Dipalmitoylphosphatidylcholine Vesicles Research Articles

Kinetics of peroxidation of linoleic acid incorporated into DPPC vesicles initiated by the thermal decomposition of 2,2′-azobis(2-amidinopropane) dihydrochloride

In a previous work [Chem. Phys. Lipids 2000 104, 49], we have derived the following rate law for the oxidation of lipids in compartmentalized systems: R T=( k 1/ k t) 0.5 k p [In] 0.5 c 0.5 [LH], where, R T is the total rate of oxidation, k 1 is the rate constant for the production of free radicals, k t and k p are the intra-particle rate constants for the termination and propagation sets, respectively, [In] is the concentration of a water-soluble initiator, c is the concentration of particles, and [LH] is the intra-particle concentration of oxidable lipid. In the present work, we have investigated on the applicability of the proposed kinetic rate law for a system where it takes place the oxidation of a reactive lipid incorporated into an inert matrix. With this purpose, we have measured the rate of oxidation of linoleic acid incorporated into dipalmitoylphosphatidylcholine vesicles initiated by the thermal decomposition of 2,2′–azobis(2-amidinopropane) dihydrochloride as a function of the initiator, particles, and intra-particle LH concentrations. The experimentally determined kinetic orders obtained were 0.54±0.02, 0.48±0.05 and 0.83±0.04 for the dependence of the oxidation rate with initiator, particles, and LH intra-particle concentrations, respectively, in agreement with those theoretically predicted. The lower value obtained for the kinetic order in LH is attributed to a change in k t with the increase in oxidable lipid intra-particle concentration. The main point to be emphazised from the results here obtained is that the kinetic rate law for the oxidation of lipids in compartmentalized systems can be significantly different than that observed when to the oxidation takes place in homogeneous solution.

Characterisation of carotenoid radical cations in liposomal environments: interaction with vitamin C

Pulse radiolysis was used to generate the radical cations of β-carotene and two xanthophylls, zeaxanthin and lutein, in unilamellar vesicles of dipalmitoylphosphatidyl choline. The rate constants for the reaction (repair) of these carotenoid radical cations with the water-soluble vitamin C were found to be similar (∼1×10 7 M −1 s −1) for β-carotene and zeaxanthin and somewhat lower (∼0.5×10 7 M −1 s −1) for lutein. The results are discussed in terms of the microenvironment of the carotenoids and suggest that for β-carotene, a hydrocarbon carotenoid, the radical cation is able to interact with a water-soluble species even though the parent hydrocarbon carotenoid is probably entirely in the non-polar region of the liposome.

Histogram method to obtain heat capacities in lipid monolayers, curved bilayers, and membranes containing peptides.

Lipid monolayer chain melting transitions were simulated using a two-state Doniach model, and experimental melting profiles of lipid vesicles were analyzed. We sampled the information of a Monte Carlo simulation into a single broad histogram containing complete information about the distribution of states. The information of the monolayer histogram was first used to calculate the melting behavior of a bilayer constructed from two uncoupled monolayers. We then fitted calorimetric heat profiles of various preparations of dipalmitoyl phosphatidylcholine vesicles. This analysis was extended to lipid bilayers. A fixed mean bilayer curvature was shown to result in a broadening of bilayer melting profiles. We furthermore used the histogram method to obtain the chain melting behavior of simple lipid-peptide mixtures.

A photophysical study of the polyene antibiotic filipin: Self-aggregation and filipin–ergosterol interaction

Filipin, a macrolide polyene antibiotic, is known to interact selectively with ergosterol, a constituent of fungi membranes. In this work, the fluorescence resonance energy transfer (FRET) between a fluorescent analog of ergosterol, dehydroergosterol (DHE), and filipin was measured in small unilamellar vesicles of dipalmitoylphosphatidylcholine at 25°C. The time-resolved FRET results were rationalized in the framework of the mean concentration model, and were complemented with steady-state fluorescence intensity, anisotropy and absorption measurements. The results point to the formation of both DHE–filipin aggregates (evidence from static quenching of DHE fluorescence by filipin) and filipin–filipin aggregates (evidence from: (i) the FRET acceptor concentration distributions; (ii) spectral changes of filipin absorption in the vesicles, the excitonic interaction suggesting a stack arrangement; (iii) filipin fluorescence self-quenching), even in presence of DHE and low antibiotic mole fractions (<1 mol%). These results point out that apparently contradictory biochemical models for the action of filipin (some based on the presence of sterols, others not) can be equally valid. Moreover, since results (ii) and (iii) are also observed when a sterol is present, both models of action can actually coexist in membranes with a low sterol content.

The Role of Surfactant Proteins in DPPC Enrichment of Surface Films

A pressure-driven captive bubble surfactometer was used to determine the role of surfactant proteins in refinement of the surface film. The advantage of this apparatus is that surface films can be spread at the interface of an air bubble with a different lipid/protein composition than the subphase vesicles. Using different combinations of subphase vesicles and spread surface films a clear correlation between dipalmitoylphosphatidylcholine (DPPC) content and minimum surface tension was observed. Spread phospholipid films containing 50% DPPC over a subphase containing 50% DPPC vesicles did not form stable surface films with a low minimum surface tension. Addition of surfactant protein B (SP-B) to the surface film led to a progressive decrease in minimum surface tension toward 1 mN/m upon cycling, indicating an enrichment in DPPC. Surfactant protein C (SP-C) had no such detectable refining effect on the film. Surfactant protein A (SP-A) had a positive effect on refinement when it was present in the subphase. However, this effect was only observed when SP-A was combined with SP-B and incubated with subphase vesicles before addition to the air bubble containing sample chamber. Comparison of spread films with adsorbed films indicated that refinement induced by SP-B occurs by selective removal of non-DPPC lipids upon cycling. SP-A, combined with SP-B, induces a selective adsorption of DPPC from subphase vesicles into the surface film. This is achieved by formation of large lipid structures which might resemble tubular myelin.

Thermal Lipid Order−Disorder Transitions in Complexes of Various Disulfide Tethered Macrocyclic Diacylglycerol Analogues and Dipalmitoyl Phosphatidyl Choline. Role of Diacylglycerol Chain Motions

The role of diacylglycerol (DAG) chain motions in binary mixtures of DAG with dipalmitoylphosphatidyl choline (DPPC) has been studied using differential scanning calorimetry (DSC). The DAGs used in this study were synthesized to have different extents of chain restriction. This was accomplished via tethering of two long hydrocarbon chains by a reducible disulfide bond at different depths in the chain. The disulfide tether provides the advantage of comparing the effect of incorporation of each of these chain restricted DAGs in DPPC vesicles with their reduced analogue, in which the S-S linkage could be opened up to -SH by in situ treatment with a reducing agent, dithiothreitol (DTT). DSC analysis with such mixtures reveals that restriction of chains at different depths in the DAGs lead to very different properties of the overall DPPC/DAG coaggregate. DAGs in which the disulfide tether is located at the middle of the chain have only partial chain restriction. Incorporation of this type of DAG in DPPC destabilizes the resulting vesicles while the end-tethered DAGs upon incorporation in DPPC vesicles lead to stabilization of the coaggregates. Similar studies with the reduced, open-chain analogues reveal that release in chain restriction leads to extra stabilization in the coaggregates.

Solvation Dynamics of DCM in Dipalmitoyl Phosphatidylcholine Lipid

Solvation dynamics of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl) 4H-pyran (DCM) in dipalmitoylphosphatidylcholine (DPPC) vesicles in water is studied using picosecond emission spectroscopy. The solvation dynamics of DCM in DPPC vesicles is found to be bi-exponential with two components of 120±20ps (20%) and 5.5±0.5ns (80%). This indicates slow relaxation of the water molecules inside the water pool of the lipid vesicles.

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·HCl), tetracaine (TC·HCl), lidocaine (LC·HCl) and procaine hydrochlorides (PC·HCl). LC·HCl and PC·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·HCl or DC·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·HCl up to 21 mmol kg −1 or DC·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·HCl (or 40–60 mmol kg −1 DC·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·HCl (or 60 mmol kg −1 DC·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.

Membrane Probe Distribution Heterogeneity: A Resonance Energy Transfer Study

Probe distribution heterogeneity was studied in model membranes (large unilamellar vesicles of dipalmitoylphosphatidylcholine) using resonance energy transfer. Two Förster pairs were used: octadecylrhodamine B (ORB; donor)/1,1',3,3,3',3'-hexamethylindotricarbocyanine (DiIC1(7); acceptor) and N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dipalmitoylphosphatidylethanolamine (NBD−PE; donor)/N-(lissamine-rhodamine B)-dipalmitoylphosphatidylethanolamine (Rh−PE; acceptor). The donors' fluorescence decays were analyzed in the framework of the mean local concentration model (see preceding article). As acceptor concentration distributions, discrete, Gaussian, and sum of two Gaussian functions were considered. In the fluid phase (50 °C), for moderate (<1%) acceptor concentrations, both donors and acceptors of the two pairs are essentially randomly distributed, and the recovered acceptor concentration distribution curves are well described by single narrow Gaussians. For the gel phase (25 °C), a sum of two Gaussians acceptor concentration distribution is necessary to describe the data. It is concluded that, in the gel phase, for the ORB/DiIC1(7) pair, acceptors partially segregate into a pseudo (defect) phase and donors are randomly dispersed in the bulk lattice. At variance, for the NBD−PE/Rh−PE pair, partial segregation of both probes into a pseudo (defect) phase occurs at 25 °C. These conclusions are supported by detailed additional photophysical measurements for these probes in membranes (steady-state energy transfer, fluorescence self-quenching in steady and transient states, and energy migration), and agree with the preceding article's simulations.

Supported Bilayers On Silica

Adsorption isotherms for bilayer-forming synthetic amphiphiles or phospholipids adsorbed from vesicles onto hydrophilic silica particles (Aerosil 0X-50) are obtained over a range of experimental conditions. Phosphatidylcholine (PC), dipalmitoylphosphatidylcholine (DPPC), and dioctadecyldimethylammonium bromide (DODAB) dispersed in 10 mM of two different buffers (Tris or Hepes) at three different pH values as small unilamellar vesicles present affinities for silica following the sequence DODAB > DPPC > PC. Deposition of phospholipid bilayers was favored by the presence of Tris as buffer, pH ≤ 7.4, and temperatures above the phase transition temperature for the phospholipid bilayer. Interaction between DPPC vesicles and silica at 65°C for 1 h efficiently leads to bilayer deposition at maximal adsorption, if Tris is the buffer used. Consistently, wettability of SiO2 planar surfaces precisely under conditions of bilayer deposition (as depicted from the isotherms) in the presence of the lipid vesicles yielded a large increase on surface hydrophobicity for DPPC at 65°C and DODAB at room temperature. Reduction of merocyanine 540 absorbance at 565 nm was used as a marker for bilayer deposition onto the silica particles. Upon interaction with the solid particle, absorbance at 565 nm displays a decrease with time that corresponds to the percentage of dye sandwiched between the bilayer and the solid particle surface and thereby hidden from the incident light. For DPPC at 65°C and DODAB at room temperature, but not for PC, occurrence of bilayer deposition depicted from the adsorption isotherms is confirmed from the three techniques employed for detection.

Antimicrobial Nonapeptide Leucinostatin A-Dependent Effects on the Physical Properties of Phospholipid Model Membranes

The influence exerted by the antimicrobial and antimicotic nonapeptide Leucinostatin A (Leu A) on a biological membrane model made up of dipalmitoylphosphatidylcholine (DPPC) was investigated. Drug–membrane interactions, studied by means of differential scanning calorimetry and Fourier-transform infrared spectroscopy, depend on the behavior of the molecule which positions its hydrophilic part toward the bilayer phospholipid polar heads, while it inserts its hydrophobic portion into the membrane phospholipid acyl chain moiety. Calorimetric experiments showed that the peptide undergoes self-aggregation within the bilayer structure when present at molar fractions higher than 0.03. Peptide–membrane interactions as a function of time were analyzed as well. The latter demonstrated that Leu A inserts rapidly into the outer bilayers of DPPC membranes. 45Ca2+ uptake by DPPC vesicles gave a reason for the ionophoric activity of Leu A against both mouse thymocytes and artificial membranes, which seems to be correlated to the self-assembling property of the peptide within the bilayers.

Fluorescence study on the interaction of a multiple antigenic peptide from hepatitis A virus with lipid vesicles.

The interaction of the multiple antigenic peptide MAP4VP3 with lipid membranes has been studied by spectroscopic techniques. MAP4VP3 is a multimeric peptide that corresponds to four units of the sequence 110-121 of the capsid protein VP3 of hepatitis A virus. In order to evaluate the electrostatic and hydrophobic components on the lipid-peptide interaction, small unilamelar vesicles of different compositions, including zwitterionic dipalmitoylphosphatidylcholine (DPPC), anionic dipalmitoylphosphatidylcholine/phatidylinositol (DPPC:PI 9:1), and cationic dipalmitoylphosphatidylcholine/stearylamine (DPPC:SA 9.5:0.5), were used as membrane models. Intrinsic tryptophan fluorescence changes and energy transfer experiments show that MAP4VP3 binds to all three types of vesicles with the same stoichiometry, indicating that the electrostatic component of the interaction is not important for binding of this anionic peptide. Steady-state polarization experiments with vesicles labeled with 1,6-diphenyl-1,3,5-hexatriene or with 1-anilino-8-naphtalene sulphonic acid indicate that MAP4VP3 induces a change in the packing of the bilayers, with a decrease in the fluidity of the lipids and an increase in the temperature of phase transition in all the vesicles. The percentage of lipid exposed to the bulk aqueous phase is around 60% in intact vesicles, and it does not change upon binding of MAP4VP3 to DPPC vesicles, indicating that the peptide does not alter the permeability of the membrane. An increase in the amount of lipid exposed to the aqueous phase in cationic vesicles indicates either lipid flip-flop or disruption of the vesicles. Binding to DPPC vesicles occurs without leakage of entrapped carboxyfluorescein, even at high mol fractions of peptide. However, a time-dependent leakage is seen with cationic DPPC/SA and anionic DPPC/PI vesicles, indicating that the peptide induces membrane destabilization and not lipid flip-flop. Resonance energy transfer experiments show that MAP4VP3 leakage from cationic vesicles is due to membrane fusion, whereas leakage from anionic vesicles is not accompanied by lipid mixing. Results show that MAP4VP3 interacts strongly with the lipid components of the membrane, and although binding is not of electrostatic nature, the bound form of the peptide has different activity depending on the membrane net charge; thus, it is membrane disruptive in cationic and anionic vesicles, whereas no destabilizing effect is seen in DPPC vesicles.

Influence of different parameters on drug release from hydrogel systems to a biomembrane model. Evaluation by differential scanning calorimetry technique

A comparative study on the drug release capacity of four water swellable polymeric systems was carried out by differential scanning calorimetry (DSC). The polymeric systems chosen were α, β-polyaspartahydrazide (PAHy) crosslinked by glutaraldehyde (GLU) (PAHy-GLU) or by ethyleneglycoldiglycidylether (EGDGE), (PAHy-EGDGE), polyvinylalcohol (PVA) crosslinked by glutaraldehyde (PVA-GLU) and α, β-poly( N-hydroxyethyl)- dl-aspartamide (PHEA) by gamma irradiation (PHEA- γ matrices). The degree of crosslinking for PAHy-GLU, PAHy-EGDGE and PVA-GLU samples was about 0.4 and 0.8. These hydrogels were characterized as free of drugs and were loaded with diflunisal (DFN) (≈2.5% w/w). Diflunisal, a non-steroidal anti-inflammatory drug, has been chosen as a model drug to be incorporated into polymeric matrices to follow the release processes of a drug from these hydrogels to a model membrane made by unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). Differential scanning calorimetry appears to be a suitable technique to follow the transfer kinetics of the drug from the controlled release system to the biomembrane model. The drug releases from all the considered polymeric hydrogels, were compared with the release observed from the drug solid form by examining the effects on the thermotropic behaviour of DPPC unilamellar vesicles. The release kinetics of the drug from hydrogels were followed at 25, 37 and 50°C to evidence the influence of temperature on the drug release and on the successive transfer to biological membrane model. Particularly, it appears evident that the total amount of drug transferred and the release rate are affected by the polymer crosslinking degree (it increases with crosslinking decrease) as well as by the nature of crosslinking agent. In fact, the drug release profiles from PAHy-GLU samples are more differentiated than those from PAHy-EGDGE. The effect of parameters correlating with the properties of starting polymer, such as water-affinity, crystallinity, glass-to-rubber transition temperature and affinity towards drug molecules, has been also evaluated.

Interaction of Ubiquinone-10 with Dipalmitoylphosphatidylcholine and Their Formation of Small Dispersed Particles

Stable aqueous dispersions of ubiquinone-10 (UQ) were obtained by cosonication with dipalmitoylphosphatidylcholine (DPPC) in the UQ mole fraction range 0.1–0.7. To clarify the dispersal mechanism, the dispersed particles were characterized, and the interaction between UQ and DPPC was investigated using several physicochemical techniques. Dynamic light scattering (DLS) measurements showed that the diameter of the dispersed particles was 50–70 nm. A limited amount of UQ was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein, and the volume in the UQ/DPPC particles decreased remarkably with the addition of UQ into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess UQ separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

Aggregation of Dipalmitoylphosphatidylcholine Vesicles Induced by Some Metal Ions with High Activity for Hydrolysis

AlCl3, YCl3, and Pb(NO3)2 induced a reversible aggregation of vesicles prepared from dipalmitoylphosphatidylcholine (DPPC). The dependence of the aggregation behavior on both the salt concentration and temperature was similar to that previously reported for BeSO4. A nature common to these four salt species is that the metal ions have a strong tendency to be hydrolyzed in aqueous solutions. The mechanism of the DPPC vesicle aggregation induced by these metal ions is considered as follows. When these metal ions are bound to the vesicular surface, they could cause a partial destruction of the hydration shell on the surface of DPPC vesicles because of their high activity for hydrolysis. This would result in the reduction of a repulsive hydration force which is responsible for the stability of phosphatidylcholine vesicles, and accordingly, would lead to an aggregation of vesicles. The dehydration of hydrated water around DPPC headgroups is suggested by the phase-transition behavior of hydrated DPPC bilayer in the presence of these metal ions.

Fluorescent Dimers of Merocyanine 540 (MC540) in the Gel Phase of Phosphatidylcholine Liposomes

Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel‐phase state. No dimer fluorescence was observed either in monopalmitoylphosphati‐dylcholine (C16PC) micelles or in liquid‐crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC‐LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC‐LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.

Formation and structure of stably dispersed particles composed of retinal with dipalmitoylphosphatidylcholine: coexistence of emulsion particles with bilayer vesicles

In order to develop an intravenous formulation of all- trans-retinal (vitamin A aldehyde, VAA) for the treatment of night blindness, VAA and dipalmitoylphosphatidylcholine (DPPC) were sonicated and the dispersions in the VAA mole fraction range of 0.1–0.7 were stable at room temperature for 3 days. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VAA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50–70 nm. A limited amount of VAA is incorporated into DPPC bilayer membranes (approximately 5 mole%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VAA/DPPC particles was decreased remarkably with the addition of VAA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO 4. These results indicate that the excess VAA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer. The monolayer–bilayer equilibrium of VAA/DPPC mixtures was estimated by measurement of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of DPPC+ core of VAA) with vesicular particles (bilayer) was critically important for the formation of the stably dispersed particles of the lipid mixture.

Physicochemical Behavior of Polylysine-[HAV-VP3 Peptide] Constructs at the Air−Water Interface

Branched-chain polypeptides based on a polylysine backbone were synthesized for use as delivery systems for biologically active molecules. To develop a new immunoadjuvant system for hepatitis A virus (HAV) antigens, a peptide fragment (110−121) of the VP3 hepatitis A protein has been linked to cationic and amphoteric polymeric polypeptides. In the present paper, we describe the physicochemical characterization of three branched polypeptide-[HAV-VP3(110−121) peptide] constructs, where the peptide is attached to the polymer side chains by disulfide linkage. The surface activity of these three conjugates has been studied as a function of time and concentration in the subphase. Moreover, its insertion into phospholipid DPPC (dipalmitoylphosphatidylcholine) monolayers has also been determined. The results show that these constructs are surface active and can insert into the lipid monolayers. AK construct is slightly more active than EAK and SAK constructs both in the presence and absence of DPPC monolayers. This behavior suggests that construct disposition at interfaces is mainly dependent on nonsubstituted side chains. Fluorescence polarization studies, performed with DPPC vesicles saturated with either DPH (1,6-diphenyl-1,3,5-hexatriene) or ANS (1-anilino-8-naphthalenesulfonic acid), indicate a strong rigidifying effect of the three constructs on the polar heads and alkyl chains of bilayers.

Interaction of retinol with dipalmitoylphosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of all- trans-retinol (vitamin A, VA) were obtained by sonication with dipalmitoylphosphatidylcholine (DPPC) in the VA mole fraction range 0.1–0.7. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50–70 nm. A limited amount of VA was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VA/DPPC particles was decreased markedly with the addition of VA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO 4. These results indicate that the excess VA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

Small angle neutron scattering and calorimetric studies of large unilamellar vesicles of the phospholipid dipalmitoylphosphatidylcholine

High-resolution differential scanning calorimetry (DSC) and small angle neutron scattering (SANS) experiments have been conducted on large unilamellar vesicles (LUV's) of the phospholipid dipalmitoylphosphatidylcholine (DPPC) in excess water. The DSC results indicate a phase transition at temperatures corresponding to the gel ${(L}_{{\ensuremath{\beta}}^{\ensuremath{'}}})$ to ripple ${(P}_{{\ensuremath{\beta}}^{\ensuremath{'}}})$ phase transition seen in multilamellar vesicles of DPPC while the SANS experiments provide direct evidence for the formation of the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase in these systems. In addition, it is shown that SANS is an effective technique for extracting structural parameters such as vesicle radius and thickness in LUV model membrane systems.