Incorporation of bovine thyroid peroxidase in liposomes

In the microsomal fraction of thyroid glands, the temperature dependence of DPH fluorescence polarization showed a discontinuity in the range of 29-33 degrees C. The transition temperatures of DMPC, DPPC and DSPC are near to the observed for the microsomal fraction. So that, thyroid peroxidase (TPO) was incorporated into liposomes made with these phospholipids. When DPH was incorporated in this peroxidase-liposome complex, a less pronounced phase transition was observed in the profiles of temperature dependence of DPH polarization, and the incorporation of the enzyme decreased the Tc. Arrhenius plots of TPO incorporated into liposomes showed discontinuities at similar temperatures observed by fluorescence polarization. The decrease of transition temperature of liposomes induced by thyroid peroxidase incorporation suggests that this enzyme seems to need a fluid medium for its enzyme activity.     

Erratum to: “Effects of Gamma Irradiation on the Physical Stability of Liposomal Phospholipids” [J. Lipos. Res. 7, (1997) 503-528]

Abstract

The effects of liposome composition and gamma irradiation on the phase transition, size, zeta potential and pH were investigated using factorial designs. In addition, the effect of irradiation on the leak-in rate of calcein was evaluated for one of the liposome composition. The liposomes were stored for 6 months in order to reveal any possible long term effects. The phospholipids used were dipalmitoyl phosphatidyl choline (DPPC) or egg phosphatidyl choline (egg PC). Charge was introduced to the liposomal bilayers by the addition of 10% dipalmitoyl phosphatidyl glycerol (DPPG) or egg phosphatidyl glycerol (egg PG). The liposome-suspensions were obtained by the extrusion method. After gamma irradiation changes in the phase transition, zeta potential and pH of the liposomes were observed. The size of the liposomes was not affected by the irradiation, but the irradiation prevented the neutral DPPC-liposomes from aggregation. This was confirmed by cryo-electron microscopy. No change in the leak-in rate was observed. During storage, a significant increase in size was observed only for the non-irradiated egg PC-liposomes. For all the liposome-suspensions composed of unsaturated phospholipids, a significant drop in pH and an increased zeta potential (more negative) was measured. Changes in the phase transition for the neutral DPPC-liposomes (non-irradiated and irradiated) were observed during gamma irradiation.     

Biophysical characterization of gold nanoparticles-loaded liposomes

Gold nanoparticles were prepared and loaded into the bilayer of dipalmitoylphosphatidylcholine (DPPC) liposomes, named as gold-loaded liposomes. Biophysical characterization of gold-loaded liposomes was studied by transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy as well as turbidity and rheological measurements. FTIR measurements showed that gold nanoparticles made significant changes in the frequency of the CH₂ stretching bands, revealing that gold nanoparticles increased the number of gauche conformers and create a conformational change within the acyl chains of phospholipids. The transmission electron micrographs (TEM) revealed that gold nanoparticles were loaded in the liposomal bilayer. The zeta potential of DPPC liposomes had a more negative value after incorporating of Au NPs into liposomal membranes. Turbidity studies revealed that the loading of gold nanoparticles into DPPC liposomes results in shifting the temperature of the main phase transition to a lower value. The membrane fluidity of DPPC bilayer was increased by loading the gold nanoparticles as shown from rheological measurements. Knowledge gained in this study may open the door to pursuing liposomes as a viable strategy for Au NPs delivery in many diagnostic and therapeutic applications.     

Temperature- and ionic strength-induced conformational changes in the lipid head group region of liposomes as suggested by zeta potential data.

Neutral liposomes composed of DMPC (dimyristoylphosphatidylcholine), DPPC (dipalmitoylphosphatidylcholine) or DSPC (distearoylphosphatidylcholine) are found to exhibit non-zero zeta potentials in an electric field even when they are dispersed in solution at pH 7.4. A model for the orientation of lipid head groups is proposed to explain the observed non-zero zeta potentials. The dependence of the zeta potential on temperature and ionic strength is analyzed via this model to obtain the information on the direction of the lipid head group in the liposome surface region. The direction of the lipid head group is found to be sensitive to the temperature and the ionic strength of the medium. At low ionic strengths, the phosphatidyl groups are located at the outer portion of the head group region. At constant temperature, as the ionic strength increases, the choline group approaches the outer region of the bilayer surface while the phosphatidyl group hides behind the surface. At the phase transition temperature of the lipid, the phosphatidyl group lies in the outer-most region of the surface and the choline group is in the inner-most region.     

IR spectroscopic study of phospholipid emulsions containing cholesteryl oleate

As models for the lipid organization of low density lipoproteins (LDL), protein-free aqueous emulsions are prepared from dimyristoyl phosphatidyl choline (DMPC), dipalmitoyl phosphatidyl choline (DPPC), and cholesteryl oleate (CO). Aqueous dispersions containing these lipids are sonicated and yield stable particles with diameters varying between 20 and 40 nm as measured through electron microscopy. IR spectroscopy shows that emulsions consisting of DMPC, DPPC, and CO at 3/1/1 and 1/1/1 ratios undergo specific thermal transitions, depending on their composition, that can be assigned to the phospholipids forming the surface layer of the emulsion particles and to core-located CO. However, at the 1/3/1 DMPC/DPPC/CO ratio this lipid system exhibits an order-disorder transition of the mixed phospholipids with no significant transition associated with core-located CO. Observation of the methylene C&bond;H and C&bond;D stretching modes of nondeuterated and deuterated lipids enables the packing characteristics and conformational order of each lipid to be monitored separately. The transition temperature changes compared to the temperatures for the analogous transitions in neat CO and CO-free phospholipid vesicles suggest the existence of interactions between CO and the above phospholipids in the ternary emulsion particles; these interactions are stronger at the 1/3/1 DMPC/DPPC/CO ratio. The results show that interactions between core and surface phases are dependent on the emulsion lipid composition and that these findings may be extended to native lipoproteins.     

The enzymatic acyl exchange of phospholipids with lipases

The acyl exchange of phospholipids with lipases was investigated. The lipase from Rhizopus delemar catalyzed the acyl exchange reaction between various phospholipids and fatty acids. When we incubated 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl choline (DPPC) and oleic acid with lipase from R. delemar, the yield of diacyl phosphatidyl choline (PC) was 25% and the fatty acid composition of the converted PC was an oleic acid content of 25% and a palmitic acid content of 75%. This reaction exhibited 1-positional specificity. Three industrial lipases from Rhizopus sp., Mucor javanicus, and Candida cylindracea had the activity of the acyl exchange of phosphatidyl choline. The lipase from R. sp. gave the best result.     

Phospholipid-enzyme interactions of chitin synthase of Coprinus cinereus

Abstract The effect of phospholipids on chitin synthase activity has been studied with digitonin-solubilized and partially purified preparations from Coprinus cinereus . When cholate was used as detergent, it inhibited enzyme activity, but this inhibition was reversed by increasing concentrations of phospholipids. Preincubation with cholate and phospholipid caused irreversible loss of activity. When sonicated with solubilized enzyme preparation, dimyristoyl phosphatidyl choline strongly stimulated activity, while dioleoyl phosphatidyl choline was inhibitory. The Arrhenius plot of the effect of temperature on enzyme activity contained breaks, characteristic of a membrane-bound enzyme. It is suggested that chitin synthase requires an annulus of phospholipids for activity.     

Interaction of tryptophan with lecithin liposomes: NMR and turbidity studies

A study on the interactions between tryptophan (Trp) and dipalmitoylphosphatidyl choline (DPPC) liposomes conducted with the NMR technique and taking turbidity measurements is reported. Trp is shown to be incorporated into the bilayer only when interaction occurs above gel-liquid transition. Disappearance of turbidity changes at the phase transition temperatures are shown to occur with Trp incorporation. 1H and 13C NMR relaxation times T1 of DPPC are seen to be reduced. Acyl chain signal intensity is shown to decrease and the corresponding line-width to increase as a function of Trp concentration. DPPC 31P [1H] Nuclear Overhauser Effect (NOE) is depressed by the presence of Trp above gel-liquid transition temperature whereas NOE remains high below phase transition temperature when Trp is present in the bilayer. Effects are shown to be the same in both H2O and in 2H2O. A membrane modification that may account for the previously observed inhibition of polysaccharide induced cell aggregation is hypothesized.     

Doxorubicin Encapsulated in Long-Circulating Thermosensitive Liposomes

Abstract

Doxorubicin (DXR) was encapsulated in long-circulating, thermosensitive liposomes (TSL, 180-200 nm in mean diameter), prepared from dipalmitoyl phosphatidyl choline (DPPC)/distearoyl phosphatidyl choline (DSPC) (9:1, m/m) and either 3 mol% of amphipathic polyethylene glycol (PEG) with 1000 in average molecular weight or 6 mol% of ganglioside GMI (GMI), with 95-98% entrapping efficiency by the pH gradient method. 57% or 45% of the entrapped DXR was released from PEG/DPPC/DSPC or G_M1/DPPC/DSPC liposomes, respectively, by incubation with 20% serum at 42°C for 5 min. Inclusion of PEG or G_M1 endowed TSL with prolonged circulation ability, resulting in increased blood levels of liposomes and decreased reticuloendothelial system (RES) uptake over 6 hours after injection. Concomitantly, high DXR level in blood was kept for long time.

Accumulation of DXR into tumor tissue of tumor-bearing mice (mouse colon carcinoma 26) by local hyperthermia after injection of DXR-long-circulating TSL was 2 times or 7 times higher than that after treatment with DXR-TSL liposomes or free DXR in combination with hyperthermia, respectively. Furthermore, the systemic treatment with DXR-long-circulating TSL and hyperthermia resulted in effective tumor growth retardation and increased survival time. These results indicate that the combination of long-circulating, thermosensitive liposomes with local hyperthermia at the tumor site could be clinically useful for delivering a wide range of chemotherapeutic agents in the treatment of solid tumors.     

Lipid-mediated regulation of pore-forming activity of syringomycin E by thyroid hormones and xanthene dyes

The effects of dipole modifiers, thyroid hormones (thyroxine and triiodothyronine) and xanthene dyes (Rose Bengal, phloxineB, erythrosin, eosinY and fluorescein) on the pore-forming activity of the lipopeptide syringomycin E (SRE) produced by Pseudomonas syringae were studied in a model bilayer. Thyroxine does not noticeably influence the steady-state number of open SRE channels (N_op), whereas triiodothyronine decreases it 10-fold at − 50 mV. Rose Bengal, phloxine B and erythrosin significantly increase N_op by 350, 100 and 70 times, respectively. Eosin Y and fluorescein do not practically affect the pore-forming activity of SRE. Recently, we showed that hormones decrease the dipole potential of lipid bilayers by approximately 60 mV at 50 μM, while Rose Bengal, phloxine B and erythrosin at 2.5 μM reduce the membrane dipole potential by 120, 80 and 50 mV, respectively. In the present study using differential scanning microcalorimetry, confocal fluorescence microscopy, the calcein release technique and measurements of membrane curvature elasticity, we show that triiodothyronine strongly affects the fluidity of model membranes: its addition leads to a significant decrease in the temperature and cooperativity of the main phase transition of DPPC, calcein leakage from DOPC vesicles, fluidization of solid domains in DOPC/DPPC liposomes, and promotion of lipid curvature stress. Thyroxine exerts a weaker effect. Xanthene dyes do not influence the phase transition of DPPC. Despite the decrease in the dipole potential, thyroid hormones modulate SRE channels predominantly via the elastic properties of the membrane, whereas the xanthene dyes Rose Bengal, phloxine B and erythrosine affect SRE channels via bilayer electrostatics.     

Control of the potassium ion-stimulated adenosine triphosphatase of pig gastric microsomes: effects of lipid environment and the endogenous activator

The K⁺-stimulated ATPase activity associated with the purified gastric microsomes from the pig gastric mucosa can be completely inactivated by treatment with 15% ethanol for 60 s at 37 °C but not at 25 °C. Sequential exposure of the microsomes to 15% ethanol at 25 and 37 °C caused the release of 2.9 and 4.3% of the total membrane phospholipids, respectively, consisting entirely of phosphatidyl choline and phosphatidyl ethanolamine. The ethanol-treated (37 °C) membrane had high basal (with Mg²⁺ as the only cation in the assay mixture) activity, which was further enhanced during reconstitution with phosphatidyl choline or phosphatidyl ethanolamine. The high basal activities could be reduced to the normal control level by assaying the enzyme in presence of the “activator protein,” partially purified from the soluble supernatant of the pig gastric cells. Phosphatidyl choline was somewhat more effective than phosphatidyl ethanolamine in the restoration of the activity of the ethanol-treated enzyme while phosphatidyl serine, phosphatidyl inositol, and sphingomyelin were without any effect. Synthetic phosphatidyl choline with various fatty acid substitutions were tested for their effectiveness in the restoration of the ethanol-inactivated enzyme. The distearoyl (18:0), dioleoyl (18:1), and dilinoleoyl (18:2) derivatives of phosphatidyl choline were almost equally effective while dipalmitoyl (16:0) phosphatidyl choline was somewhat less effective in the reconstitution process. Cholesterol appeared to interfere with phosphatidyl choline in the restoration of the activity of ethanol-treated enzyme. The fatty acid composition of phosphatidyl choline and phosphatidyl ethanolamine extracted by 15% ethanol at 37 °C was clearly different than those of the total microsome. Our data suggest that the phospholipids extracted by 15% ethanol at 37 °C are derived primarily from the immediate lipid environment of the enzyme and ATP together with Mg²⁺ and K⁺ help the partially delipidated enzyme to retain the appropriate conformation for the subsequent reconstitution. Furthermore, ethanol appears to either release or inactivate the membrane-associated activator protein, demonstrated to be essential for the K⁺-stimulated activity of the pig gastric ATPase.     

Phosphatidyl alcohols: Effect of head group size on domain forming properties and interactions with sterols

In this study, we have examined the membrane properties and sterol interactions of phosphatidyl alcohols varying in the size of the alcohol head group coupled to the sn-3-linked phosphate. Phosphatidyl alcohols of interest were dipalmitoyl derivatives with methanol (DPPMe), ethanol (DPPEt), propanol (DPPPr), or butanol (DPPBu) head groups. The Phosphatidyl alcohols are biologically relevant, because they can be formed in membranes by the phospholipase D reaction in the presence of alcohol. The melting behavior of pure phosphatidyl alcohols and mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or cholesterol was assessed using high sensitivity differential scanning calorimetry (DSC). DPPMe had the highest melting temperature (∼ 49 °C), whereas the other phosphatidyl alcohols had similar melting temperatures as DPPC (∼ 40-41 °C). All phosphatidyl alcohols, except DPPMe, also showed good miscibility with DPPC. The effects of cholesterol on the melting behavior and membrane order in multilamellar bilayer vesicles were assessed using steady-state anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and DSC. The ordering effect of cholesterol in the fluid phase was lower for all phosphatidyl alcohols as compared to DPPC and decreased with increasing head group size. The formation of ordered domains containing the phosphatidyl alcohols in complex bilayer membranes was determined using fluorescence quenching of DPH or the sterol analogue cholesta-5,7,(11)-trien-3-beta-ol (CTL). The phosphatidyl alcohols did not appear to form sterol-enriched ordered domains, whereas DPPMe, DPPEt appeared to form ordered domains in the temperature window examined (10-50 °C). The partitioning of CTL into bilayer membranes containing phosphatidyl alcohols was to a small extent increased for DPPMe and DPPEt, but in general, sterol interactions were weak or unfavorable for the phosphatidyl alcohols. Our results show that the biophysical and sterol interacting properties of phosphatidyl alcohols, having identical acyl chain structures, are markedly dependent on the size of the head group.     

The occurrence of phosphatidyl choline exchange protein in leaves

The transfer of phosphatidyl choline between liposomes was stimulated by the protein fractions from spinach leaves, etiolated and greening leaves of $\text{Avena}$ seedlings. This is confirmed by the transfer of [¹⁴C]phosphatidyl choline or spin-labeled phosphatidyl choline between donor and acceptor liposomes. ESR spectrum changes also indicated that no spin-labeled phosphatidyl choline was released from donor liposomes by spinach leaf protein unless acceptor liposomes were present. [¹⁴C]phospholipids were transferred from liposomes to both spinach chloroplasts and $\text{Avena}$ etiochloroplasts by phosphatidyl choline exchange protein from germinated castor bean endosperms and further from liposomes to spinach chloroplasts by spinach leaf protein. These results support the view that phosphatidyl choline in the plastid is supplied from the synthesis site, the endoplasmic reticulum, by phospholipid exchange protein.     

Serum of normal subjects contains IgG with antibody-like specificity for phospholipids

Using counterimmunoelectrophoresis, sera from normal subjects display precipitin activity against liposomes made by phosphatidic acid, phosphatidyl ethanolamine, phosphatidyl serine and cardiolipin. No activity is detected by using liposomes made with phosphatidyl choline, sphingomyelin and phosphatidyl inositol. The same pattern of precipitation is obtained when pure IgG from normal subjects is tested against phospholipids. No precipitin activity is found by using pure IgM. The precipitin reaction is prevented by preincubating liposomes with Fab obtained from normal IgG.     

Chlorpromazine interaction with glycerophospholipid liposomes studied by magic angle spinning solid state (13)C-NMR and differential scanning calorimetry

Phosphatidylserine (PS) extracted from pig brain and synthetic dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were used to make DPPC/DMPC and DPPC/PS large unilamellar liposomes with a diameter of approximately 1 microm. Chlorpromazine-HCl (CPZ), an amphipathic cationic psychotropic drug of the phenothiazine group, is known to partition into lipid bilayer membranes of liposomes with partition coefficients depending on the acyl chain length and to alter the bilayer structure in a manner depending on the phospholipid headgroups. The effects of adding CPZ to these membranes were studied by differential scanning calorimetry and proton cross polarization solid state magic angle spinning (13)C-nuclear magnetic resonance spectroscopy (CP-MAS-(13)C-NMR). CP-MAS-(13)C-NMR spectra of the DPPC (60%)/DMPC (40%) and the DPPC (54%)/DMPC (36%)/CPZ (10%) liposomes, show that CPZ has low or no interaction with the phospholipids of this neutral and densely packed bilayer. Conversely, the DPPC (54%)/PS (36%)/CPZ (10%) bilayer at 25 degrees C demonstrates interaction of CPZ with the phospholipid headgroups (PS). This CPZ interaction causes about 30% of the acyl chains to enter the gauche conformation with low or no CPZ interdigitation among the acyl chains at this temperature (25 degrees C). The DPPC (54%)/PS (36%)/CPZ (10%) bilayer at a sample temperature of 37 degrees C (T(C)=31.2 degrees C), shows CPZ interdigitation among the phospholipids as deduced from the finding that approximately 30% of the phospholipid acyl chains carbon resonances shift low-field by 5-15 ppm.     

Phosphatidyl inositol: myo-Inositol exchange enzyme from rat liver: Partial purification and characterization

The enzyme which catalyzes CDP-diglyceride-independent incorporation of myo-inositol into phosphatidyl inositol was solubilized from rat liver microsomes by sodium cholate and was partially purified by ammonium sulfate fractionation and sucrose density gradient centrifugation. Addition of phospholipids during purification and assay procedures prevented irreversible loss of the enzyme activity to some extent. The resulting preparation contained about 3.7% of the protein and 35% of the original activity of the microsomal fraction. The activity of the enzyme preparation was strongly enhanced by addition of phosphatidyl inositol. The enzyme required Mn²⁺ for activity. The K_m for myo-inositol was 4 × 10^?5m. The pH optimum was 7.4. The activity was inhibited by thiol-reactive reagents and also to some extent by inosose-2 but not by scyllitol. Phosphorus-containing acidic substances such as acidic phospholipids and nucleotides were generally inhibitory. It was found that the preparation catalyzed liberation of inositol moiety from phosphatidyl inositol in a manner dependent on the concentration of free myo-inositol and also on Mn². The K_m of this reaction for free myo-inositol was estimated to be 7 × 10^?5m. This result indicates that CDP-diglyceride-independent incorporation, which has been assumed to show inositol exchange reaction, actually represents an exchange reaction between the myo-inositol moiety of phosphatidyl inositol and free myo-inositol. Phosphatidyl choline and phosphatidyl ethanolamine did not play a role as acceptor of the exchange reaction.     

AFM study of the thermotropic behaviour of supported DPPC bilayers with and without the model peptide WALP23

Temperature-controlled Atomic Force Microscopy (TC-AFM) in Contact Mode is used here to directly image the mechanisms by which melting and crystallization of supported, hydrated DPPC bilayers proceed in the presence and absence of the model peptide WALP23. Melting from the gel L_β′ to the liquid-crystalline L_α phase starts at pre-existing line-type packing defects (grain boundaries) in absence of the peptide. The exact transition temperature is shown to be influenced by the magnitude of the force exerted by the AFM probe on the bilayer, but is higher than the main transition temperature of non-supported DPPC vesicles in all cases due to bilayer–substrate interactions. Cooling of the fluid L_α bilayer shows the formation of the line-type defects at the borders between different gel-phase regions that originate from different nuclei. The number of these defects depends directly on the rate of cooling through the transition, as predicted by classical nucleation theory.The presence of the transmembrane, synthetic model peptide WALP23 is known to give rise to heterogeneity in the bilayer as microdomains with a striped appearance are formed in the DPPC bilayer. This striated phase consists of alternating lines of lipids and peptide. It is shown here that melting starts with the peptide-associated lipids in the domains, whose melting temperature is lowered by 0.8–2.0 °C compared to the remaining, peptide-free parts of the bilayer. The stabilization of the fluid phase is ascribed to adaptations of the lipids to the shorter peptide. The lipids not associated with the peptide melt at the same temperature as those in the pure DPPC supported bilayer.     

Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers

In order to understand the effect of polar head group modification on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidyl-N-methylethanolamine (DPMePE), dipalmitoylphosphatidyl-N,N-dimethylethanolamine (DPMe₂PE) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were observed by differential scanning calorimetry and high-pressure optical methods. The temperatures of the so-called main transition from the gel (L_β) or ripple gel (P_β′) phase to the liquid crystalline (L_α) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary, dT/dp, was in the range of 0.220-0.264 K MPa⁻¹ depending on the number of methyl groups in the head group of lipids. The main-transition temperatures of N-methylated DPPEs decreased with increasing size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities of the main transition between the phospholipids. With respect to the transition from the subgel (L_c) phase to the lamellar gel (L_β or L_β′) phase, the transition temperatures were also elevated by applying pressure. In the case of DPPE bilayer the L_c/L_β transition appeared at a pressure higher than 21.8 MPa. At a pressure below 21.8 MPa the L_c/L_α transition was observed at a temperature higher than the main-transition temperature. The main (L_β/L_α) transition can be recognized as the transformation between metastable phases in the range from ambient pressure to 21.8 MPa. Polymorphism in the gel phase is characteristic of DPPC bilayer membrane unlike other lipid bilayers used in this study: the L_β′, P_β′ and pressure-induced interdigitated gel (L_βI) phases were observed only in the DPPC bilayer. Regarding the bilayers of DPPE, DPMePE and DPMe₂PE, the interdigitation of acyl chain did not appear even at pressures as high as 200 MPa.     

Energy transfer in artificial membrane systems. Singlet-singlet energy transfer from alloxazines to isoalloxazines in dipamitoyl phosphatidylcholine liposomes and dialkylammonium chloride vesicles

The singlet-singlet energy transfer from alloxazines to isoalloxazines has been investigated in dipalmitoyl phosphatidylcholine (DPPC) liposomes and dioctadecyltrimethylammonium chloride (2C₁₈NC) vesicles to clarify the role of the artificial membranes in the energy transfer phenomenon. The structures of the artificial membranes were divided into two types: the single-walled (sonicated DPPC) and the multi-compartment vesicles (unsonicated DPPC and sonicated 2C₁₈NC). In the DPPC single-walled liposomes, the energy of the donor lost by quenching is efficiently transferred to the acceptor via the Förster-type dipole-dipole interaction. In the case of multi-compartment liposomes of DPPC, the mean distance between donor and acceptor is so small because the external surface of a bilayer is in the vicinity of the internal surface of another bilayer. As a consequence, efficiencies both of energy transfer and of energy loss were greater than those in single-walled liposomes. The fluid property of the 2C₁₈NC bilayer allowed the preferential collisional quenching. The marked reduction in the efficiencies of both energy transfer and energy loss were attributed to the elongation of donor-acceptor distances due to the increase of the size of liposome.     

Dye permeability at phase transitions in single and binary component phospholipid bilayers

By encapsulating a pH-sensitive dye, phenol red, in multilamellar liposomes of DMPC, DPPC and DMPC/DPPC mixtures, the permeability of these phospholipid bilayers to dye as a function of temperature has been studied. For both DMPC and DPPC liposomes, dye release begins well below the main gel-to-liquid-crystalline phase transition (24°C and 42°C, respectively) at temperatures corresponding to the onset of the pretransition (about 14°C and 36°C, respectively) with DPPC liposomes exhibiting a permeability anomaly at the main phase transition (42°C). The perturbation occurring in the bilayer structure that allows the release of encapsulated phenol red (approx. 5 Å diameter) is not sufficient to permit the release of encapsulated haemoglobin (approx. 20 Å diameter, negatively charged). In liposomes composed of a range of DMPC/DPPC mixtures, dye release commences at the onset of the pretransition range (determined by optical absorbance measurements) and increases with increasing temperature until the first appearance of liquid crystalline phase after which no further dye release occurs. Interestingly, the dye retaining properties of DMPC and DPPC liposomes well below their respective pretransition temperature regions are very different: DMPC liposomes release much encapsulated dye at incubation temperatures of 5°C whilst DPPC liposomes do not.