Direct measurement of the transbilayer movement of phospholipids by sum-frequency vibrational spectroscopy

The direct measurement of the transbilayer movement of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in a planar supported lipid bilayer (PSLB) at the fused silica/D2O interface was obtained with sum-frequency generation (SFG) vibrational spectroscopy. The intrinsic sensitivity of SFG to the symmetry of an interface was used to measure the asymmetric distribution of DSPC and perdeuterated DSPC (DSPC-d83) lipids in asymmetrically prepared DSPC/DSPC-d83 PSLBs. Changes in the membrane lipid composition due to exchange between leaflets was monitored by measuring the decay in the CH3 symmetric stretch intensity at 2875 cm-1 with time. The activation energy for transverse motion was determined directly from spectral relaxation measurements at several temperatures and was determined to be 206 +/- 18 kJ/mol. At room temperature (25 degrees C) the half-time of lipid flip-flop was calculated to be approximately 25 days. At 51 degrees C, only 7 degrees C below the main phase-transition temperature of DSPC, the half-time decreases to 25 min. These results have important implications for understanding the transbilayer movement of lipids in biological membranes.     

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.     

Structure of a gel phase lipid bilayer prepared by the Langmuir-Blodgett/Langmuir-Schaefer method characterized by sum-frequency vibrational spectroscopy

The structure of a planar supported lipid bilayer (PSLB) prepared by the Langmuir-Blodgett (LB)/Langmuir-Schaefer (LS) method was investigated by sum-frequency vibrational spectroscopy (SFVS). By using asymmetric lipid bilayers composed of selectively deuterated 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) lipids, the orientation of the fatty acid chains and phosphocholine headgroups has been determined independently for both leaflets of the bilayer. The alkyl chains of the lipids were found to be orientated approximately 13 degrees +/- 4 degrees from the surface normal for both leaflets. The lipid chains in both leaflets also contain some gauche content, which is consistent with previous NMR and FTIR studies of similar lipid systems. More importantly, the relative number of gauche defects does not seem to be influenced by the deposition method, LB versus LS. The headgroup orientation for the lipid film in contact with the silica support was determined to be 69 degrees +/- 3 degrees , whereas that in contact with the aqueous phase was 66 degrees +/- 4 degrees from the surface normal. The SFVS results indicate that the structure of the DSPC lipid film in contact with the solid support and the film adjacent to the aqueous phase are nearly identical in structure. These results suggesting the LB/LS deposition method do indeed produce symmetric lipid bilayers. These studies further add to the growing information on the efficacy of PSLBs as suitable models for biological membrane studies.     

Membrane orientation of MSI-78 measured by sum frequency generation vibrational spectroscopy

Antimicrobial peptides (AMPs) selectively disrupt bacterial cell membranes to kill bacteria whereas they either do not or weakly interact with mammalian cells. The orientations of AMPs in lipid bilayers mimicking bacterial and mammalian cell membranes are related to their antimicrobial activity and selectivity. To understand the role of AMP-lipid interactions in the functional properties of AMPs better, we determined the membrane orientation of an AMP (MSI-78 or pexiganan) in various model membranes using sum frequency generation (SFG) vibrational spectroscopy. A solid-supported single 1,2-dipalmitoyl-an-glycero-3-[phospho-rac-(1-glycerol)] (DPPG) bilayer or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) bilayer was used as a model bacterial cell membrane. A supported 1,2-dipalmitoyl-an-glycero-3-phosphocholine (DPPC) bilayer or a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer was used as a model mammalian cell membrane. Our SFG results indicate that the helical MSI-78 molecules are associated with the bilayer surface with ～70° deviation from the bilayer normal in the negatively charged gel-phase DPPG bilayer at 400 nM peptide concentration. However, when the concentration was increased to 600 nM, MSI-78 molecules changed their orientation to make a 25° tilt from the lipid bilayer normal whereas multiple orientations were observed for an even higher peptide concentration in agreement with toroidal-type pore formation as reported in a previous solid-state NMR study. In contrary, no interaction between MSI-78 and a zwitterionic DPPC bilayer was observed even at a much higher peptide concentration (～12,000 nM). These results demonstrate that SFG can provide insights into the antibacterial activity and selectivity of MSI-78. Interestingly, the peptide exhibits a concentration-dependent membrane orientation in the lamellar-phase POPG bilayer and was also found to induce toroidal-type pore formation. The deduced lipid flip-flop from SFG signals observed from lipids also supports MSI-78-induced toroidal-type pore formation.     

Dynamic properties of bicellar lipid mixtures observed by rheometry and quadrupole echo decay

In bicellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), the transition from isotropic reorientation to partial orientational order, on warming, is known to coincide with a sharp increase in viscosity. In this work, cone-and-plate rheometry, (2)H NMR spectroscopy, and quadrupole echo decay observations have been used to obtain new insights into the dynamics of phases observed in bicellar DMPC/DHPC mixtures. Samples with 25% of the DMPC component deuterated were used to correlate rheological measurements with phase behavior observed by (2)H NMR spectroscopy. Mixtures containing only normal DMPC (DMPC/DHPC) or only chain perdeuterated DMPC (DMPC-d(54)/DHPC) were used to refine rheology and quadrupole echo decay measurements respectively. The viscosity peaked at 4-9 Pa·s, just above the isotropic-to-nematic transition, and then dropped as samples were warmed through the nematic-to-lamellar transition. Quadrupole echo decay times above the nematic-to-lamellar transition were significantly longer than typically observed in the liquid crystalline phase of saturated lipid multilamellar vesicles. This may indicate a damping of slow bilayer undulations resulting from the coupling of opposite bilayer surfaces by DHPC-lined pores.     

Influence of ester and ether linkage in phospholipids on the environment and dynamics of the membrane interface: a wavelength-selective fluorescence approach

We have monitored the environment and dynamics of the membrane interface formed by the ester-linked phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the ether-linked phospholipid 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) utilizing the wavelength-selective fluorescence approach and using the fluorescent membrane probe 2-(9-anthroyloxy)stearic acid (2-AS). This interfacially localized probe offers a number of advantages over those which lack a fixed location in the membrane. When incorporated in membranes formed by DPPC and DHPC, 2-AS exhibits red edge excitation shift (REES) of 14 and 8 nm, respectively. This implies that the rate of solvent reorientation, as sensed by the interfacial anthroyloxy probe, in ester-linked DPPC membranes is slow compared to the rate of solvent reorientation in ether-linked DHPC membranes. In addition, the fluorescence polarization values of 2-AS are found to be higher in DHPC membranes than in DPPC membranes. This is further supported by wavelength-dependent changes in fluorescence polarization and lifetime. Taken together, these results are useful in understanding the role of interfacial chemistry on membrane physical properties.     

Nanoscale patterning in mixed fluorocarbon-hydrocarbon phospholipid bilayers

A growing body of literature suggests that fluorocarbons can direct self-assembly within hydrocarbon environments. We report here the fabrication and characterization of supported lipid bilayers (SLBs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a synthetic, fluorocarbon-functionalized analogue, 1. AFM investigation of these model membranes reveals an intricate, composition-dependent domain structure consisting of approximately 50 nm stripes interspersed between approximately 1 microm sized domains. Although DSC of 1 showed a phase transition near room temperature, DSC of DPPC:1 mixtures exhibited complex phase behavior suggesting domain segregation. Finally, temperature-dependent AFM of DPPC:1 bilayers shows that, while the stripe structures can be melted above the Tm of 1, the stripes and domains result from immiscibility of the hydrocarbon and fluorocarbon lipid gel phases. Fluorination appears to be a promising strategy for chemical self-assembly in two dimensions. In particular, because no modification is made to the lipid headgroups, it may be useful for nanopatterning biologically relevant ligands on bilayers in vitro or in living cells.     

Ethanol-induced reorganization of the liquid-ordered phase: enhancement of cholesterol-phospholipid association

This paper records what is believed to be the first evidence for the reorganization of the liquid-ordered phase by ethanol. Specifically, ethanol has been found to significantly enhance sterol-phospholipid association in liquid-ordered bilayers derived from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) plus cholesterol and also 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) plus cholesterol. The evidence for such reorganization comes from a series of nearest-neighbor recognition (NNR) experiments that have been carried out, where low concentrations of equilibrating lipid dimers (i.e., "reporter molecules") have been used to detect changes in the phase composition of host membranes made from varying mixtures of DPPC/cholesterol, and also DSPC/cholesterol, in the presence and in the absence of ethanol. These findings have important biological implications, which are briefly discussed.     

Effect of low amounts of cholesterol on the swelling behavior of floating bilayers

The effect of the addition of 1, 2, 4, and 6 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) floating bilayers has been investigated by neutron reflectivity. All samples exhibited fully stable and reversible gel and fluid phases. Around the main lipid phase transition temperature, DPPC double bilayers exhibit large increases in the water layer separating the bilayers and the upper bilayer roughness. The inclusion of low amounts of cholesterol reduced the swelling of the water layer between the bilayers and the upper bilayer roughness and progressively widened the temperature range over which swelling occurs. Results from asymmetric bilayers are also reported. A higher amount of cholesterol in the lower bilayer induces a smaller swelling of the water layer between the bilayers than in the symmetric case. Finally, the effect of the inclusion of a leaflet of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) was investigated. The presence of a leaflet with a higher gel-transition temperature (T(m)) modifies the phase behavior of the lower T(m) leaflet.     

Structure and thermotropic phase behavior of fluorinated phospholipid bilayers: a combined attenuated total reflection FTIR spectroscopy and imaging ellipsometry study

Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39 degrees as compared to 21 degrees for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon-hydrocarbon interactions in the center of the bilayer due to chain staggering.     

Temperature-sensitive nonionic vesicles prepared from Span 80 (sorbitan monooleate)

Different types of nonionic vesicles were prepared from commercial Span 80 (also called sorbitan monooleate), as an inexpensive, biocompatible alternative to conventional phospholipid-based vesicles (liposomes). The vesicles were characterized by different techniques and comparison was made with vesicles formed from POPC (1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine) or DOPC (1,2-dioleoyl- sn-glycero-3-phosphocholine). Dynamic light scattering measurements, electron microscopy analyses, and two types of fusion assays indicate that Span 80 vesicles are stable for at least 7 days at 4 or 25 degrees C, while storage at 42 degrees C causes irreversible vesicle fusion. This indicates that Span 80 vesicles are thermoresponsive with vesicle fusion occurring at elevated temperature. This property may be related to headgroup dehydration and is certainly not directly linked to the phase transition temperature (Tm) of the vesicles, since the Tm is below -30 degrees C, as determined by differential scanning calorimetry (DSC). The measured Tm value for Span 80 vesicles is lower than in the case of DOPC or POPC, correlating with a higher fluidity of Span 80 vesicles as compared to POPC or DOPC vesicles, as determined with DPH (1,6-diphenyl-1,3,5-hexatriene) as fluorescent membrane probe. High fluidity correlates with increased leakage of entrapped water-soluble dye molecules. Addition of cholesterol and soybean phosphatidylcholine lowers the extent of leakage, allowing a tuning of the bilayer permeability.     

Interaction of antimicrobial arginine-based cationic surfactants with liposomes and lipid monolayers

The present work examines the relationship between the antimicrobial activity of novel arginine-based cationic surfactants and the physicochemical process involved in the perturbation of the cell membrane. To this end, the interaction of these surfactants with two biomembrane models, namely, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar lipid vesicles (MLVs) and monolayers of DPPC, 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] sodium salt (DPPG), and Escherichia coli total lipid extract, was investigated. For the sake of comparison, this study included two commercial antimicrobial agents, hexadecyltrimethylammonium bromide and chlorhexidine dihydrochloride. Changes in the thermotropic phase transition parameters of DPPC MLVs in the presence of the compounds were studied by differential scanning calorimetry analysis. The results show that variations in both the transition temperature (Tm) and the transition width at half-height of the heat absorption peak (deltaT1/2) were consistent with the antimicrobial activity of the compounds. Penetration kinetics and compression isotherm studies performed with DPPC, DPPG, and E. coli total lipid extract monolayers indicated that both steric hindrance effects and electrostatic forces explained the antimicrobial agent-lipid interaction. Overall, in DPPC monolayers single-chain surfactants had the highest penetration capacity, whereas gemini surfactants were the most active in DPPG systems. The compression isotherms showed an expansion of the monolayers compared with that of pure lipids, indicating an insertion of the compounds into the lipid molecules. Owing to their cationic character, they are incorporated better into the negatively charged DPPG than into zwitterionic DPPC lipid monolayers.     

Isomerization dynamics of photochromic spiropyran molecular switches in phospholipid bilayers

Transient absorption spectroscopy was used to investigate the dynamics of the photochromic indolinobenzospiropyran reaction in toluene solution and in phosphatidylcholine bilayers (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)). After excitation with UV light, colorless (R/S)-2-(3',3'-dimethyl-6-nitro-3'H-spiro[chromene-2,2'-indol]-1'-yl)ethanol derivatives are converted to colored merocyanine products in high yield; Phi = 0.45 in DMPC liposomes. We find that the reaction occurs in the bilayer aliphatic region in the gel (P(beta)(')) and liquid (L(alpha)) phases. The Arrhenius activation energy for the isomerization in DMPC bilayers was approximately 3.5 times larger in the liquid phase (L(alpha), E(a) = 26.0 +/- 1.0 kJ mol(-1)) than that in the gel phase (P(beta)('), E(a) = 7.3 +/- 1.6 kJ mol(-1)). Analysis of the isomerization rate constant temperature dependence allows an estimation of the bilayer viscosity and free volume properties in the L(alpha) phase.     

The interaction of a peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with DPPC model membranes: a DSC study

Depending on their hydrophobicity, peptides can interact differently with lipid membranes inducing dramatic modifications into their host systems. In the present paper, the interaction of a synthetic peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membranes has been investigated by differential scanning calorimetry (DSC), adopting three different experimental approaches. In the first, the peptide is forced to be included into the hydrocarbon region of the lipid bilayer, by codissolving it with the lipid giving rise to mixed multilamellar vesicles–peptide systems; in the second, this system is passed through an extruder, thus producing large unilamellar vesicles–peptide systems; in the third, it is allowed to interact with the external surface of the membrane.

The whole of the DSC results obtained have shown that the incorporation of the peptide into the lipid bilayer by means of the first method induces a decrease in the enthalpy of the gel–liquid crystal transition of the membrane and a shift of the transition to the lower temperatures, thus resembling, in spite of its prevalently hydrophilic nature, the behavior of transbilayer hydrophobic peptides. The extrusion of these systems creates unilamellar vesicles free of peptides but of smaller size as evidenced by the decreased cooperativity of the transition. The peptide, added externally to the DPPC model membrane, has no effect on the phase behavior of the bilayer.

These findings suggest that the effect of the interaction of scrambled hydrophobic/hydrophilic peptides into lipid bilayers strongly affects the thermotropic behavior of the host membrane depending on the preparation method of the lipid/peptide systems. The whole of the results obtained in the present paper can be useful in approaching studies of bioactive peptides/lipids systems.     

Material properties of matrix lipids determine the conformation and intermolecular reactivity of diacetylenic phosphatidylcholine in the lipid bilayer

Photopolymerizable phospholipid DC(8,9)PC (1,2-bis-(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine) exhibits unique assembly characteristics in the lipid bilayer. Because of the presence of the diacetylene groups, DC(8,9)PC undergoes polymerization upon UV (254 nm) exposure and assumes chromogenic properties. DC(8,9)PC photopolymerization in gel-phase matrix lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monitored by UV-vis absorption spectroscopy occurred within 2 min after UV treatment, whereas no spectral shifts were observed when DC(8,9)PC was incorporated into liquid-phase matrix 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Liquid chromatography-tandem mass spectrometry analysis showed a decrease in the amount of DC(8,9)PC monomer in both DPPC and POPC environments without any change in the matrix lipids in UV-treated samples. Molecular dynamics (MD) simulations of DPPC/DC(8,9)PC and POPC/DC(8,9)PC bilayers indicate that the DC(8,9)PC molecules adjust to the thickness of the matrix lipid bilayer. Furthermore, the motions of DC(8,9)PC in the gel-phase bilayer are more restricted than in the fluid bilayer. The restricted motional flexibility of DC(8,9)PC (in the gel phase) enables the reactive diacetylenes in individual molecules to align and undergo polymerization, whereas the unrestricted motions in the fluid bilayer restrict polymerization because of the lack of appropriate alignment of the DC(8,9)PC fatty acyl chains. Fluorescence microscopy data indicates the homogeneous distribution of lipid probe 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-lissamine rhodamine B sulfonyl ammonium salt (N-Rh-PE) in POPC/DC(8,9)PC monolayers but domain formation in DPPC/DC(8,9)PC monolayers. These results show that the DC(8,9)PC molecules cluster and assume the preferred conformation in the gel-phase matrix for the UV-triggered polymerization reaction.     

Cholesterol induced suppression of large swelling of water layer in phosphocholine floating bilayers

The effect of the addition of 10 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) floating bilayers has been investigated by means of neutron reflectivity measurements. The large swelling of the water layer between the two bilayers found in pure phosphocholine systems around the lipid main phase transition is greatly reduced in the 9:1 DPPC:cholesterol mixture. The analysis of the structure of the bilayer reveals that in the gel phase cholesterol induces the presence of a high rms roughness that disappears in the fluid phase.     

Phase Transitions and Structural Changes in DPPC Liposomes Induced by a 1-Carba-Alpha-Tocopherol Analogue

Steady-state emission spectroscopy of 1-anilino-8- naphthalene sulfonate (ANS) and 1,6-diphenyl-1,3,5-hexatriene (DPH), fluorescence anisotropy, and DSC methods were used to characterize the interactions of the newly synthesized 1-carba-alpha-tocopherol (CT) with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane. The DSC results showed significant perturbations in the DPPC structure for CT concentrations as low as 2 mol%. The main phase transition peak was broadened and shifted to lower temperatures in a concentration-dependent manner, and pretransition was abolished. Increasing CT concentrations induced the formation of new phases in the DPPC structure, leading to melting at lower temperatures and, finally, disruption of the ordered DPPC structure. Hydration and structural changes of the DPPC liposomes using ANS and DPH fluorescent probes, which are selectively located at different places in the bilayer, were studied. With the increased concentration of CT molecules in the DPPC liposomes, structural changes with the simultaneous formation of different phases of such mixture were observed. Temperature studies of such mixtures revealed a decrease in the temperature of the main phase transition and fluidization at decreasing temperatures related to increasing hydration in the bilayer. Contour plots obtained from concentration–temperature data with fluorescent probes allowed for identification of different phases, such as gel, ordered liquid, disordered liquid, and liquid crystalline phases. The CT molecule with a modified chromanol ring embedded in the bilayer led to H-bonding interactions, expelling water molecules from the interphase, thus introducing disorder and structural changes to the highly ordered gel phase.     

Effect of pressure on the Prodan fluorescence in bilayer membranes of phospholipids with varying acyl chain lengths

The fluorescence spectra of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were observed as a function of pressure for the bilayer membrane systems of dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC). The wavelength of the emission maximum, lambdamax, was found to be 480, 430, and 500 nm for the liquid crystalline (Lalpha), ripple gel (P'beta), and pressure-induced interdigitated gel (LbetaI) phase, respectively. Since the lambdamax reflects the solvent property around the probe molecules, we could speculate on the location of the Prodan molecules in the bilayer membranes; in the Lalpha phase of the lipid bilayer, the Prodan molecules distribute around the phosphate of the lipids (i.e. the polar region). The Lalpha/P'beta phase transition caused the Prodan molecules to move into the less polar region near the glycerol backbone. The fluorescence intensity of the Prodan in the P'beta phase was dependent on the chain length of the lipids and on pressure; the shorter the chain length of the lipid, the stronger the fluorescence intensity of the Prodan. Moreover, for the DLPC bilayer membrane system, the fluorescence intensity at 430 nm increased with increasing pressure, indicating that the partition of Prodan into the DLPC bilayer membrane is promoted by applying pressure. In the case of the DPPC and DSPC bilayers, as the pressure increased further, the pressure-induced interdigitation caused the Prodan molecules to squeeze out of the glycerol backbone region and to move the hydrophilic region near the bilayer surface. The ratio of fluorescence intensity at 480 nm to that at 430 nm, F480/F430, showed a sharp change at the phase-transition pressure. In the case of the DPPC and DSPC bilayers, the values of F480/F430 showed an abrupt increase above a certain pressure higher than the Lalpha/P'beta transition pressure, which corresponds to the interdigitation from the P'beta to the LbetaI phase. The plot of F480/F430 versus pressure is available for recognition of the bilayer phase transitions, especially the bilayer interdigitation.     

Langmuir balance investigation of superoxide dismutase interactions with mixed-lipid monolayers

Higher than theoretical encapsulation efficiencies in liposomes of the cytoplasmic protein, superoxide dismutase (SOD), were previously observed. The high encapsulation of SOD led to the consideration of lipid-protein interactions and the embedding of SOD in the lipid bilayer. Difficulty in other methods such as dynamic scanning calorimetry due to cholesterol obscuring the measurements brought about the interest for a modified Langmuir monolayer relaxation study. A novel method was devised to distinguish between different lipid compositions that formed either a favorable or an unfavorable environment for SOD. Normalized monolayer relaxations with SOD were compared between mixed-lipid compositions containing 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and cholesterol (Chol). Lipid-monolayer relaxation with and without SOD in the subphase was plotted over 30 min to determine if the protein was altering the lipid-monolayer relaxation. The monolayer relaxation with SOD was normalized to the monolayer relaxation without SOD over the 30 min period. The results indicated that lipid length and mole percent of cholesterol were important parameters that must be adjusted in order to support a favorable environment for SOD interaction with the lipid. It was determined that hydrophobic interactions were dominant over electrostatic forces; thus, SOD was embedding into the lipid monolayer. Additionally, this study was correlated to a previous liposome study and proved that lipid-protein interactions were the reason for the higher encapsulation efficiencies. The significance of this method is that it (1) provides a connection between lipid-protein interactions observed in monolayers and bilayers and (2) establishes a simple and effective manner to test lipid compositions for lipid-protein interaction that will aid in optimization of liposome encapsulation efficiency.     

Role of trehalose in prevention of giant vesicle adsorption and encapsulated solute leakage in anhydrobiotic preservation

Anhydrobiotic preservation has the potential to allow the processing and storage of mammalian cells in a state of suspended animation at ambient conditions in trehalose glasses; however, stresses--particularly to the lipid bilayer--during desiccation and rehydration have thus far prevented the full realization of the promise of this technique. Giant gel-phase 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and liquid-crystalline-phase 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) vesicles provide a model cell system with which to elucidate the role of trehalose in surface-lipid bilayer interactions, as well as the part played by lipid phase. In the absence of trehalose, DSPC liposomes adsorbed to polystyrene, producing irreversible structural changes and apparent leakage of all intravesicular solute upon drying and rehydration. Addition of trehalose significantly reduced vesicle adsorption with only transient intravesicular solute leakage for the rehydrated vesicles; however, at very low moisture contents, the vesicles underwent permanent structural changes. In contrast to the results with DSPC vesicles, DLPC vesicles largely avoided adsorption and exhibited high intravesicular solute retention when dried and rehydrated even in the absence of trehalose, despite significant internal structural changes.