol and of water for membranes are similar; consequently, free ethanol concentrations are insensitive to the presence of tissue despite significant ethanol binding. A fraction of the binding sites possess submillimolar affinities for ethanol consistent with published functional studies, both in vitro and in vivo, that reported submillimolar efficacies for ethanol.

Pathways involved in trifluoperazine-, dibucaine- and praziquantel-induced hemolysis

This work elucidates differences in the hemolytic pathway developed by the antipsychotic trifluoperazine (TFP), the local anesthetic dibucaine (DBC) and the antihelminthic praziquantel (PZQ). Their partition coefficients (P) were measured at pH 7.4 between n-octanol, microsomes, liposomes, erythrocyte ghosts and n-octanol/water. The effective drug:lipid molar ratios for the onset of membrane solubilization (ReSAT) and complete hemolysis (ReSOL) were calculated from the experimental P values and compared with a classical surface-active compound treatment Lichtenberg, D. Biochim. Biophys. Acta 821 (1985) 470-478[. The contribution of charged/uncharged forms of TFP and DBC for the hemolytic activity was also analyzed. In all cases the hemolytic phenomena could be related to the monomeric drug insertion into the membrane. Only for TFP at isosmotic condition lysis occurs at concentrations beyond the CMC of the drug, indicating that micellization facilitates TFP hemolytic effect, while DBC and PZQ reach a real membrane saturation at their monomeric form.

Retention-activity relationship studies of benzodiazepines by micellar liquid chromatography

Solute partitioning into lipid bilayers of biological membranes is the basis for drug and metabolite uptake, passive transport across membranes and bioaccumulation. In order to emulate in vitro the partitioning process in the biomembranes, different approaches have been proposed. The use of micellar solutions as mobile phases in reversed-phase liquid chromatography (micellar liquid chromatography, MLC) has proven to be valid in the prediction of the biological activities of local anesthetics, catecholamines and barbiturates. In this paper we focus our attention on benzodiazepines. The retention of benzodiazepines using different concentrations of Brij35 as micellar mobile phase in modified C(18) stationary phase was studied. Quantitative retention-activity relationships (QRAR) in MLC were investigated for these compounds. An adequate correlation between the capacity factors (log k) and the toxicity (log LD(50)) and anxiolitic activity (log ED(50)) of benzodiazepines was obtained.

Association of ethanol with lipid membranes containing cholesterol, sphingomyelin and ganglioside: a titration calorimetry study

The association of ethanol at physiologically relevant concentrations with lipid bilayers of different lipid composition has been investigated by use of isothermal titration calorimetry (ITC). The liposomes examined were composed of combinations of lipids commonly found in neural cell membranes: dimyristoyl phosphatidylcholine (DMPC), ganglioside (GM(1)), sphingomyelin and cholesterol. The calorimetric results show that the interaction of ethanol with fluid lipid bilayers is endothermic and strongly dependent on the lipid composition of the liposomes. The data have been used to estimate partitioning coefficients for ethanol into the fluid lipid bilayer phase and the results are discussed in terms of the thermodynamics of partitioning. The presence of 10 mol% sphingomyelin or ganglioside in DMPC liposomes enhances the partitioning coefficient by a factor of 3. Correspondingly, cholesterol (30 mol%) reduces the partitioning coefficient by a factor of 3. This connection between lipid composition and partitioning coefficient correlates with in vivo observations. Comparison of the data with the molecular structure of the lipid molecules suggests that ethanol partitioning is highly sensitive to changes in the lipid backbone (glycerol or ceramide) while it appears much less sensitive to the nature of the head group.

Contribution of trifluoperazine/lipid ratio and drug ionization to hemolysis

The interaction of the antipsychotic drug trifluoperazine (TFP) with membranes was investigated in terms of lipid phase perturbation. TFP partition coefficients (P) were measured by phase separation between octanol/water and model membranes/water. The profile of P values at pH 7.4 was: microsomes (7172+/-1229)>liposomes (1916+/-341)>erythrocyte ghosts (1380+/-429)>octanol (452+/-55). Hemolytic experiments showed a biphasic, protective (at lower concentrations) and hemolytic effect above the CMC (42 microM at pH 7.4) of the phenothiazine. By applying classical treatments for surface active compounds to the hemolytic curves, we could calculate P values in whole erythrocyte cells. The preferential binding of uncharged to charged TFP in the membrane was discussed, since it results in a ionization constant (pKapp) different from that observed in the aqueous phase (pK). The TFP ionization constant was decreased from 8.1 (in water) to 7.62 in the presence of membranes and almost the same ratio of charged/uncharged TFP species is present at physiologic pH. Taking into account the DeltapK, we calculated the average TFP partition coefficient between egg phosphatidylcholine liposomes and water, at pH 7.4 (Paverage=1432), which was well correlated with the measured one (Plip=1916). Paverage is highly influenced by the uncharged TFP species and the real base/acid ratio under physiologic conditions was discussed in terms of its possible role in the biological activity of TFP.

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 beta-adrenoceptor antagonist compounds (beta-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 beta-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 (rIAM.PC.DD = 0.8710 vs. rIAM.PC.MG = 0.9538), the C18 HPLC retention factors (rIAM.PC.DD = 0.8408 vs. rIAM.PC.MG = 0.9380), the liposome partition coefficients (rIAM.PC.DD = 0.8887 vs. rIAM.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.

Probing DNA-cationic lipid interactions with the fluorophore trimethylammonium diphenyl-hexatriene (TMADPH)

The aim of this study is to get a better understanding of DNA-cationic lipid complex formation and its characterization through the properties of the lipid assembly, using fluorescent probes known to have different locations in the vesicle bilayer, 1,6-diphenylhexa-1,3,5-triene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMADPH). The location of these two fluorescent probes in the membrane differs; the positive charge of TMADPH is localized close to the water/lipid interface and its fluorophore is present in the upper part of the acyl chain region while DPH (lacking polar group) is embedded deeper in the hydrophobic part of the bilayer. Unilamellar vesicles ( approximately 100 nm size) composed of N-(1-(2, 3-dioleoyloxy)-propyl)-N,N,N-trimethylammonium chloride (DOTAP) and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) as a helper lipid (at 1 : 1 mole ratio) were used as a model of cationic liposomes. Both linear and circular DNA gave almost identical results. DNA-/L+ (mole charge ratio of DNA negatively-charged phosphate to positively-charged lipid) ratios have large effects on the measured parameters. The effects monitored through TMADPH are much more striking than those obtained through the use of DPH, suggesting that the major DNA-lipid interaction occurs at the lipid/water interface. The fact that DNA induced much larger changes in TMADPH fluorescence intensity in H2O than in D2O suggests that the changes in the exposure of TMADPH to water and solvent relaxation effects are involved in the interaction. At DNA-/L+>/=1, fluorescence intensity increased concomitantly with a small increase in TMADPH fluorescence anisotropy without much affect in the size of the complex. At DNA-/L+<0.6, fluorescence quenching proportional to DNA-/L+ occurred, as well as a large increase in TMADPH fluorescence anisotropy and in complex size. These results suggest that at low DNA-/L+, negatively-charged DNA condenses positively-charged lipid headgroups, thereby inducing formation of lipid-ordered domains. This phase separation results in membrane defects at the lipid/water interface and increased exposure of the hydrophobic upper parts of the acyl chains to water, as indicated by the quenching of TMADPH. This leads to instability and aggregation/fusion of the DNA-lipid complexes. On the other hand, at DNA-/L+>/=1, the condensing effect is smaller, involving homogeneous lateral condensation of all the lipids, leading to a reduction in water content near the probe, and the DNA-lipid complexes are relatively small and stable.

Ethanol binding to a model carbohydrate, glycogen

The binding affinity of ethanol for carbohydrates is unknown. Glycoconjugates are postulated to be sensitive targets of ethanol action. The glycogen content of muscle, liver, and brain is sensitive to ethanol. To explore whether carbohydrates as a class have a specific affinity to bind ethanol, we measured the binding of ethanol and other small molecules to the carbohydrate glycogen. Ethanol binding was found to be weak. The polar alcohol, glycerol, bound to glycogen with a greater affinity than ethanol did. Other small polar molecules (methanol, sucrose, acetate, glycine, and dimethyl sulfoxide) also bound more strongly than ethanol did. Ethanol and glycerol binding were concentration independent. No evidence of saturable or specific sites for these alcohols was obtained. Water binding was determined and was in agreement with hydrodynamic measures. Water binding exceeded the binding of all solutes studied. The loosely structured water of hydration in glycogen apparently was able to accommodate polar solutes, but tended to exclude ethanol and, to a lesser extent, methanol. We conclude that carbohydrates as a class exhibit no strong affinity or specificity for ethanol.

Saturable ethanol binding in rat liver microsomes

The binding of ethanol to rat liver microsomes is shown to be saturable at clinically relevant ethanol concentrations, whereas this effect is not observed in extracted microsomal phospholipids. Brief exposure of the microsomes to heat abolishes saturable ethanol binding. Equilibrium binding data analysis, although only approximate in this context, suggests the presence of at least two groups of specific sites: high capacity sites with affinities near the pharmacological range and low capacity sites at lesser levels. The results indicate that the specificity of ethanol for tissue is considerably greater than previously recognized.

Alcohol binding to liposomes by 2H NMR and radiolabel binding assays: does partitioning describe binding?

Implicit within the concept of membrane-buffer partition coefficients of solutes is a nonspecific solvation mechanism of solute binding. However, (2)H NMR studies of the binding of (2)H(6)-ethanol and [1-(2)H(2)] n-hexanol to phosphatidylcholine vesicles have been interpreted as evidence for two distinct alcohol binding modes. One binding mode was reported to be at the membrane surface. The second mode was reported to be within the bilayer interior. An examination of the (2)H NMR binding studies, together with direct radiolabel binding assays, shows that other interpretations of the data are more plausible. The results are entirely consistent with partitioning (nonspecific binding) as the sole mode of alcohol binding to liposomes, in accord with our previous thermodynamic interpretation of alcohol action in phosphatidylcholine liposomes.

Development of a combined NMR paramagnetic ion-induced line-broadening/dynamic light scattering method for permeability measurements across lipid bilayer membranes

A combined method using NMR line-broadening for permeant lifetime determination and dynamic light scattering for vesicle size determination has been developed for the measurement of permeability coefficients of ionizable permeants across phospholipid:cholesterol large unilamellar vesicles. The method has been validated by examining its reproducibility and the influence of various factors that might affect the permeability measurements. The vesicle hydrodynamic diameter was varied between 0.1 and 0.2 micron by extruding multilamellar vesicles through polycarbonate membranes with different pore sizes (0.03-0.2 microns). For these large unilamellar vesicles, the normalized size distributions analyzed by the CONTIN method had standard deviations < 0.36, which led to errors in permeability coefficients < 10% as predicted from a theoretical model developed here. The permeability coefficient for acetic acid is independent of its concentration, vesicle hydrodynamic diameter, the concentration of Pr3+, and ionic strength over the ranges 0.01-0.2 M, 0.1-0.2 microns, 0.004-0.04 M, and 0.03-0.3, respectively. Membrane/water and decane/water partition coefficient measurements of acetic acid indicate that the effects of permeant binding onto the bilayer membrane and self-association are negligible within the permeant concentration range 0.01-0.2 M. The addition of Pr3+ ions induces vesicle fusion with rates increasing with temperature and decreasing with cholesterol concentration in the membranes. While the intravesicular resonance intensity for acetic acid decreases continuously with time due to vesicle fusion under certain conditions, the corresponding line width and chemical shift remain constant over the same period, highlighting an important advantage of this NMR method over those based on detecting net flux in response to a concentration gradient as there is no means in the latter experiments of discerning vesicle leakiness from passive diffusion rates. The effective chemical nature of a dimristoylphosphatidylcholine:cholesterol bilayer barrier microenvironment was explored by comparing the transport of two permeants, D-(-)-mandelic acid and phenylacetic acid, to their relative bulk solvent/water partition coefficients using three reference solvents (n-decane, 1,9-decadiene, and isoamyl alcohol). Using the NMR line-broadening method, the permeability coefficients for these two permeants were determined to be (2.9 +/- 0.4) x 10(-4) cm/s and (3.9 +/- 0.7) x 10(-2) cm/s, respectively, at 294 K and Xchol = 0.3. The incremental free energy of transport for the additional OH group in D-(-)-mandelic acid, delta delta G0 = +2.9 kcal/mol, resembles most closely that for the transfer of this group from water to 1,9-decadiene, suggesting that the barrier domain resides in the acyl chain region and is slightly more polar/polarizable than a saturated hydrocarbon, possibly due to the presence of a double bond in cholesterol and/or the proximity of the barrier domain to the hydrophilic interface.

Effect of ethanol-induced lipid interdigitation on the membrane solubility of Prodan, Acdan, and Laurdan

The effect of ethanol-induced lipid interdigitation on the partition coefficient (Kp) of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and its two derivatives, 6-acetyl-2-(dimethylamino)naphthalene (Acdan) and 6-lauroyl-2-(dimethylamino)naphthalene (Laurdan), in L-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles has been examined by a precipitation method over the ethanol concentration range of 0-1.8 M. At 20 degrees C and in the absence of ethanol, the Kp values for Acdan, Prodan, and Laurdan are 2.0 x 10(3), 2.8 x 10(4), and 4.7 x 10(6), respectively. This result suggests that the Kp of Prodan and its derivatives is not simply a linear function of the polymethylene units. As DPPC undergoes the ethanol-induced phase transition from the noninterdigitated to the fully interdigitated gel state, Kp for Prodan and Acdan decreases by a factor of 5 and 2, respectively, whereas Kp for Laurdan exhibits no detectable changes with ethanol. The differences in Kp are in parallel with the differences in the fluorescence emission spectra of these probes over the ethanol concentration range examined. Previous fluorescence and infrared data indicated that membrane perturbation caused by the probes increases in the order: Laurdan > Prodan > Acdan. Thus, the degree of membrane perturbation also seems to be in parallel with Kp. Among these three probes, Prodan fluorescence reflects most correctly the ethanol-induced lipid interdigitation. In conclusion, the partitioning of small solutes in lipid membranes is significantly reduced by ethanol-induced lipid interdigitation, probably as a result of an increased membrane surface density due to the increased intramolecular lipid acyl chain ordering and a tighter overall intermolecular packing.

Radiosensitization and radioprotection of E. coli by alcohols

The survival of E. coli K12 strain AB1157 and the isogenic repair-deficient mutant E. coli AB2480 (recA13, uvrA6) was measured after gamma-irradiation in the presence of various alcohols as well as after incubation and subsequent removal of the alcohols before irradiation. Irradiation in the presence of alcohols leads to the already known protection effect, which has been attributed to OH radical scavenging. However, it was not possible to explain the protection solely in terms of the reactivity of the OH radical with the various alcohols, because addition of some of the alcohols did not yield the expected survival values. It was found that incubation with and subsequent removal of various alcohols before irradiation led to radiosensitization. The degree of radiosensitization increases with the hydrophobicity of the alcohol. In the case of glycerol no radiosensitization was observed. We can conclude that alcohols protect predominantly by OH radical scavenging. The comparatively small protection of cell survival by the more hydrophobic alcohols can be attributed to the sensitizing effect of these alcohols.

Effects of the membrane action of tetralin on the functional and structural properties of artificial and bacterial membranes

Tetralin is toxic to bacterial cells at concentrations below 100 mumol/liter. To assess the inhibitory action of tetralin on bacterial membranes, a membrane model system, consisting of proteoliposomes in which beef heart cytochrome c oxidase was reconstituted as the proton motive force-generating mechanism, and several gram-positive and gram-negative bacteria were studied. Because of its hydrophobicity, tetralin partitioned into lipid membranes preferentially (lipid/buffer partition coefficient of tetralin is approximately 1,100). The excessive accumulation of tetralin caused expansion of the membrane and impairment of different membrane functions. Studies with proteoliposomes and intact cells indicated that tetralin makes the membrane permeable for ions (protons) and inhibits the respiratory enzymes, which leads to a partial dissipation of the pH gradient and electrical potential. The effect of tetralin on the components of the proton motive force as well as disruption of protein-lipid interaction(s) could lead to impairment of various metabolic functions and to low growth rates. The data offer an explanation for the difficulty in isolating and cultivating microorganisms in media containing tetralin or other lipophilic compounds.

Partitioning of (+-)-5,6-dihydro-6-phenyl-2-n-alkyl-imidazo- [2,1-b]thiazoles into large unilamellar liposomes: a steady-state fluorescence quenching study

The interaction of the tetramisole derivative (+-)-5,6-dihydro-6-phenyl-imidazo[2,1-b]thiazole and a number of its 2-n-alkyl homologues (-ethyl through -n-pentyl and -n-heptyl) with large unilamellar phosphatidylcholine/phosphatidylethanolamine/dipalmitoylphosphatidic acid (2:1:0.06, w/w) vesicles was studied by means of steady-state fluorescence quenching using 8-(2-anthryl)octanoic acid as membrane probe. Linear Stern-Volmer plots were obtained for each derivative, indicating dynamic quenching. The slopes of the plots decreased with increasing liposomal concentration. For four short-chain homologues (-H, -ethyl, -n-propyl and -n-butyl), the respective membrane partition coefficients Kp and bimolecular quenching rate constants kq were determined from the plots of the reciprocal of the apparent quenching rate constant (kappq)-1 against the lipid volume fraction alpha L of the liposomes. The partition coefficients increased with increasing chain-length of the tetramisoles. A linear relationship was found between the free energy of partitioning and the number of methylene units of the homologues (-delta G degrees per methylene group = 1.6 +/- 0.1 kJ mol-1). For the n-pentyl and n-heptyl derivatives, the fluorescence quenching technique did not allow one to determine their membrane partition coefficients. Analysis of the fluorescence intensity measurements with Scatchard plots gave further evidence for the partitioning nature of the tetramisole derivatives' association with the liposomal membranes.

Partitioning behaviour of 1-hexanol into lipid membranes as studied by deuterium NMR spectroscopy

Deuterium nuclear magnetic resonance (NMR) spectroscopy was used to study the partitioning behaviour of 1-hexanol specifically deuterated in the alpha-position into model lipid bilayers. In all systems studied, the observed deuterium NMR lineshapes were time-dependent. Initially, 1-hexanol-d2 gave rise to an isotropic deuterium resonance with a different chemical shift from that of aqueous 1-hexanol-d2. After equilibration over a period of days, a broader spectral component characteristic of a spherically-averaged powder-pattern was observed. The quadrupole anisotropy of the 1-hexanol-d2 giving rise to the broad spectrum depended upon the cholesterol content of the membrane. From quantitation of the anisotropic to isotropic deuterium NMR spectra, the partition coefficients of 1-hexanol-d2 in a number of bilayer systems (asolectin and phosphatidylcholine bilayers (the latter with and without cholesterol] were determined. The partitioning of 1-hexanol-d2 into red blood cell membranes, and a suspension of lipids extracted from red blood cell membranes, was also examined. It is suggested that 1-hexanol, and probably other lipophiles, can partition to either the bilayer surface or the bilayer interior in a time-dependent manner.