Effect of n-alcohols and glycerol on the pretransition of dipalmitoylphosphatidylcholine

Effect of polyols on the DMPC lipid monolayers and bilayers

In this study, the effect of polyols, erythritol, xylitol, mannitol, on a model membrane systems composed of DMPC was investigated using differential scanning calorimetry and Fourier transform infrared spectroscopy. Generally, it is considered that polyols possess strong hydrophilic properties, and either does not interact with the hydrophobic environment at all, or these interactions are very weak. To better understand the mutual interactions between polyols and the lipid system, the Langmuir technique was used to examine the molecular organization of monolayers and to calculate their thickness in the presence of polyols at the subphase. The detailed description of the interactions between polyols and DMPC molecules was complemented by the analysis of the morphology of monolayers with the application of Brewster angle microscopy. From ATR FTIR, the significant spectral shift is observed only for the PO₂^- stretching band, which correlates strongly with the polyol chain-length. The longer the polyol chain, the weaker the observed interactions with lipid molecules. The most important findings, obtained from thickness measurements, reveal that short-chain polyols may prevent the formation of bilayers by the DMPC molecules under high surface pressure. The changes in the organization of DMPC monolayers on the surface, as visualized by Brewster angle microscopy, showed that the domains observed for phospholipid film spread on pure water differ substantially from those containing polyols in the subphase.

Proliposome Powders for the Generation of Liposomes: the Influence of Carbohydrate Carrier and Separation Conditions on Crystallinity and Entrapment of a Model Antiasthma Steroid

Formulation effects on the entrapment of beclometasone dipropionate (BDP) in liposomes generated by hydration of proliposomes were studied, using the high-density dispersion medium deuterium oxide in comparison to deionized water (DW). Proliposomes incorporating BDP (2 mol% of the lipid phase consisting of soya phosphatidylcholine (SPC) and cholesterol; 1:1) were manufactured, using lactose monohydrate (LMH), sorbitol or D-mannitol as carbohydrate carriers (1:5 w/w lipid to carrier). Following hydration of proliposomes, separation of BDP-entrapped liposomes from the unentrapped (free) BDP at an optimized centrifugation duration of 90 min and a centrifugation force of 15,500g were identified. The dispersion medium was found to have a major influence on separation of BDP-entrapped liposomes from the unentrapped drug. Entrapment efficiency values were higher than 95% as estimated when DW was used. By contrast, the entrapment efficiency was 19.69 ± 5.88, 28.78 ± 4.69 and 34.84 ± 3.62% upon using D₂O as a dispersion medium (for LMH-, sorbitol- and D-mannitol-based proliposomes, respectively). The similarity in size of liposomes and BDP crystals was found to be responsible for co-sedimentation of liposomes and free BDP crystals upon centrifugation in DW, giving rise to the falsely high entrapment values estimated. This was remedied by the use of D₂O as confirmed by light microscopy, nuclear magnetic resonance (¹HNMR), X-ray diffraction (XRD) and entrapment studies. This study showed that carrier type has a significant influence on the entrapment of BDP in liposomes generated from proliposomes, and using D₂O is essential for accurate determination of steroid entrapment in the vesicles.

Ligand partitioning into lipid bilayer membranes under high pressure: Implication of variation in phase-transition temperatures

The variation in phase-transition temperatures of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane by adding two membrane-active ligands, a long-chain fatty acid (palmitic acid (PA)) and an inhalation anesthetic (halothane (HAL)), was investigated by light-transmittance measurements and fluorometry. By assuming the thermodynamic colligative property for the bilayer membrane at low ligand concentrations, the partitioning behavior of these ligands into the DPPC bilayer membrane was considered. It was proved from the differential partition coefficients between two phases that PA has strong affinity with the gel (lamellar gel) phase in a micro-molal concentration range and makes the bilayer membrane more ordered, while HAL has strong affinity with the liquid crystalline phase in a milli-molal concentration range and does the bilayer membrane more disordered. The transfer volumes of both ligands from the aqueous solution to each phase of the DPPC bilayer membrane showed that the preferential partitioning of the PA molecule into the gel (lamellar gel) produces about 20% decrease in transfer volume as compared with the liquid crystalline phase, whereas that of the HAL molecule into the liquid crystalline phase does about twice increase in transfer volume as compared with the gel (ripple gel) phase. Furthermore, changes in thermotropic and barotropic phase behavior of the DPPC bilayer membrane by adding the ligand was discussed from the viewpoint of the ligand partitioning. Reflecting the contrastive partitioning of PA and HAL into the pressure-induced interdigitated gel phase among the gel phases, it was revealed that PA suppresses the formation of the interdigitated gel phase under high pressure while HAL promotes it. These results clearly indicate that each phase of the DPPC bilayer membrane has a potential to recognize various ligand molecules.

Temperature-dependence of cationic lipid bilayer intermixing: possible role of interdigitation

In this work, we investigated the properties of a fusogenic cationic lipid, diC14-amidine, and show that this lipid possesses per se the capacity to adopt either an interdigitated structure (below and around its transition temperature) or a lamellar structure (above the transition temperature). To provide experimental evidence of this lipid bilayer organization, phospholipids spin-labeled at different positions of the hydrocarbon chain were incorporated into the membrane and their electron spin resonance (ESR) spectra were recorded at different temperatures. For comparison, similar experiments were performed with dimyristoyl phosphatidylcholine, a zwitterionic lipid (DMPC) which adopts a bilayer organization over a broad temperature range. Lipid mixing between diC14-amidine and asolectin liposomes was more efficient below (10-15 °C) than above the transition temperature (above 25 °C). This temperature-dependent "fusogenic" activity of diC14-amidine liposomes is opposite to what has been observed so far for peptides or virus-induced fusion. Altogether, our data suggest that interdigitation is a highly fusogenic state and that interdigitation-mediated fusion occurs via an unusual temperature-dependent mechanism that remains to be deciphered.

Effects of pentanol isomers on the phase behavior of phospholipid bilayer membranes

Differential scanning calorimetry (DSC) was used to analyze the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of pentanol isomers. The concentration of each pentanol isomer needed to induce the interdigitated phase was determined by the appearance of a biphasic effect in the main transition temperatures, the onset of a hysteresis associated with the main transition from the gel-to-liquid crystalline phase, and the disappearance of the pretransition. Lower threshold concentrations were found to correlate with isomers of greater alkyl chain length while branching of the alkyl chain was found to increase biphasic behavior. The addition of a methyl group to butanol systems drastically decreased threshold concentrations. However, as demonstrated in the DPPC/neopentanol system, branching of the alkyl chain away from the -OH group lowers the threshold concentration while maintaining a biphasic effect.

Properties of phosphatidylcholine in the presence of its monofluorinated analogue

In aqueous solution, the monofluorinated phospholipid 1-palmitoyl-2-[16-fluoropalmitoyl]sn-glycero-3-phosphocholine (F-DPPC) interdigitates without the use of inducing agents. To understand the thermal and physical properties of this unique lipid, F-DPPC was combined with the non-fluorinated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC). Differential scanning calorimetry (DSC) was used to determine the miscibility and thermotropic phase behavior of these binary lipid mixtures. In addition, the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) and a DPH-labeled analogue of DPPC, 2-(3-(diphenylhexatrienyl) propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (beta-DPH HPC, aka DPH-PC or DPHpPC), were used to detect interdigitation. In F-DPPC, the fluorescence intensity of both probes decreased a similar amount and to a degree that is consistent with an interdigitated system. We also determined that there are two separate effects of increasing the ratio of F-DPPC in the DPPC/F-DPPC system. With low amounts of F-DPPC, there is little evidence that the system is heavily interdigitated. Instead, we hypothesize that the introduction of F-DPPC provides nucleation sites that alter the kinetics, reversibility, and temperature of the main transition (T(m)). At higher mol% of F-DPPC, we propose that interdigitated F-DPPC-rich domains form to create a phase-segregated system. While DPPC/F-DPPC was highly miscible, the DAPC/F-DPPC system was significantly less miscible. Additionally, we observed that DAPC/F-DPPC samples have reduced solubility in water, which affected the acquisition of fluorescence data. However, our DSC results indicate the existence of DAPC-rich and F-DPPC-rich components. Furthermore, this data support that the mixing was disruptive to lipid packing and that the presence of DAPC hinders the interdigitation of F-DPPC.

Effects of butanol isomers on dipalmitoylphosphatidylcholine bilayer membranes

Differential scanning calorimetry and (31)P-NMR were used to study the effects of butanol isomers on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers. The threshold concentration for the onset of interdigitation for each isomer was determined by the disappearance of the pretransition and the onset of a large hysteresis between the heating and cooling scans of the gel-to-liquid main transition. The threshold concentration was found to correlate with increased solubility of the isomers in the aqueous phase, led by tert-butanol. However, as the solution concentration of tert-butanol increased, there was an abrupt shrinking of the hysteresis, initially with well-resolved shoulder peaks indicating mixed phases. The eventual disappearance of the shoulder peaks was correlated with a breakdown of the multilamellar structure identified using (31)P-NMR.

High sensitivity differential scanning calorimetry investigation of the interaction between liposomes, lactate dehydrogenase and tyrosinase

High sensitivity differential scanning calorimetry (HSDSC) has been used to study the interaction of the model proteins lactate dehydrogenase (LDH) and tyrosinase with dimyristoylphosphatidylcholine (DMPC) liposomes, and relate this to the thermal and physical stability of the proteins. On heating, both LDH and tyrosinase denatured irreversibly in a time-dependent manner and modified the phase transition behaviour of DMPC liposomes at all concentrations investigated. The most marked effects occurred for the pretransition rather than the main phospholipid phase transition. The effects on the bilayer are likely to result from electrostatic interactions of the hydrophilic proteins with the head-groups of DMPC molecules, whilst due to their hydrophilic nature they do not penetrate into the bilayer. Tyrosinase is more highly ionised than LDH at the pH of the investigation, which may explain why tyrosinase has a greater effect than LDH on the HSDSC scans at mg/ml protein concentrations.

Simulating induced interdigitation in membranes

In this study we introduce a mesoscopic lipid-water-alcohol model. Dissipative particle dynamics (DPD) simulations have been used to investigate the induced interdigitation of bilayers consisting of double-tail lipids by adding alcohol molecules to the bilayer. Our simulations nicely reproduce the experimental phase diagrams. We find that alcohol can induce an interdigitated structure where the common bilayer structure changes into monolayer in which the alcohol molecules screen the hydrophobic tails from the water phase. At low concentrations of alcohol the membrane has domains of the interdigitated phase that are in coexistence with the common membrane phase. We compute the effect of the chain length of the alcohol on the phase behavior of the membrane and show that the stability of the interdigitated phase depends on the length of the alcohol. We show that we can reproduce the experimental hydrophobic thickness of the bilayer for various combinations of lipids and alcohols. We use our model to clarify some of the experimental questions related to the structure of the interdigitated phase and put forward a simple model that explains the alcohol chain length dependence of the stability of this interdigitated phase.

Effects of ethanol on lipid bilayers with and without cholesterol: the distearoylphosphatidylcholine system

Differential scanning calorimetry (DSC) and fluorescence spectroscopy are useful techniques for investigating the phase transitions of phospholipid bilayers. In this study, these methods have been extended to determine the effects of ethanol on DSPC and DSPC/2 mol.% cholesterol bilayers. The biphasic effect of the main transition was observed on the DSC heating scans above 0.60 M ethanol. In addition, the concentration at which the biphasic effect occurs is not significantly changed in the presence of 2 mol.% cholesterol. For the fluorescence studies, 1,6-diphenyl-1,3,5-hexatriene (DPH) has been incorporated into the bilayer to monitor the phase transitions through the displacement of DPH. This fluorescent probe is used to directly determine the onset of interdigitation in the bilayer systems as indicated by a large decrease in the DPH fluorescence intensity. The addition of cholesterol lowered and broadened the transition temperatures of the phosphatidylcholine (PC) system. However, 2 mol.% cholesterol did not have a significant effect on the induction of the interdigitated phase in DSPC as observed from the small difference in ethanol threshold concentration for the two systems. This suggests that DSPC forms a more stable interdigitated gel phase than other PCs with shorter acyl chains.

Partition coefficients of charged and uncharged local anesthetics into dipalmitoylphosphatidylcholine bilayer membrane: estimation from pH dependence on the depression of phase transition temperatures

Effects of the local anesthetics, dibucaine, bupivacaine and lidocaine on the phase transition temperatures of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane were studied by the optical method. We focus our attention on pH dependence of the depression of main transition and pretransition temperatures. The temperatures of both transitions of DPPC bilayer membrane were depressed by the addition of anesthetics; the higher the value of pH, the larger the depression of main transition temperature and/or pretransition temperature by anesthetics. By extending the colligative thermodynamic framework to the depression of main transition temperature by an anesthetic, we can estimate the differential partition coefficient, which is defined by the difference in partition coefficients of an anesthetic into the ripple gel and liquid crystal phases. The difference in partition coefficient between the lamellar and ripple gel phases can also be estimated from the depression of pretransition temperature. Since the differential partition coefficients include both contributions of the charged and uncharged anesthetics, we could estimate the partition coefficients of the charged and uncharged anesthetic into the membranes from the pH dependence of differential partition coefficients. The liquid crystalline membrane of DPPC bilayer was more receptive to the uncharged local anesthetics than the charged species. The partition coefficients of the charged and uncharged anesthetics into the liquid crystalline phase of DPPC bilayer membrane were 3540 and 249000 (for dibucaine), 1120 and 83900 (for bupivacaine), 256 and 11700 (for lidocaine), respectively. The transfer free energy of uncharged anesthetics from the aqueous phase to the liquid crystalline membrane was well correlated to the local anesthetic potency.

Evaluation of 1-naphthol as a convenient fluorescent probe for monitoring ethanol-induced interdigitation in lipid bilayer membrane

In this work we have tried to evaluate the usefulness of 1-naphthol as an excited state proton transfer fluorescent probe for studying the ethanol-induced interdigitation in lipid bilayer membranes. When ethanol concentration in lipisome is progressively increased, the neutral form fluorescence of 1-naphthol is found to decrease with corresponding increase in the anionic form intensity. This behavior is in contrast to that observed in the absence of lipid where a reverse effect is noticed. Modification of lipid bilayer is known to occur in the presence of ethanol, which increases the packing density of the membrane. Due to this induction of interdigitated gel phase, redistribution of naphthol between the inner core and interfacial region of the lipid bilayer takes places, accounting for the reduction in neutral form fluorescence intensity. The partition coefficient values and the quenching studies also support the redistribution of 1-naphthol in the liposome membrane. The neutral form fluorescence of 1-naphthol successfully monitors the shift in phase transition temperature due to ethanol-induced interdigitation. It also explains the prevention of interdigitation in lipid bilayer at high cholesterol concentration.

Effect of local anesthetics on the bilayer membrane of dipalmitoylphosphatidylcholine: interdigitation of lipid bilayer and vesicle-micelle transition

The phase transitions of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane were observed by means of differential scanning calorimetry (DSC) as a function of the concentration of local anesthetics, dibucaine (DC x HCl), tetracaine (TC x HCl), lidocaine (LC x HCl) and procaine hydrochlorides (PC x HCl). LC x HCl and PC x HCl depressed monotonously the temperatures of the main- and pre-transition of DPPC bilayer membrane. The enthalpy changes of both transitions decreased slightly with an increase in anesthetic concentration up to 160 mmol kg(-1). In contrast, the addition of TC x HCl or DC x HCl, having the ability to form a micelle by itself, induced the complex phase behavior of DPPC bilayer membrane including the vesicle-to-micelle transition. The depression of both temperatures of the main- and pre-transition, which is accompanied with a decrease in enthalpy, was observed by the addition of TC x HCl up to 21 mmol kg(-1) or DC x HCl up to 11 mmol kg(-1). The pretransition disappeared when these concentrations of anesthetic were added, and the interdigitated gel phase appeared above these concentrations. The appearance of the interdigitated gel phase, instead of the ripple gel phase, brings about the stabilization of the gel phase by 1.8-2.4 kcal mol(-1). In the concentration range of 70-120 mmol kg(-1) TC x HCl (or 40-60 mmol kg(-1) DC x HCl), the enthalpy of the main transition exhibited a drastic decrease, resulting in the virtual disappearance of the main transition. This process includes the decrease in vesicle size with increasing anesthetic concentration, resulting in the mixed micelle of DPPC and anesthetics. Therefore, in this range of anesthetic concentration, the DPPC vesicle solubilized an anesthetic which coexists with the DPPC-anesthetic mixed micelle. Above the concentration of 120 mmol kg(-1) TC x HCl (or 60 mmol kg(-1) DC x HCl), there exists the DPPC-anesthetic mixed micelle. Two types of new transitions concerned with the mixed micelle of DPPC and micelle-forming anesthetics were observed by DSC.

Effects of alcohols on lipid bilayers with and without cholesterol: the dipalmitoylphosphatidylcholine system

Differential scanning calorimetry is a useful method to study the thermotropic phase transitions of a phospholipid bilayer. In the present study DSC is used to determine the effects of methanol and ethanol on DPPC and DPPC/2 mol% cholesterol bilayers. The biphasic effect of the main transition and the presence of an extra peak on the DSC cooling scans were observed above certain alcohol concentrations. In the presence of 2% cholesterol, the concentration at which the biphasic effect occurs is increased by both short-chain alcohols. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is used as a fluorescent probe to directly determine the onset of interdigitation in these systems as reflected by a drop in the DPH fluorescence intensity.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Restricted motion of photoexcited bacteriorhodopsin in purple membrane containing ethanol

The molecular motion of retinal within the purple membrane was investigated by flash-induced absorption anisotropies with or without ethanol. In the absence of ethanol, the measured anisotropies at several wavelengths exhibited almost the same slow decay. This slow decay was attributed to only the rotation of purple membrane sheet itself in the aqueous suspension. In the presence of ethanol, however, we observed the wavelength-dependent anisotropies. The fluidity of the purple membrane, investigated with a fluorescence anisotropy method, was increased by the addition of ethanol. These facts indicated that the characteristic motion of bacteriorhodopsin is induced in perturbed purple membrane with ethanol. The data analysis was performed, taking account of the overlapping of absorption from ground-state bacteriorhodopsin and photointermediates. The results showed that the rotational motion of photointermediates within the membrane was more restricted than that of nonexcited bacteriorhodopsin. The addition of ethanol facilitated the rotation of nonexcited protein, whereas it did not significantly affect the motion of photointermediates. The restricted motion of photointermediates is probably caused by a conformational change in them, which may hinder the rotation of monomer protein and/or induce the interaction between photointermediate and neighboring proteins.

Amphiphilic sites for general anesthetic action? Evidence from 129Xe-[1H] intermolecular nuclear Overhauser effects

Because a strong correlation exists between the potency of general anesthetics and their ability to dissolve in oil, a lipophilic site of action is often assumed. We show here that a lipophilic molecule may preferentially target less lipophilic sites after interaction with a membrane takes place. Xenon, a chemically inert and structureless general anesthetic, was chosen as an unbiased molecular probe for assessment of its dynamic distribution. Site-selective intermolecular 129Xe-[1H] nuclear Overhauser effects were used to measure the specific interaction between xenon and protons in different regions in a phosphatidylcholine lipid membrane. It was evident that xenon-membrane interaction was directed toward the amphiphilic head region, with significant involvement of interfacial water, despite xenon's apolar and highly lipophilic nature in the gas phase. This result may suggest the importance of amphiphilicity in association with anesthetic action.

Ethanol-enhanced permeation of phosphatidylcholine/ phosphatidylethanolamine mixed liposomal membranes due to ethanol-induced lateral phase separation

Effects of ethanol on permeability of large unilamellar vesicles (ca. 160 nm in diameter), composed of dipalmitoyl phosphatidylcholine/dilauroyl phosphatidylethanolamine (DLPE) mixture, were studied by monitoring leakage of the fluorescent dye, calcein, entrapped in the inner aqueous phase of the vesicles. In the presence of 2.1 M ethanol, permeabilities of membranes in various phases were G (bilayer gel) phase > L (bilayer liquid-crystalline) phase with a high mole fraction of DLPE and (I (ethanol-induced interdigitated gel phase) + G) phase > (I + L) at 20 mol % DLPE. Arrhenius plots of the leakage rate constants demonstrated that the permeability was greater with 2.1 M ethanol than without ethanol, especially in the temperature above 33 degrees C, suggesting that the presence of ethanol can induce lateral phase separation of liposomal membranes and cause them to have a high permeability even if they are stable and have low permeability in its absence.

Induction of lipid-protein mismatch by xenobiotics: kinetic cooperativity

Lipophilic inhibitors such as general anaesthetics or drugs can conceivably act by displacing boundary lipid molecules that are required by many functional membrane proteins. The resulting lipid-protein mismatch has been analyzed previously in terms of multiple site kinetics (Sandermann H. (1993) Biochim. Biophys. Acta 1150, 130-133). Expressions for kinetic cooperativity are now derived, and data for the inhibition of dog kidney Na+,K+-ATPase and Escherichia coli lactose permease by organic solvents are presented and analyzed. Half-maximal inhibitor concentrations were without diagnostic value because they were within the general range of critical solvent concentrations known for general anaesthesia and several membraneous and non-membraneous systems, as well as two specific liposomal parameters. The kinetic cooperativity of inhibition was of much higher diagnostic value because the cooperativity values for the solvent inhibition of Na+,K+-ATPase and lactose permease were characteristic for the lipid displacement mechanism, in contrast to cooperativity values of protein kinase C and luciferase. The latter enzymes are known not to require a boundary lipid layer, so that the degree of kinetic cooperativity provides a new diagnostic tool to distinguish between modes of action of lipophilic inhibitors.

Phase stability of phosphatidylcholines in dimethylsulfoxide solutions

The temperature dependence of the phase stability of dispersions of dimyristoyl, dipalmitoyl, and distearoyl derivatives of phosphatidylcholines in excess aqueous dimethylsulfoxide has been examined by differential scanning calorimetry and synchrotron x-ray diffraction methods. There was a close correlation between the enthalpic transitions and the structural changes associated with the pre- and main transitions of the phospholipids in the range of concentrations up to mole fractions of dimethylsulfoxide in water of 0.1333. The temperature of the pre- and main transitions of the three phospholipids were found to increase linearly with increasing mole fraction of dimethylsulfoxide. The difference in phase stability between the lamellar gel and ripple phases induced by increasing dimethylsulfoxide concentration resulted in disappearance of the ripple phase and direct transition between lamellar gel and lamellar liquid-crystal phases. The effect of changing the properties of the solvent by the addition of dimethylsulfoxide on the dimensions of dipalmitoylphosphatidylcholine and solvent layers of the bilayer repeat structure has been determined from electron density distribution calculations. The lamellar repeat spacing recorded at 25 degrees C decreased from 6.36 nm in aqueous dispersion to 6.04 nm in a dispersion containing a mole fraction of 0.1105 dimethylsulfoxide. The results indicate that dipole interactions between solvent and phospholipid and dielectric properties of the solvent are important factors in the determination of the structure of saturated phosphatidylcholines.

Increased permeability of phase-separated liposomal membranes with mixtures of ethanol-induced interdigitated and non-interdigitated structures

It has been suggested by many workers using model membranes that the interdigitated structure formation, in which the acyl chains fully interpenetrate the hydrocarbon chains of the opposing monolayer, plays an important role in regulating many functions of biomembranes. In the present study the control of permeability was focused on as one of the biomembrane functions, and the effects of ethanol on the permeability of large unilamellar vesicles made by the extrusion technique (LUVET) (average diameter: about 250 nm), composed of dipalmitoyl or egg yolk phosphatidylcholines, were studied by monitoring the leakage of fluorescent dye, calcein, entrapped in the inner aqueous phase of the LUVET. The permeability was estimated from the apparent rate constant of calcein leakage at 25 degrees C. Large permeabilities were observed in the region of 0.6 M to 1.3 M ethanol, with a concentration dependence. In this range of ethanol concentrations the normal bilayer and interdigitated structure coexist and the membrane is in a phase-separated state. The large permeability is due to the instability of the boundary regions, the interdigitated membrane being characterized by a thinner structure and more rigid hydrocarbon regions in the layer than its non-interdigitated counter part. These results suggest the possibility of biomembrane-permeability regulation by interdigitated membrane formation.

Effects of short-chain alcohols on the phase behavior and interdigitation of phosphatidylcholine bilayer membranes

Solute effects on the polymorphism and phase transitions in the suspensions of dipalmitoylphosphatidylcholine (DPPC) were studied by means of carboxyfluorescein (CF) and phosphatidylethanolamine rhodamine (PERho) fluorescence, differential scanning calorimetry, and X-ray diffraction. Specifically, the shifts of the lipid chain-melting phase transition, pretransition and subtransition temperature as a function of the bulk alcohol concentration were determined calorimetrically. The chain-melting phase transition temperature, Tm, was found to depend on the chain-length of the added alcohol: for short-chain alcohols (up to n-propanol), Tm first decreases and then increases with increasing alcohol concentration. For longer-chain alcohols, however, Tm decreases over the whole investigated alcohol concentration range. The pretransition and the subtransition temperature of DPPC both decrease monotonously (but non-linearly) with increasing alcohol concentration, but the former transition disappears at some characteristic, chain-length dependent alcohol concentration, cL beta i. This point in the solute-dependent phase diagram of DPPC is diagnostic of the complete hydrocarbon interdigitation. It was determined for a series of short-chain alcohols ranging from methanol through to 1-hexanol. A quantitative formula for the calculation of such limiting alcohol concentration is introduced. This formula relates the cL beta i values to the free energy of transfer of alcohols from the aqueous sub-phase into the DPPC sub-phase. By using the concept of an apparent chain-length this formalism can also be used for the alcohols with polar OH-groups at the second or third position on the hydrocarbon chain. Alcohol-induced hydrocarbon interdigitation in the phospholipid bilayers is thus shown to result chiefly from the solute-induced perturbation (lateral expansion) in the lipid headgroup region. Longer-chain alcohols, which balance this effect by disordering the phospholipid chains, therefore do not induce chain interdigitation.

Ethanol-induced aggregation and fusion of small phosphatidylcholine liposome: participation of interdigitated membrane formation in their processes

The mechanism for the ethanol-induced aggregation/fusion of uniform-sized small liposomes comprised of dipalmitoyl (DPPC) or egg yolk (eggPC) phosphatidylcholines was studied by measuring the average size using a photon correlation spectroscopy, by observing directly the states in the liposomal solutions using freeze-fracture electron microscopy and by attempting resonance energy transfer using flurophore-labeled phospholipids. Abrupt increases in the apparent size of DPPC liposomes were observed in the presence of above 44 mg/ml ethanol, where microscopically plateau membranes form interdigitated structure, in which the acyl chains fully interpenetrate the hydrocarbon chains of the apposing monolayer. On the contrary, in the eggPC liposome, where the membranes cannot form interdigitated structures even in the presence of high concentration of ethanol, such intense aggregation and fusion were not observed, suggesting their intimate relation to the interdigitated structure formation. The formation of interdigitated structures in the adhering region leads to an increase in the interfacial area and an exposure of hydrophobic acyl chain terminal on the surface area, and enhances hydrophobic interactions between two interdigitated bilayers. Thus, the resultant interdigitated structure makes the aggregated state stable and partially initiates the bilayer mixing between the two apposed membranes, leading to fusion.

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.

Alcohol induction of interdigitation in distearoylphosphatidylcholine: fluorescence studies of alcohol chain length requirements

Although it is now well established that the fully interdigitated phase is induced in saturated like-chain phosphatidylcholines (PCs) by a variety of amphipathic molecules including alcohols, no systematic study of the properties of the inducing molecules has been reported. To elucidate the stereochemical features that lead to the alcohol induction of interdigitation in PCs, we have investigated the induction of interdigitation in distearoylphosphatidylcholine (DSPC) by a series of alcohols. Our previously established DPH (1,6-diphenyl-1,3,5-hexatriene) fluorescence intensity method has been expanded (P. Nambi, E. S. Rowe, and T. M. McIntosh (1988), Biochemistry 27:9175-9182) and used to determine which of the alcohols induce interdigitation and to determine the threshold concentrations for each. We have found that each of the n-alcohols up to heptanol and several branched alcohols are capable of inducing interdigitation in DSPC; octanol and nonanol do not appear to induce interdigitation by these criteria. The threshold concentrations for interdigitation for each of these alcohols up to heptanol were found to be correlated with the membrane: buffer partition coefficients. The mole fraction of bound alcohol at the threshold concentration was similar for each of the alcohols up to pentanol. These results are discussed in terms of a general mechanism of the formation of the interdigitated phase.

Local anesthetics destabilize lipid membranes by breaking hydration shell: infrared and calorimetry studies

Differential scanning calorimetry (DSC) showed that local anesthetics decreased the pretransition (L beta'-->P beta') temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes four- to five-fold more than the main transition (P beta'-->L alpha) temperature. Because pretransition is mainly a change in the hydrophilic head property (tilted-rippled), the stronger effect on the pretransition suggests that the primary action site of local anesthetics is the lipid-water interface. The interfacial effect was analyzed by Fourier-transform infrared spectroscopy (FTIR) in water-in-oil (CCl4) reversed micelles. FTIR showed that the local anesthetics released hydrogen-bonded water molecules from the phosphate (P = O bands) and glycerol (sn-2 C = O) moieties. The N-H stretching band of the local anesthetics was deconvoluted into two bands: hydrogen bonded to the phosphate moiety of the lipid and free (unbound to lipid). The formation constants between lipid P = O and anesthetic N-H were estimated in CCl4 from the spectral changes: 110 M-1 for lidocaine and 250 M-1 for dibucaine. This small difference in the formation constants cannot explain the ten-fold stronger effect on the phase-transition temperature of dibucaine over lidocaine. By comparing the local anesthetic adsorption to the air/water interface in the presence and absence of lipid monolayers, we have previously shown (Lin et al. (1980) Biochim. Biophys. Acta 598, 51-65) that lipid-anesthetics interaction involves three forces: lipophilic effect, hydrophobic effect, and anesthetic-anesthetic interaction. The anesthetic potency depends mainly on the hydrophobic effect (the difference in the standard molar free energies of local anesthetics in water and at the interface) and anesthetic-anesthetic interaction energy. The anesthetic-anesthetic interaction means cooperativity of local anesthetics for the interfacial density: local anesthetics condense at the membrane surface when there are enough anesthetic molecules present at the interface to attract more anesthetics. The present data suggest that anesthetic action is directed to the interface between water and macromolecule, whether it is lipid membranes or proteins.

Effects of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine vesicles

6-Carboxyfluorescein was employed to examine the effect of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles. Proton permeability was measured by monitoring the decrease of 6-carboxyfluorescein fluorescence after a pH gradient from 3.5 (outside the vesicle) to 8.0 (inside the vesicle) was established. At 20 degrees C and below 1.2 M ethanol, the fluorescence decrease is best described by a single exponential function. Above 1.2 M ethanol, the intensity decrease is better described by a two-exponential decay law. Using the fitted rate constants and the vesicle radii determined from light-scattering measurements, the proton permeability coefficient, P, in DPPC vesicles was calculated as a function of ethanol concentration. At 20 degrees C, P increases monotonically with increasing ethanol content up to 1.0 M, followed by an abrupt increase at 1.2 M. The vesicle size also exhibits a sudden increase at around 1.2 M ethanol, which has been shown to result from vesicle aggregation rather than vesicle fusion. The abrupt increases in P and in vesicle size occur at the concentration region close to the critical ethanol concentration for the formation of the fully interdigitated gel state of DPPC. At 14 degrees C, the abrupt change in P shifts to 1.9-2.0 M ethanol, completely in accordance with the ethanol-temperature phase diagram of interdigitated DPPC. Effects of methanol and benzyl alcohol on lipid interdigitation have also been examined. At 20 degrees C, DPPC large unilamellar vesicles exhibit a dramatic change in P at 3 M methanol and at 40 mM benzyl alcohol. These concentrations come close to the critical methanol and benzyl alcohol concentrations for the formation of fully interdigitated DPPC structures determined previously by others. It can be concluded that proton permeability increases dramatically as DPPC is transformed from the noninterdigitated gel to the fully interdigitated gel state by high concentrations of alcohol. This marked increase in proton permeability can be attributed to the combined effect of the changes in membrane thickness and surface charge density, due to the ethanol-induced lipid interdigitation. The possible effects of the increased proton permeability caused by ingested ethanol on gastric mucosal membranes are discussed.

Effect of unilamellar vesicle size on ethanol-induced interdigitation in dipalmitoylphosphatidylcholine

Unilamellar liposomes are widely used as model membranes to represent and study the properties of biological membranes and as potential drug delivery systems. It is well established that ethanol and other amphiphiles induce the interdigitated L beta I phase in multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). However, all of the work on this phase has been performed using the hand shaken multilamellar preparations. In the present report, we have studied the induction of interdigitation in a series of unilamellar vesicles prepared by sonication and by extrusion. The methods used to characterize the vesicles were freeze fracture electron microscopy and quasielastic light scattering (QELS). Two fluorescence methods were used to detect interdigitation, the DPH fluorescence quenching method (Nambi, P., Rowe, E.S. and McIntosh, T.J. (1988) Biochemistry 27, 9175-9182) and the pyrene-PC fluorescence method (Komatsu, H. and Rowe, E.S. (1991) Biochemistry 30: 2463-2470). It was found that sonicated vesicles are not stable in the presence of interdigitating concentrations of ethanol; they form higher aggregates at all temperatures examined. The behavior of the extruded vesicles was different from that of the SUV; each size studied was stable in the presence of ethanol, although they exhibited an increase in size. It was shown that extruded vesicles having a 200-nm or greater diameter become interdigitated in the presence of ethanol. The threshold concentration for interdigitation in vesicles is greater than that for MLVs and it decreases with increasing vesicle size, approaching the MLV value for the largest vesicles.

Ethanol effects on the stratum corneum lipid phase behavior

The stratum corneum is considered to be the diffusional barrier of mammalian skin for water and most solutes. The intercellular lipid multilayer domains of the stratum corneum are believed to be the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers in the presence of ethanol is frequently conceived to result in enhanced permeation. Current investigations address the effect of ethanol on the phase behavior in terms of stratum corneum lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. Phospholipid multilamellar vesicles were also studied as model systems. There appeared to be no effect of ethanol on either the solid-solid phase transition or the gel phase interchain coupling of the stratum corneum lipids. However, there was a reduction in the mobility of the alkyl chains in the presence of ethanol. Possible mechanistic relationships between the current FTIR spectroscopic results with available literature data of ethanol induced lipophilic solute penetration enhancement through the skin are discussed.

Influence of Ca2+ and ethanol on the aggregation and thermal phase behaviour of L-dihexadecylphosphatidylcholine liposomes

The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid] ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca(2+)-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8 degrees in the pretransition temperature Tp in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in Tp and finally the disappearance of the pretransition. Concomitantly a slight elevation in Tm occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.

Lateral conductance parallel to membrane surfaces: effects of anesthetics and electrolytes at pre-transition

The effects of dilute salts and anesthetics were studied on the impedance dispersion in the dipalmitoylphosphatidylcholine (DPPC) liposomes. Below the pre-transition temperature, the apparent activation energy for conductance in DPPC-H2O without salts was equivalent to pure water, 18.2 kJ mol-1. This suggests that the mobile ions (H3O+ and OH-) interact negligibly with the lipid surface below the pre-transition temperature. At pre-transition temperature, the apparent activation energy of the conductance decreased by the increase in the DPPC concentrations. The effects of various salts (LiCl, NaCl, KCl, KBr, and KI) on the apparent activation energy of the conductance were studied. Changes in anions, but not in cations, affected the activation energy. The order of the effect was Cl- less than Br- less than I-. Cations appear to be highly immobilized by hydrogen bonding to the phosphate moiety of DPPC. The smaller the ionic radius, the more ions are fixed on the surface at the expense of the free-moving species. The apparent activation energy of the transfer of ions at the vesicle surface was estimated from the temperature-dependence of the dielectric constant, and was 61.0 kJ mol-1 in the absence of electrolytes. In the presence of electrolytes, the order of the activation energy was F- greater than Cl- greater than Br- greater than I-. When the ionic radius is smaller, these anions interact with the hydration layer at the vesicle surface and the ionic transfer may become sluggish. In the absence of electrolytes, the apparent activation energy of the dielectric constant decreased by the increase in halothane concentrations. In the presence of electrolytes, however, the addition of halothane increased the apparent activation energy. We propose that the adsorption of halothane on the vesicle surface produces two effects: (1) destruction of the hydration shell, and (2) increase in the binding of electrolytes to the vesicle surface. In the absence of electrolytes, the first effect predominates and the apparent activation energy is decreased. In the presence of electrolytes, the latter effect predominates and the apparent activation energy is increased.

Permeability of lipid membranes revised in relation to freeze-thaw processes

Water and solute activity gradients created during freeze-thaw processes produce water and solute fluxes across the cell membrane resulting in volume changes. Under these conditions, osmotic and thermal stresses affect the curvature, the phase behavior, and the surface properties of the lipid bilayer. These structural changes are not considered by the classical formalisms describing permeability of lipid membranes to water and nonelectrolytes such as the Nernst-Planck equation, Eyring's absolute rate theory, and Kedem-Katchalsky's thermodynamic of irreversible processes approach. In this paper, the influence of such changes on the glycerol permeation kinetics are reported. The results indicate that osmotic and chemical effects of the cryoprotectant on the membrane properties affect the rate of volume swelling depending on whether the membrane is in the gel or in the liquid crystalline state.

Titration calorimetric and differential scanning calorimetric studies of the interactions of n-butanol with several phases of dipalmitoylphosphatidylcholine

The interactions of n-butanol with dipalmitoylphosphatidylcholine (DPPC) were studied using titration calorimetry and differential scanning calorimetry (DSC). DSC results indicated that n-butanol induces the interdigitated phase in DPPC above 10 mg/mL butanol. A new application of titration calorimetry for measuring partition coefficients of nonsaturating solutes into lipids was developed. The partition coefficients and the heat of binding of n-butanol into DPPC were measured for the L beta', P beta', L alpha, and L beta I phases of DPPC. The partition coefficients were temperature dependent and ranged from 70 to 110 for the L beta I phase, from 170 to 183 for the L alpha phase, and similar to that for the L beta I phase in the P beta' phase. The binding to the L beta' phase could not be detected, giving an upper limit for this partition coefficient of 23. The enthalpies for binding to the L beta I and L alpha phases were 1.0 and 1.5 kcal/mol, respectively. The van't Hoff enthalpy was in good agreement with the calorimetric enthalpy for the partitioning into the L alpha phase; however, it was greater than the calorimetric enthalpy for the L beta I phase, suggesting that the interaction of n-butanol with this phase is cooperative in some way.

Effect of insulin on the lytic action of lysophosphatidylcholine in lipid bilayers

The effect of insulin on the bilayer properties of dimyristoylphosphatidylcholine liposomes at the gel and the liquid crystalline state was measured by differential scanning calorimetry and absorbance at 450 nm. It is found that insulin promotes a decrease in the enthalpy of the gel-liquid crystalline transition without displacing the transition temperature. Under these conditions the lytic action of monomyristoylphospatidylcholine is enhanced, decreasing the critical lytic concentrations to values comparable to the bilayer at the gel state. The effect of the lysoderivate on liposomes in contact with increasing concentrations of insulin promotes a reorganization of the lipids into smaller particles as inferred from fluorescence dequenching, turbidity and exclusion chromatography assay. It is concluded that the action of lysoderivates can be enhanced, at temperatures above the transition temperature, by proteins that without spanning the lipid bilayers can perturb the bilayer interface.

Effect of cholesterol on the ethanol-induced interdigitated gel phase in phosphatidylcholine: use of fluorophore pyrene-labeled phosphatidylcholine

It is now recognized that many amphiphilic molecules such as ethanol can induce the formation of the fully interdigitated gel phase (L beta I) in phosphatidylcholines (PC's). In the present study, we have developed a simple detection method for the L beta I phase using pyrene-labeled PC (PyrPC), which is a PC analogue with covalently coupled pyrene moiety at the end of one of its acyl chains. The intensity ratio of its fluorescence vibrational bands is a reflection of the polarity of the environment of the fluorophore. We have tested this fluorophore in several established interdigitated lipid systems, including 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (1,2-DPPC) in the presence of high concentrations of ethanol and 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) and 1,3-dipalmitoyl-sn-glycero-2-phosphocholine (1,3-DPPC) in the absence of any additives. We have found in each of these systems that the ratio of the intensities of band III (387.5 nm) to band I (376.5 nm) is sensitive to the lipid phase change from the noninterdigitated L beta' phase to the interdigitated L beta I phase. By comparison of the III/I ratios for PyrPC in the lipid systems with the III/I ratios for methylpyrene in organic solvents, it was shown that the polarity of the PyrPC environment in the L beta I phase is similar to that of pentanol or ethanol. Using this method, we investigated the effect of cholesterol on the ethanol induction of the interdigitated gel phase in 1,2-DPPC. We found that the ethanol induction of the interdigitated gel phase is prevented by the presence of 20 mol % cholesterol.