Thermally-induced order-order transition of DNA-cationic surfactant complexes

Polyanionic DNA interacts with cationic amphiphiles to form electrostatic complexes exhibiting rich self-assembled structures. This type of complex has been considered as a nonviral carrier in gene therapy and as a template for nanostructure construction. Here we report a thermally-induced phase transition of the complexes of DNA with the mixtures of a cationic surfactant, dodecyltrimethyl bromide (DTAB), and a neutral lipid, dioleoylphosphatidylethanolamine (DOPE), in fully hydrated state. An order-order transition between a multilamellar (L(c)alpha) phase and an inverted hexagonal (H(c)II) phase was found to occur with the transition temperature adjustable by the DTAB-to-DNA base pair molar ratio (x) and DOPE-to-DTAB molar ratio (m). The stability of the L(c)alpha phase was enhanced at lower m and x, as the L(c)alpha-to-H(c)II transition temperature increased with the decreases of these two parameters. The suppression of -to- transition at lower x was attributed to the lower entropic gain from the counterion release due to the presence of uncomplexed DNA in the bulk solution.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Tail ordering due to headgroup hydrogen bonding interactions in surfactant monolayers at the water-oil interface

Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head- and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH(2)OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH(3)(CH(2))(29)OH) and triacontanoic acid (CH(3)(CH(2))(28)COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.     

Geometric constraints at the surfactant headgroup: effect on lipase activity in cationic reverse micelles

The primary objective of the present article is to understand how the geometric constraints at the surfactant head affect the lipase activity in the reverse micellar interface. To resolve this issue, surfactants were designed and synthesized, and activity was measured in /water/isooctane/n-hexanol reverse micellar systems at z ([alcohol]/[surfactant])=5.6, pH 6.0 (20 mM phosphate), 25 degrees C across a varying range of W0 ([water]/[surfactant]) using p-nitrophenylalkanoates as the substrate. It was observed that lipase activity increases from surfactants to with the increment in surface area per molecule (Amin) because of the substitution by the bulky tert-butyl group at the polar head. However, the activity was found to be similar for despite an enhancement in the hydrophilic moieties at the interface. This unchanged lipase activity is presumably due to the comparable surface area of to originating from the rigidity at the surfactant head. Noticeably, the enzyme activity improved from with the simultaneous increment of both the hydroxyl group and the flexibility of the headgroup whereas that for increased exclusively with the flexibility of the headgroup. The common parameter in both groups of surfactants and is the flexibility of the headgroup, which possibly enhance Amin and consequently the lipase activity. Thus, the geometric constraints at the surfactant headgroup play a crucial role in modulating the lipase activity profile probably because of the variation in interfacial area.     

Structure of DNA-cationic surfactant complexes at hydrophobically modified and hydrophilic silica surfaces as revealed by neutron reflectometry

In this article, we discuss the structure and composition of mixed DNA-cationic surfactant adsorption layers on both hydrophobic and hydrophilic solid surfaces. We have focused on the effects of the bulk concentrations, the surfactant chain length, and the type of solid surface on the interfacial layer structure (the location, coverage, and conformation of the DNA and surfactant molecules). Neutron reflectometry is the technique of choice for revealing the surface layer structure by means of selective deuteration. We start by studying the interfacial complexation of DNA with dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) on hydrophobic surfaces, where we show that DNA molecules are located on top of a self-assembled surfactant monolayer, with the thickness of the DNA layer and the surfactant-DNA ratio determined by the surface coverage of the underlying cationic layer. The surface coverages of surfactant and DNA are determined by the bulk concentration of the surfactant relative to its critical micelle concentration (cmc). The structure of the interfacial layer is not affected by the choice of cationic surfactant studied. However, to obtain similar interfacial structures, a higher concentration in relation to its cmc is required for the more soluble DTAB surfactant with a shorter alkyl chain than for CTAB. Our results suggest that the DNA molecules will spontaneously form a relatively dense, thin layer on top of a surfactant monolayer (hydrophobic surface) or a layer of admicelles (hydrophilic surface) as long as the surface concentration of surfactant is great enough to ensure a high interfacial charge density. These findings have implications for bioanalytical and nanotechnology applications, which require the deposition of DNA layers with well-controlled structure and composition.     

Role of the surfactant headgroup on the counterion specificity in the micelle-to-vesicle transition through salt addition

A transition from micelles to vesicles is reported when salts are added to a catanionic micellar solution composed of sodium dodecylcarboxylate (SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. The counterion binding and increase in aggregate size was monitored by mass spectrometry, rheology and dynamic light scattering measurements, whereas the vesicles were characterized by freeze-fracture and cryo-transmission microscopy experiments. The effect of counterions on the formation of vesicles was studied and compared to a previously studied catanionic system with a sulfate head group, SDS/DTAB. As in the latter case, no anion specificity was found, while large differences in the hydrodynamic radii of the formed objects were observed, when the cation of the added salt was varied. A classification of the cations could be made according to their ability to increase the measured hydrodynamic radii. It is observed that, if the sulfate headgroup of the anionic surfactant is replaced by a carboxylic group, the order of the ions is reversed, i.e. it follows the reversed Hofmeister series. Different morphologies are observed as the ionic strength of the system is increased. The aggregates are analogous to those found in the SDS/DTAB system.     

Effect of headgroup size on permeability of Newton black films

Gas permeability and thin-film interferometry are used as a tool to elucidate the orientation of polymeric headgroups in free-standing foam films. Nonionic polyoxyethylene (EO) surfactants were used to stabilize the foam films, keeping the size of the hydrophobic part constant (C12) and varying the size of the hydrophilic (EO numbers) part. The effect of headgroup size on the gas permeability of Newton black foam films was studied. Thickness, contact angle, and surface tension were measured to understand the permeation mechanism. Increase of film thickness and surface tension was observed while increasing the headgroup size, but the contact angle remains small and constant. Upon increasing the headgroup size, the permeability decreases showing that the headgroups provide a resistance to permeation. For smaller headgroups, the permeability follows a linear dependence on the film thickness, whereas for larger headgroups, the permeability essentially deviates from linearity. We use the conventional "coil model" of the EO chains to explain the observed results providing a detailed picture of the orientation of this important molecule in a confined volume of foam films.     

Interaction between DNA and cationic surfactants: effect of DNA conformation and surfactant headgroup

The interactions between DNA and a number of different cationic surfactants, differing in headgroup polarity, were investigated by electric conductivity measurements and fluorescence microscopy. It was observed that, the critical association concentration (cac), characterizing the onset of surfactant binding to DNA, does not vary significantly with the architecture of the headgroup. However, comparing with the critical micelle concentration (cmc) in the absence of DNA, it can be inferred that the micelles of a surfactant with a simple quaternary ammonium headgroup are much more stabilized by the presence of DNA than those of surfactants with hydroxylated head-groups. In line with previous studies of polymer-surfactant association, the cac does not vary significantly with either the DNA concentration or its chain length. On the other hand, a novel observation is that the cac is much lower when DNA is denaturated and in the single-stranded conformation, than for the double-helix DNA. This is contrary to expectation for a simple electrostatically driven association. Thus previous studies of polyelectrolyte-surfactant systems have shown that the cac decreases strongly with increasing linear charge density of the polyion. Since double-stranded DNA (dsDNA) has twice as large linear charge density as single-stranded DNA (ssDNA), the stronger binding in the latter case indicates an important role of nonelectrostatic effects. Both a higher flexibility of ssDNA and a higher hydrophobicity due to the exposed bases are found to play a role, with the hydrophobic interaction argued to be more important. The significance of hydrophobic DNA-surfactant interaction is in line with other observations. The significance of nonelectrostatic effects is also indicated in significant differences in cac between different surfactants for ssDNA but not for dsDNA. For lower concentrations of DNA, the conductivity measurements presented an "anomalous" feature, i.e., a second inflection point for surfactant concentrations below the cac; this feature was not displayed at higher concentrations of DNA. The effect is attributed to the presence of a mixture of ss- and dsDNA molecules. Thus the stability of dsDNA is dependent on a certain ion atmosphere; at lower ion concentrations the electrostatic repulsions between the DNA strands become too strong compared to the attractive interactions, and there is a dissociation into the individual strands. Fluorescence microscopy studies, performed at much lower DNA concentrations, demonstrated a transformation of dsDNA from an extended "coil" state to a compact "globule" condition, with a broad concentration region of coexistence of coils and globules. The onset of DNA compaction coincides roughly with the cac values obtained from conductivity measurements. This is in line with the observed independence of cac on the DNA concentration, together with the assumption that the onset of binding corresponds to an initiation of DNA compaction. No major changes in either the onset of compaction or complete compaction were observed as the surfactant headgroup was made more polar.     

Effect of headgroup on the supramolecular morphology of diacetylenic aldonamides

This report describes the initial characterization of supramolecular morphology of diacetylenic aldonamides where the headgroup has been systematically varied from hexose, pentose, to tetrose. The electron-dense diacetylenes allow direct electron microscopic imaging of the assembly morphology without the aid of staining reagents. The electron micrographs reveal a dramatic change in supramolecular morphology upon the simple changing the headgroup from D-galactose to L-arabinose. Possible reasons for the change in morphology are discussed.     

Enantioselectivity of vesicle-forming chiral surfactants in capillary electrophoresis. Role of the surfactant headgroup structure

Two vesicle-forming single-tailed amino acid derivatized surfactants sodium N-[4-n-dodecyloxybenzoyl]-L-leucinate (SDLL) and sodium N-[4-n-dodecyloxybenzoyl]-L-isoleucinate (SDLIL) have been synthesized and used as pseudo-stationary phase in micellar electrokinetic chromatography to evaluate the role of steric factor of amino acid headgroup and hydrophobic/hydrophilic interactions for enantiomeric separations. The aggregation behavior of the surfactants has been studied in aqueous buffered solution using surface tension and fluorescence probe techniques. Results of these studies have suggested formation of vesicles in aqueous solutions. Microenvironment of the vesicle, which determines the depth of penetration of the analytes into vesicle was determined by fluorescence probe technique using pyrene, N-phenyl-1-naphthylamine (NPN), and 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe molecules. Atropisomeric compounds (+/-)-1,1'-bi-2-naphthol (BOH), (+/-)-1,1'-binaphthyl-2,2'-diamine (BDA), (+/-)-1,1'-binaphthyl-2,2'-diylhydrogen phosphate (BNP) and Tr?ger's base (TB) and chiral compound benzoin (BZN) has been enantioseparated. The separations were optimized with respect to surfactant concentration, pH, and borate buffer concentration. SDLL was found to provide better resolution for BOH, BNP, and BZN. On the other hand, SDLIL offers better resolution for BDA. The chromatographic results have been discussed in the light of the aggregation behavior of the surfactants and the interaction of the solutes with the vesicles.     

Temperature-induced vesicle aggregation in catanionic surfactant systems: the effects of the headgroup and counterion

The peculiar nature of temperature-induced vesicle aggregation (TIVA) in some catanionic surfactant systems is systematically investigated. On the basis of a general analysis of the intervesicular interactions, the main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction among the exposed hydrophobic part of the surfactant headgroups. The addition of an oppositely charged hydrophobic salt to the catanionic vesicle systems is also found to promote the occurrence of TIVA. In fact, TIVA can be induced in ordinary catanionic vesicle systems by the addition of an oppositely charged hydrophobic counterion.     

Effect of surfactant head group size on polyelectrolyte-surfactant interactions: steady-state and time-resolved fluorescence study

Interactions between sodium poly(acrylate) (NaPAA) and dodecyltrimethyl/ethyl/propyl/butylammonium bromide (C12NM, C12NE, C12NP, and C12NB) were studied. Variation of the physicochemical properties of the surfactants and polyelectrolyte-surfactant mixtures, such as critical micelle concentration (cmc), critical aggregation concentration (cac), micellar micropolarity, aggregation number, and pyrene lifetime, were determined by steady-state and time-resolved fluorescence methods. It is shown that the surfactant head group size has a striking effect on the interaction between surfactant and polyelectrolyte. The interaction is weakened gradually when the surfactant head group is increased from trimethyl to tripropyl, which might be owing to the increase of the steric hindrance between the polyelectrolyte chain and micellar surface. But when the head group is tributyl, the interaction is enhanced and stronger than that between C12NP and NaPAA. This might be explained by the self-association of the C12NB head groups.     

单烷基乙二胺稀水溶液头基相互作用自组装形成 双分子膜

提出普通表面活性剂(单链两亲分子)亲水头基相互作用诱导疏水尾链平行聚集形成双分子膜的新机制。设计和合成了系列单烷基取代乙二胺C~nH~2~n~+~1NHC~2H~4NH~2(n=8,12,14,16,18)。通过电镜形态,分散液凝胶/液晶相变和对应铸膜的二维双层结构,表明单链两亲分子头基相互作用和脂链引入刚性片断一样,两者形成的双分子膜具有类似的结构和性能;展示了各体系取代乙二胺双层结构和性能的密切联系。指出了广泛认同的单链两亲分子形成双分子膜必须引入刚性片断的单一成膜机制的片面性,为组装新一类功能头基表面活性剂双分子膜独辟蹊径。     

Short-range interactions between non-ionic surfactant layers

Short-range interactions between surfactant and lipid layers are of great importance in technical applications in complex fluids such as foams, dispersions and emulsions, as well as in the formulation and performance of dispersants, detergents and flocculants. It is also of utmost importance in biological systems where interactions between biomembranes influence a range of processes. The field of short-range interactions has been thoroughly investigated during the past 30 years, following the emergence of a number of techniques to measure interaction forces. Thus, our understanding has increased considerably and it is timely to summarize relevant knowledge accumulated in this area. In this review we focus on the nature of short-range interactions between non-ionic and zwitterionic surfactant and lipid layers exposing their polar groups to the surrounding medium. We discuss the complex interplay of short-range (van der Waals, hydration, steric and other) forces based on recent theoretical and experimental results.     

The interactions of surfactant viologens with cyclodextrins

N-ethyl-N-hexadecyl-4,4-bipyridinium bromide (C₁₆VBr₂) andN-ethyl-N-octadecyl-4,4-bipyridinium bromide (C₁₈VBr₂) were used as electroactive probes to assess the interactions between surfactants and cyclodextrins. Cyclic voltammetry, visible spectroscopy, fluorescence spectroscopy and surface tension techniques were used to detect the formation of complexes between the surfactant viologen probes and- and-cyclodextrins. The voltammetric results suggest the formation of inclusion compounds in which the hydrophobic tail of the surfactant viologens penetrate the cyclodextrin cavity. The dimerization of the viologen cation radicals is essentially suppressed by the presence of-cyclodextrin (ACD) while no effects are observed in the presence of-cyclodextrin (BCD). The observed results are best explained by the relative solubility in aqueous media of each of the inclusion complexes in the several accessible viologen oxidation states.     

Characterization of epoxy–surfactant interactions

We have investigated epoxies based on the diglycidyl ether of bisphenol A (DGEBA) cured with 2-ethyl-4-methylimidazole (EMI-24) in the presence of the nonionic surfactant Triton X-100. A goal was to determine if the viscoelastic properties of the epoxy–surfactant system differed when prepared in bulk form, as opposed to being cast as a thin film on the surface of E-glass cloth. Such a combination of materials has generated great interest for potential use in the construction of laminated circuit boards. Using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atomic force microscopy (AFM), it was determined that the surfactant acts as a plasticizer and is miscible with the epoxy system in concentrations up to 15% by weight. The glass transition temperature (T_g) depression of the epoxy due to the surfactant was accurately described by the Fox equation. DMA master curves were constructed in the frequency domain. The temperature dependence of the shift factors was used to determine the fragility of each of the samples studied. It was found that the fragility (cooperativity) of the epoxy decreased as the concentration of surfactant increased, presumably due to a reduction of intermolecular constraints. The fragility of the combined epoxy–surfactant system increased when cast on the surface of the E-glass cloth. Results for our model epoxy–surfactant resin were in excellent agreement with those obtained using a commercially available aqueous waterborne epoxy resin. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2781–2792, 1998     

Effect of the polar headgroup of phospholipids on their interaction with actin

It is generally admitted that actin filaments are anchored to a membrane by membranar actin-binding-proteins. However, we found that actin may also interact directly with membrane phospholipids. The actin-phospholipid complex has been investigated at the air-water interface using a film balance technique. In order to probe the effect of the phospholipid headgroup on the actin-phospholipid interaction, we focus mainly on phospholipids that have the same acyl chain length but different headgroups. For all the phospholipids, the apparent area per molecule (the total surface divided by the number of lipid molecules) increases after the injection of the protein into the subphase, which suggests an intercalation of actin between the phospholipid molecules. This effect seems to be more important for DMPE and DMPS than for DMPG, suggesting that the headgroup plays an important role in this intercalation. The critical surface pressure associated to the liquid expanded-liquid condensed (LE-LC) phospholipid transition increases with the concentration of G-actin and thus suggests that G-actin acts as an impurity, simply competing as a surfactant at the air-water interface. On the other hand, F-actin affects the LE to LC transition of phospholipids differently. In this case, the LE to LC transition is broader and F-actin slightly decreases the critical surface pressure, which suggests that electrostatic interactions are involved.     

Effect of lipid molecule headgroup mismatch on non steroidal anti-inflammatory drugs induced membrane fusion

Membrane fusion is an essential process guiding many important biological events, which most commonly requires the aid of proteins and peptides as fusogenic agents. Small drug induced fusion at low drug concentration is a rare event. Only three drugs, namely, meloxicam (Mx), piroxicam (Px), and tenoxicam (Tx), belonging to the oxicam group of non steroidal anti-inflammatory drugs (NSAIDs) have been shown by us to induce membrane fusion successfully at low drug concentration. A better elucidation of the mechanism and the effect of different parameters in modulating the fusion process will allow the use of these common drugs to induce and control membrane fusion in various biochemical processes. In this study, we monitor the effect of lipid headgroup size mismatch in the bilayer on oxicam NSAIDs induced membrane fusion, by introducing dimyristoylphosphatidylethanolamine (DMPE) in dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles (SUVs). Such headgroup mismatch affects various lipid parameters which includes inhibition of trans-bilayer motion, domain formation, decrease in curvature, etc. Changes in various lipidic parameters introduce defects in the membrane bilayer and thereby modulate membrane fusion. SUVs formed by DMPC with increasing DMPE content (10, 20, and 30 mol %) were used as simple model membranes. Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were used to characterize the DMPC-DMPE mixed vesicles. Fluorescence assays were used to probe the time dependence of lipid mixing, content mixing, and leakage and also used to determine the partitioning of the drugs in the membrane bilayer. How the inhibition of trans-bilayer motion, heterogeneous distribution of lipids, decrease in vesicle curvature, etc., arising due to headgroup mismatch affect the fusion process has been isolated and identified here. Mx amplifies these effects maximally followed by Px and Tx. This has been correlated to the enhanced partitioning of the hydrophobic Mx compared to the more hydrophilic Px and Tx in the mixed bilayer.     

Effect of lipid headgroup composition on the interaction between melittin and lipid bilayers

The effect of the lipid polar headgroup on melittin-phospholipid interaction was investigated by cryo-TEM, fluorescence spectroscopy, ellipsometry, circular dichroism, electrophoresis and photon correlation spectroscopy. In particular, focus was placed on the effect of the lipid polar headgroup on peptide adsorption to, and penetration into, the lipid bilayer, as well as on resulting colloidal stability effects for large unilamellar liposomes. The effect of phospholipid headgroup properties on melittin-bilayer interaction was addressed by comparing liposomes containing phosphatidylcholine, -acid, and -inositol at varying ionic strength. Increasing the bilayer negative charge leads to an increased liposome tolerance toward melittin which is due to an electrostatic arrest of melittin at the membrane interface. Balancing the electrostatic attraction between the melittin positive charges and the phospholipid negative charges through a hydration repulsion, caused by inositol, reduced this surface arrest and increased liposome susceptibility to the disruptive actions of melittin. Furthermore, melittin was demonstrated to induce liposome structural destabilization on a colloidal scale which coincided with leakage induction for both anionic and zwitterionic systems. The latter findings thus clearly show that coalescence, aggregation, and fragmentation contribute to melittin-induced liposome leakage, and that detailed molecular analyses of melittin pore formation are incomplete without considering also these colloidal aspects.     

Effects of lipid chain unsaturation and headgroup type on molecular interactions between paclitaxel and phospholipid within model biomembrane

Molecular interactions between paclitaxel, an anticancer drug, and phospholipids of various chain unsaturations and headgroup types were investigated in the present study by Langmuir film balance and differential scanning calorimetry. Both the lipid monolayer at the air-water interface and the lipid bilayer vesicles (liposomes) were employed as model cell membranes. It was found that, regardless of the difference in molecular structure of the lipid chains and headgroup, the drug can form nonideal, miscible systems with the lipids at the air-water interface over a wide range of paclitaxel mole fractions. The interaction between paclitaxel and phospholipid within the monolayer was dependent on the molecular area of the lipids at the interface and can be explained by intermolecular forces or geometric accommodation. Paclitaxel is more likely to form thermodynamically stable systems with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) than with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Investigation of the drug penetration into the lipid monolayer showed that DPPC and DEPC have higher incorporation abilities for the drug than DPPE and DSPC. A similar trend was also evidenced by DSC investigation with liposomes. While little change of DSC profiles was observed for the DPPE/paclitaxel and DSPC/paclitaxel liposomes, paclitaxel caused noticeable changes in the thermographs of DPPC and DEPC liposomes. Paclitaxel was found to cause broadening of the main phase transition without significant change in the peak melting temperature of the DPPC bilayers, which demonstrates that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer, i.e., in the region of the C1-C8 carbon atoms of the acyl chain or binding at the polar headgroup site of the lipids. However, it may penetrate into the deeper hydrophobic zone of the DEPC bilayers. These findings provide useful information for liposomal formulation of anticancer drugs as well as for understanding drug-cell membrane interactions.