Shifts in chain-melting transition temperature of liposomal membranes by polymer-grafted lipids

The chain-melting transition temperature of dipalmitoyl phosphatidylcholine (DPPC) bilayer membranes containing poly(ethylene glycol)-grafted dipalmitoyl phosphatidylethanolamine (PEG-DPPE) was determined by optical turbidity measurements. The dependence on content, X_p, of PEG-DPPE lipid was studied for different polar headgroup sizes, n_p, of the polymer lipid, throughout the lamellar phase of the mixtures with DPPC. Mean-field theory for the polymer brush regime predicts that the downward shift in transition temperature should vary with polymer size and content as n_pX_p^5/3 (∼n_pX_p^11/6 for scaling theory). Any shift induced by the charge on PEG-lipids is independent of polymer size. These predictions are reasonably borne out for the longer polymer lipids (PEG molecular masses 750, 2000 and 5000 Da). Transition temperature shifts in the lamellar phase, before the onset of micellisation, are in the region of −1 to −2 °C (±0.1-0.2 °C) in reasonable accord with theoretical estimates of the lateral pressure exerted by the polymer brush. Shifts of this size are significant to the design of liposomes for controlled release of contents by mild hyperthermia.     

Thermodynamics of lipid interactions in complex bilayers

The mutual interactions between lipids in bilayers are reviewed, including mixtures of phospholipids, and mixtures of phospholipids and cholesterol (Chol). Binary mixtures and ternary mixtures are considered, with special emphasis on membranes containing Chol, an ordered phospholipid, and a disordered phospholipid. Typically the ordered phospholipid is a sphingomyelin (SM) or a long-chain saturated phosphatidylcholine (PC), both of which have high phase transitions temperatures; the disordered phospholipid is 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC). The unlike nearest-neighbor interaction free energies (ω_AB) between lipids (including Chol), obtained by an variety of unrelated methods, are typically in the range of 0-400 cal/mol in absolute value. Most are positive, meaning that the interaction is unfavorable, but some are negative, meaning it is favorable. It is of special interest that favorable interactions occur mainly between ordered phospholipids and Chol. The interpretation of domain formation in complex mixtures of Chol and phospholipids in terms of phase separation or condensed complexes is discussed in the light of the values of lipid mutual interactions.     

Rapid phase change of lipid microdomains in giant vesicles induced by conversion of sphingomyelin to ceramide

To understand the role of sphingomyelinase (SMase) in the function of biological membranes, we have investigated the effect of conversion of sphingomyelin (SM) to ceramide (Cer) on the assembly of domains in giant unilamellar vesicles (GUVs). The GUVs were prepared from mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), N-palmitoly-d-erythro-sphingosine (C₁₆Cer), N-palmitoyl-d-erythro-sphingosylphosphorylcholine (C₁₆SM) and cholesterol. The amounts of DOPC, sum of C₁₆Cer and C₁₆SM, and cholesterol were kept constant (the ratio of these four lipids is shown as 1:X:1-X:1 (molar ratio), i.e., X is C₁₆Cer/(C₁₆Cer + C₁₆SM)). Shape and distribution of domains formed in the GUVs were monitored by a fluorescent lipid, Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (0.1 mol%). In GUVs containing low C₁₆Cer (X = 0 and 0.25), round-shaped domains labeled by the fluorescent lipid were present, suggesting coexistence of liquid-ordered and disordered domains. In GUVs containing intermediate Cer concentration (X = 0.5), the fluorescent domain covered most of GUV surface, which was surrounded by gel-like domains. Differential scanning calorimetry of multilamellar vesicles prepared in the presence of higher Cer concentration (X ≥ 0.5) suggested existence of a Cer-enriched gel phase. Video microscopy showed that the enzymatic conversion of SM to Cer caused rapid change in the domain structure: several minutes after the SMase addition, the fluorescent region spread over the GUV surface, within which regions with darker contrast existed. Image-based measurement of generalized polarization (GP) of 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan), which is related to the acyl chain ordering of the lipids, was performed. Before the SMase treatment domains with high (0.65) and low (below 0.4) GP values coexisted, presumably reflecting the liquid-ordered and disordered domains; after the SMase treatment regions with intermediate GP values (0.5) and smaller regions with higher GP values (0.65) were present. Generation of Cer thus caused a phase transition from liquid-ordered and disordered phases to a gel and liquid phase.     

The Mechanism of Collapse of Heterogeneous Lipid Monolayers

Collapse of homogeneous lipid monolayers is known to proceed via wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous monolayers with phase coexistence, the mechanism of collapse remains unclear. Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid and liquid-solid domains using molecular dynamics simulations. The MARTINI coarse-grained model was employed to simulate monolayers of ∼80 nm in lateral dimension for 10–25 μs. The monolayer minimum surface tension decreased in the presence of solid domains, especially if they percolated. Liquid-ordered domains facilitated monolayer collapse due to the spontaneous curvature induced at a high cholesterol concentration. Upon collapse, bilayer folds formed in the liquid (disordered) phase; curved domains shifted the nucleation sites toward the phase boundary. The liquid (disordered) phase was preferentially transferred into bilayers, in agreement with the squeeze-out hypothesis. As a result, the composition and phase distribution were altered in the monolayer in equilibrium with bilayers compared to a flat monolayer at the same surface tension. The composition and phase behavior of the bilayers depended on the degree of monolayer compression. The monolayer-bilayer connection region was enriched in unsaturated lipids. Percolation of solid domains slowed down monolayer collapse by several orders of magnitude. These results are important for understanding the mechanism of two-to-three-dimensional transformations in heterogeneous thin films and the role of lateral organization in biological membranes. The study is directly relevant for the function of lung surfactant, and can explain the role of nanodomains in its surface activity and inhibition by an increased cholesterol concentration.     

Interactions of glycerol monooleate and dimethylsulphoxide with phospholipids A differential scanning calorimetry and 31P-NMR study

1. A comparative study has been made of the effects of the fusogens glycerol monooleate and dimethylsulphoxide on the polymorphic phase behaviour of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylethanolamine by differential scanning calorimetry and ³¹P-NMR techniques. 2. Glycerol monooleate induces a reduction in the temperature, cooperativity and enthalpy of the gel to liquid-crystal transitions of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylethanolamine, whereas dimethylsulphoxide induces an increase in the temperature and enthalpy and a reduction in the cooperativity of the gel to liquid-crystal transitions for those same phospholipids. 3. Glycerol monooleate induces the formation of isotropic and hexagonal (H_II) phases when mixed with either dipalmitoyl phosphatidylcholine or dipalmitoyl phosphatidylethanolamine. By contrast, in the presence of dimethylsulphoxide, those same phospholipids retain the lamellar configuration observed in the absence of fusogen. 4. These results are discussed in terms of the mechanisms of chemically induced cell fusion.     

Cholesterol in condensed and fluid phosphatidylcholine monolayers studied by epifluorescence microscopy

Epifluorescence microscopy was used to investigate the effect of cholesterol on monolayers of dipalmitoylphosphatidylcholine (DPPC) and 1 -palmitoyl-2-oleoyl phosphatidylcholine (POPC) at 21 +/- 2 degrees C using 1 mol% 1-palmitoyl-2-[12-[(7-nitro-2-1, 3-benzoxadizole-4-yl)amino]dodecanoyl]phosphatidylcholine (NBD-PC) as a fluorophore. Up to 30 mol% cholesterol in DPPC monolayers decreased the amounts of probe-excluded liquid-condensed (LC) phase at all surface pressures (pi), but did not effect the monolayers of POPC, which remained in the liquid-expanded (LE) phase at all pi. At low pi (2-5 mN/m), 10 mol% or more cholesterol in DPPC induced a lateral phase separation into dark probe-excluded and light probe-rich regions. In POPC monolayers, phase separation was observed at low pi when > or =40 mol% or more cholesterol was present. The lateral phase separation observed with increased cholesterol concentrations in these lipid monolayers may be a result of the segregation of cholesterol-rich domains in ordered fluid phases that preferentially exclude the fluorescent probe. With increasing pi, monolayers could be transformed from a heterogeneous dark and light appearance into a homogeneous fluorescent phase, in a manner that was dependent on pi and cholesterol content. The packing density of the acyl chains may be a determinant in the interaction of cholesterol with phosphatidylcholine (PC), because the transformations in monolayer surface texture were observed in phospholipid (PL)/sterol mixtures having similar molecular areas. At high pi (41 mN/m), elongated crystal-like structures were observed in monolayers containing 80-100 mol% cholesterol, and these structures grew in size when the monolayers were compressed after collapse. This observation could be associated with the segregation and crystallization of cholesterol after monolayer collapse.     

The melting of pulmonary surfactant monolayers.

Monomolecular films of phospholipids in the liquid-expanded (LE) phase after supercompression to high surface pressures (pi), well above the equilibrium surface pressure (pi(e)) at which fluid films collapse from the interface to form a three-dimensional bulk phase, and in the tilted-condensed (TC) phase both replicate the resistance to collapse that is characteristic of alveolar films in the lungs. To provide the basis for determining which film is present in the alveolus, we measured the melting characteristics of monolayers containing TC dipalmitoyl phosphatidylcholine (DPPC), as well as supercompressed 1-palmitoyl-2-oleoyl phosphatidylcholine and calf lung surfactant extract (CLSE). Films generated by appropriate manipulations on a captive bubble were heated from < or =27 degrees C to > or =60 degrees C at different constant pi above pi(e). DPPC showed the abrupt expansion expected for the TC-LE phase transition, followed by the contraction produced by collapse. Supercompressed CLSE showed no evidence of the TC-LE expansion, arguing that supercompression did not simply convert the mixed lipid film to TC DPPC. For both DPPC and CLSE, the melting point, taken as the temperature at which collapse began, increased at higher pi, in contrast to 1-palmitoyl-2-oleoyl phosphatidylcholine, for which higher pi produced collapse at lower temperatures. For pi between 50 and 65 mN/m, DPPC melted at 48-55 degrees C, well above the main transition for bilayers at 41 degrees C. At each pi, CLSE melted at temperatures >10 degrees C lower. The distinct melting points for TC DPPC and supercompressed CLSE provide the basis by which the nature of the alveolar film might be determined from the temperature-dependence of pulmonary mechanics.     

Molecular Organization of Cholesterol in Polyunsaturated Membranes: Microdomain Formation

The molecular organization of cholesterol in phospholipid bilayers composed of 1,2-diarachidonylphosphatidylcholine (20:4-20:4PC), 1-stearoyl-2-arachidonylphosphatidylcholine (18:0-20:4PC), and 20:4-20:4PC/18:0-20:4PC (1/1 mol) was investigated by solid-state ²H NMR and by low- and wide-angle x-ray diffraction (XRD). On the basis of distinct quadrupolar powder patterns arising from [3α-²H₁]cholesterol intercalated into the membrane and phase separated as solid, solubility χ_chol^NMR = 17 ± 2 mol% and tilt angle α₀ = 25 ± 1° in 20:4-20:4PC were determined. The corresponding values in 18:0-20:4PC were χ_chol^NMR ≥ 50 mol% and α₀ = 16 ± 1°. Cholesterol solubility determined by XRD was χ_chol^XRD = 15 ± 2 mol% and χ_chol^XRD = 49 ± 1 mol% for 20:4-20:4PC and 18:0-20:4PC, respectively. XRD experiments show that the solid sterol is monohydrate crystals presumably residing outside the bilayer. The ²H NMR spectrum for equimolar [3α-²H₁]cholesterol added to mixed 20:4-20:4PC/18:0-20:4PC (1/1 mol) membranes is consistent with segregation of cholesterol into 20:4-20:4PC and 18:0-20:4PC microdomains of <160 Å in size that preserve the molecular organization of sterol in the individual phospholipid constituents. Our results demonstrate unambiguously that cholesterol has low affinity to polyunsaturated fatty acids and support hypotheses of lateral phase separation of membrane constituents into sterol-poor/polyunsaturated fatty acid-rich and sterol-rich/saturated fatty acid-rich microdomains.     

Effect of the structure of lipids favoring disordered domain formation on the stability of cholesterol-containing ordered domains (lipid rafts): identification of multiple raft-stabilization mechanisms

Despite the importance of lipid rafts, commonly defined as liquid-ordered domains rich in cholesterol and in lipids with high gel-to-fluid melting temperatures (T_m), the rules for raft formation in membranes are not completely understood. Here, a fluorescence-quenching strategy was used to define how lipids with low T_m, which tend to form disordered fluid domains at physiological temperatures, can stabilize ordered domain formation by cholesterol and high-T_m lipids (either sphingomyelin or dipalmitoylphosphatidylcholine). In bilayers containing mixtures of low-T_m phosphatidylcholines, cholesterol, and high-T_m lipid, the thermal stability of ordered domains decreased with the acyl-chain structure of low-T_m lipids in the following order: diarachadonyl > diphytanoyl > 1-palmitoyl 2-docosahexenoyl = 1,2 dioleoyl = dimyristoleoyl = 1-palmitoyl, 2-oleoyl (PO). This shows that low-T_m lipids with two acyl chains having very poor tight-packing propensities can stabilize ordered domain formation by high-T_m lipids and cholesterol. The effect of headgroup structure was also studied. We found that even in the absence of high-T_m lipids, mixtures of cholesterol with PO phosphatidylethanolamine (POPE) and PO phosphatidylserine (POPS) or with brain PE and brain PS showed a (borderline) tendency to form ordered domains. Because these lipids are abundant in the inner (cytofacial) leaflet of mammalian membranes, this raises the possibility that PE and PS could participate in inner-leaflet raft formation or stabilization. In bilayers containing ternary mixtures of PO lipids, cholesterol, and high-T_m lipids, the thermal stability of ordered domains decreased with the polar headgroup structure of PO lipids in the order PE > PS > phosphatidylcholine (PC). Analogous experiments using diphytanoyl acyl chain lipids in place of PO acyl chain lipids showed that the stabilization of ordered lipid domains by acyl chain and headgroup structure was not additive. This implies that it is likely that there are two largely mutually exclusive mechanisms by which low-T_m lipids can stabilize ordered domain formation by high-T_m lipids and cholesterol: 1), by having structures resulting in immiscibility of low-T_m and high-T_m lipids, and 2), by having structures allowing them to pack tightly within ordered domains to a significant degree.     

Multiple phase equilibria in binary mixtures of phospholipids

Approximate phase diagrams describing lateral phase separations are given for binary mixtures of dimyristoyl phosphatidylcholine with dipalmitoyl phosphatidylcholine, distearoyl phosphatidylcholine, and dipalmitoyl phosphatidylethanolamine. These diagrams are based in part on freeze-fracture electron microscopic data presented here, as well as other earlier spin-label, calorimetric, and X-ray data. These phase diagrams represent an improvement over previous studies in that both solid phases (P_β' and L_β') of the phosphatidylcholines are included. Further consideration is given to the problem of binary mixtures in which there are two P_β' phases that do not form a continuous range of solid solutions.     

Amphidinol 3 preferentially binds to cholesterol in disordered domains and disrupts membrane phase separation

Amphidinol 3 (AM3), a polyhydroxy-polyene metabolite from the dinoflagellate Amphidinium klebsii, possesses potent antifungal activity. AM3 is known to interact directly with membrane sterols and permeabilize membranes by forming pores. Because AM3 binds to sterols such as cholesterol and ergosterol, it can be assumed that AM3 has some impact on lipid rafts, which are membrane domains rich in sphingolipids and cholesterol. Hence, we first examined the effect of AM3 on phase-separated liposomes, in which raft-like ordered and non-raft-like disordered domains are segregated. Consequently, AM3 disrupted the phase separation at 22 μM, as in the case of methyl-β-cyclodextrin, a well-known raft-disrupter that extracts sterol from membranes. The surface plasmon resonance measurements and dye leakage assays show that AM3 preferentially recognizes cholesterol in the disordered membrane, which may reflect a weaker lipid-cholesterol interaction in disordered membrane than in ordered membrane. Finally, to gain insight into the AM3-induced coalescence of membrane phases, we measured membrane fluidity using fluorescence correlation spectroscopy, demonstrating that AM3 significantly increases the order of disordered phase. Together, AM3 preferentially binds to the disordered phase rather than the ordered phase, and enhances the order of the disordered phase, consequently blending the separated phases.     

Solid-state NMR study of membrane interactions of the pore-forming cytolysin, equinatoxin II

Equinatoxin II (EqtII) is a pore-forming protein from Actinia equina that lyses red blood cell and model membranes. Lysis is dependent on the presence of sphingomyelin (SM) and is greatest for vesicles composed of equimolar SM and phosphatidylcholine (PC). Since SM and cholesterol (Chol) interact strongly, forming domains or “rafts” in PC membranes, ³¹P and ²H solid-state NMR were used to investigate changes in the lipid order and bilayer morphology of multilamellar vesicles comprised of different ratios of dimyristoylphosphatidylcholine (DMPC), SM and Chol following addition of EqtII. The toxin affects the phase transition temperature of the lipid acyl chains, causes formation of small vesicle type structures with increasing temperature, and changes the T₂ relaxation time of the phospholipid headgroup, with a tendency to order the liquid disordered phases and disorder the more ordered lipid phases. The solid-state NMR results indicate that Chol stabilizes the DMPC bilayer in the presence of EqtII but leads to greater disruption when SM is in the bilayer. This supports the proposal that EqtII is more lytic when both SM and Chol are present as a consequence of the formation of domain boundaries between liquid ordered and disordered phases in lipid bilayers leading to membrane disruption.     

Fluoroscence studies of chloropyll α incorporated into lipid mixtures,and the interpretation of “phase” diagrams

The fluorescence of chloropyll α incorporated into liposomes of mixtures of phosphatidylcholines and phosphatidylethanolamines is reprted. Plots of fluorescence intensities against temperature show breaks at characteristic temperatures which can be attributed to the onset and completion of solid phase lipid formation. These temperatures can be plotted to give diagrams analogous to the phase diagrams obtained for macroscopic systems. Complications due to “small-system effects” are discussed, and the experimental diagrams are compared with theoretical phase digrams calculated for ideal mixing. Introduction of cholesterol leads to a reduction in fluorescence intensity, most readily explained by a1 : 1 lipid :cholesterol interaction with exclution of monomeric, fluorescent, chlorophyll a. Interaction of divalent ions with mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylserine leads to exclution oc chlorophyll a from the phosphatidylserine.     

Interaction of human serum albumin with membranes containing polymer-grafted lipids: spin-label ESR studies in the mushroom and brush regimes

The adsorption of human serum albumin (HSA) to dipalmitoyl phosphatidylcholine (DPPC) bilayer membranes containing poly(ethylene glycol)-grafted dipalmitoyl phosphatidylethanolamine (PEG-DPPE) was studied as a function of content and headgroup size of the polymer lipid. In the absence of protein, conversion from the low-density mushroom regime to the high-density brush regime of polymer-lipid content is detected by the change in ESR outer hyperfine splitting, 2A_max, of chain spin-labelled phosphatidylcholine in gel-phase membranes. The values of 2A_max remain constant in the mushroom regime, but decrease on entering the brush regime. Conversion between the two regimes occurs at mole fractions X_PEG(m→b)≈0.04, 0.01-0.02 and 0.005-0.01 for PEG-DPPE with mean PEG molecular masses of 350, 2000 and 5000 Da, respectively, as expected theoretically. Adsorption of HSA to DPPC membranes is detected as a decrease of the spin label 2A_max hyperfine splitting in the gel phase. Saturation is obtained at a protein/lipid ratio of ca. 1:1 w/w. In the presence of polymer-grafted lipids, HSA adsorbs to DPPC membranes only in the mushroom regime, irrespective of polymer length. In the brush regime, the spin-label values of 2A_max are unchanged in the presence of protein. Even in the mushroom regime, protein adsorption progressively becomes strongly attenuated as a result of the steric stabilization exerted by the polymer lipid. These results are in agreement with theoretical estimates of the lateral pressure exerted by the grafted polymer in the brush and mushroom regimes, respectively.     

Mode of binding of cytochrome b5 to phospholipid bilayers in lamellar structure

X-ray diffraction studies were made on the multilamellar systems produced by incubation of phospholipid bilayers and the membrane protein, cytochrome b₅, or non-membrane proteins (albumin, ovalbumin and β-lactoglobulin A) at pH 8.1 in aqueous 5 mM CaCl₂ solutions.Detergent-extracted cytochrome b₅ (soluble aggregate) forms two types of lamellar phase with dipalmitoyl phosphatidylcholine bilayers, depending upon the incubation temperature. One type, which has a repeat distance of 114Å, is formed above 34°C, where the binding of cytochrome b₅ to the bilayers is hydrophobic. The other type, with a repeat distance of 153 Å, is formed below 34°C, where the binding is electrostatic. It is also suggested that cytochrome b₅ is monomeric in the former phase but remains aggregated in the latter phase.When dimyristoyl phosphatidylcholine is used, the boundary temperature for the two types shifts to 12°C. These boundary temperatures coincide with the thermal pretransition points of hydrated dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, respectively.Trypsin-treated cytochrome b₅ (monomeric) and the three non-membrane proteins exhibit only binding of the electrostatic type to the bilayers, independently of the incubation temperature. The observed repeat distances suggest that in these cases two layers of protein molecules are incorporated between the bilayers.     

Cholesterol's interactions with serine phospholipids — A comparison of N-palmitoyl ceramide phosphoserine with dipalmitoyl phosphatidylserine

In this study we have prepared ceramide phosphoserine (CerPS) and examined its sterol-interacting properties. CerPS is a hydrogen-bonding sphingolipid, but its head group differs from that found in sphingomyelin (SM). Based on diphenylhexatriene steady-state anisotropy measurements, we observed that fully hydrated N-palmitoyl CerPS had a gel-to-liquid crystalline phase transition temperature of about 51 °C in 50 mM sodium phosphate buffer (pH 7.4). This was close to the T_m measured for 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) bilayers (T_m 50.5 °C). Based on cholestatrienol (CTL) quenching experiments in liquid disordered ternary bilayers (containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphcholine; POPC), cholesterol/CTL formed sterol-enriched ordered domains with CerPS. These had similar thermostability as the sterol domains formed with N-palmitoyl SM. Cholesterol failed to form sterol-enriched ordered domains with DPPS under comparable conditions. Based on the equilibrium partitioning of CTL, we observed that the affinity of sterol for bilayers containing POPC/CerPS/cholesterol (6:3:1 by mol) was much higher than the affinity measured for control fluid POPC/cholesterol (9:1 by mol) bilayers, but slightly less than seen for comparable PSM-containing bilayers. We conclude that the phosphoserine head group was less efficient than the phosphocholine head group in stabilizing sterol/sphingolipid interaction. However, hydrogen bonding apparently can overcome some of the negative effects of the phosphoserine head group, since CerPS interacted more favorably with cholesterol compared to DPPS.     

Suppression of Lα/Lβ Phase Coexistence in the Lipids of Pulmonary Surfactant

To determine how different constituents of pulmonary surfactant affect its phase behavior, we measured wide-angle x-ray scattering (WAXS) from oriented bilayers. Samples contained the nonpolar and phospholipids (N&PL) obtained from calf lung surfactant extract (CLSE), which also contains the hydrophobic surfactant proteins SP-B and SP-C. Mixtures with different ratios of N&PL and CLSE provided the same set of lipids with different amounts of the proteins. At 37°C, N&PL by itself forms coexisting L_α and L_β phases. In the L_β structure, the acyl chains of the phospholipids occupy an ordered array that has melted by 40°C. This behavior suggests that the L_β composition is dominated by dipalmitoyl phosphatidylcholine (DPPC), which is the most prevalent component of CLSE. The L_β chains, however, lack the tilt of the L_β′ phase formed by pure DPPC. At 40°C, WAXS also detects an additional diffracted intensity, the location of which suggests a correlation among the phospholipid headgroups. The mixed samples of N&PL with CLSE show that increasing amounts of the proteins disrupt both the L_β phase and the headgroup correlation. With physiological levels of the proteins in CLSE, both types of order are absent. These results with bilayers at physiological temperatures indicate that the hydrophobic surfactant proteins disrupt the ordered structures that have long been considered essential for the ability of pulmonary surfactant to sustain low surface tensions. They agree with prior fluorescence micrographic results from monomolecular films of CLSE, suggesting that at physiological temperatures, any ordered phase is likely to be absent or occupy a minimal interfacial area.     

Regulation of membrane proteins by dietary lipids: effects of cholesterol and docosahexaenoic acid acyl chain-containing phospholipids on rhodopsin stability and function

Purified bovine rhodopsin was reconstituted into vesicles consisting of 1-stearoyl-2-oleoyl phosphatidylcholine or 1-stearoyl-2-docosahexaenoyl phosphatidylcholine with and without 30 mol % cholesterol. Rhodopsin stability was examined using differential scanning calorimetry (DSC). The thermal unfolding transition temperature (T_m) of rhodopsin was scan rate-dependent, demonstrating the presence of a rate-limited component of denaturation. The activation energy of this kinetically controlled process (E_a) was determined from DSC thermograms by four separate methods. Both T_m and E_a varied with bilayer composition. Cholesterol increased the T_m both the presence and absence of docosahexaenoic acid acyl chains (DHA). In contrast, cholesterol lowered E_a in the absence of DHA, but raised E_a in the presence of 20 mol % DHA-containing phospholipid. The relative acyl chain packing order was determined from measurements of diphenylhexatriene fluorescence anisotropy decay. The T_m for thermal unfolding was inversely related to acyl chain packing order. Rhodopsin kinetic stability (E_a) was reduced in highly ordered or disordered membranes. Maximal kinetic stability was found within the range of acyl chain order found in native bovine rod outer segment disk membranes. The results demonstrate that membrane composition has distinct effects on the thermal versus kinetic stabilities of membrane proteins, and suggests that a balance between membrane constituents with opposite effects on acyl chain packing, such as DHA and cholesterol, may be required for maximum protein stability.     

Visualization of lateral phases in cholesterol and phosphatidylcholine monolayers at the air/water interface — a comparative study with two different reporter molecules

This study has compared two chemically distinct NBD-lipids with regard to their partitioning properties into lateral phases of pure and mixed cholesterol/phosphatidylcholine monolayers. Pure NBD-cholesterol (22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3-ol), which has the NBD-function in the sterol side chain (at carbon 22), gave a liquid-expanded force-area isotherm on water at 22°C (having a compressibility of 0.005 to 0.007 m/mN), although epifluorescence microscopy of the compressed NBD-cholesterol monolayer revealed that it had a solid-like surface texture. When the compressed NBD-cholesterol monolayer was allowed to expand, it fragmented into large flakes (tens to hundreds of μm in width) which eventually dissolved into a liquid state. The force-area isotherm of pure NBD-phosphatidylcholine (1-hexadecanoyl-2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecyl-sn-glycero-3-phosphocholine) was also liquid-expanded. When a compressed (30 mN/m) monolayer of NBD-phosphatidylcholine was examined by microscopy, it displayed many bright crystalline spots (about 50 μm across) which appeared to form when the monolayer was allowed to stabilize at this lateral surface pressure. These bright spots disappeared when the monolayer was expanded. When the surface texture of a pure cholesterol monolayer was examined, both probes (at 1 mol%) partitioned very similarly in the sterol monolayer. At low lateral surface pressures (1 and 5 mN/m) the probes appeared to be excluded from the cholesterol phase, forming very bright liquid-like areas against a uniformly black cholesterol phase. At 30 mN/m, NBD-phosphatidylcholine appeared to distribute increasingly into the cholesterol phase, whereas NBD-cholesterol still did not to mix with cholesterol. The characteristic surface texture of the liquid-expanded to liquid-condensed lateral phase transition of pure dipalmitoyl phosphatidylcholine (DPPC) monolayers could be visualized identically with both probes, indicating that these were similarly excluded from the liquid-condensed solid phase of DPPC. Finally, in mixed monolayers containing cholesterol and DPPC (molar ratio 33:67), both probes (at 1 mol%) revealed a similar surface texture of the monolayers (examined at a lateral surface pressure of 0.5 mN/m), suggesting that these partitioned similarly between the different lateral phases present in the mixed monolayer. In conclusion, although the two NBD-probes differed from each other in chemical and physical properties, both acted like ‘impurities’ when admixed into pure or mixed monolayers, and appeared to be equally excluded from lateral phases in which the packing density was high.     

The effect of hashish compounds on phospholipid phase transition

The interaction of hashish compounds, Δ¹-tetrahydrocannabinol and cannabidiol, with dipalmitoyl phosphatidylcholine was investigated using differential scanning calorimetry. Both drugs affect the transition of dipalmitoyl phosphatidylcholine from the gel to liquid crystalline state, decreasing both the melting temperature and the enthalpy of melting. At a drug to dipalmitoyl phosphatidylcholine ratio of approx. 1:5, two peaks appear in the transition profile, suggesting a phase separation in the drug dipalmitoyl phosphatidylcholine mixture.