Influence of cholesterol on the phase transition of lipid bilayers: a temperature-controlled force spectroscopy study

Cholesterol (Chol) plays the essential function of regulating the physical properties of the cell membrane by controlling the lipid organization and phase behavior and, thus, managing the membrane fluidity and its mechanical strength. Here, we explore the model system DPPC:Chol by means of temperature-controlled atomic force microscopy (AFM) imaging and AFM-based force spectroscopy (AFM-FS) to assess the influence of Chol on the membrane ordering and stability. We analyze the system in a representative range of compositions up to 50 mol % Chol studying the phase evolution upon temperature increase (from room temperature to temperatures high above the T(m) of the DPPC bilayer) and the corresponding (nano)mechanical stability. By this means, we correlate the mechanical behavior and composition with the lateral order of each phase present in the bilayers. We prove that low Chol contents lead to a phase-segregated system, whereas high contents of Chol can give a homogeneous bilayer. In both cases, Chol enhances the mechanical stability of the membrane, and an extraordinarily stable system is observed for equimolar fractions (50 mol % Chol). In addition, even when no thermal transition is detected by the traditional bulk analysis techniques for liposomes with high Chol content (40 and 50 mol %), we demonstrate that temperature-controlled AFM-FS is capable of identifying a thermal transition for the supported lipid bilayers. Finally, our results validate the AFM-FS technique as an ideal platform to differentiate phase coexistence and transitions in lipid bilayers and bridge the gap between the results obtained by traditional methods for bulk analysis, the theoretical predictions, and the behavior of these systems at the nanoscale.     

Interaction of propyl paraben with dipalmitoyl phosphatidylcholine bilayer: a differential scanning calorimetry and nuclear magnetic resonance study

The influence of the preservative, propyl paraben (PPB) on the biophysical properties of dipalmitoyl phosphatidyl choline (DPPC) vesicles, both in multilamellar vesicle (MLV) and unilamellar vesicle (ULV) forms, has been studied using DSC and (¹H and ³¹P) NMR. The mechanism by which PPB interacts with DPPC bilayers was found to be independent of the morphological organization of the lipid bilayer. Incorporation of PPB in DPPC vesicles causes a significant depression in the transition temperature and enthalpy of both the pre-transition (PT) and the gel to liquid crystalline transition. The presence of the PPB also reduces the co-operativity of these transitions. However, at high PPB concentration the PT disappears. DSC and NMR findings indicate that: (i) PPB is bound strongly to the lipid bilayer leading to increased headgroup fluidity due to reduced headgroup–headgroup interaction and (ii) the PPB molecules are intercalated between the DPPC polar headgroups with its alkyl chain penetrate into the co-operative region. MLV incorporated with high PPB concentration shows additional transitions whose intensity increases with increasing PPB concentration. This phase segregation observed could probably be due to co-existence of PPB-rich and PPB-poor phospholipid domains within the bilayers. The effect of inclusion of cholesterol in the PPB-free and PPB-doped DPPC dispersion was also studied. Equilibration studies suggest that PPB molecules are very strongly bound and remain intercalated between the polar headgroup for prolonged time.     

Influence of perfluorinated compounds on the properties of model lipid membranes

The influence of selected perfluorinated compounds (PFCs), perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS), on the structure and organization of lipid membranes was investigated using model membranes-lipid monolayers and bilayers. The simplest model--a lipid monolayer--was studied at the air-water interface using the Langmuir-Blodgett technique with surface pressure and surface potential measurements. Lipid bilayers were characterized by NMR techniques and molecular dynamics simulations. Two phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), characterized by different surface properties have been chosen as components of the model membranes. For a DPPC monolayer, a phase transition from the liquid-expanded state to the liquid-condensed state can be observed upon compression at room temperature, while a DMPC monolayer under the same conditions remains in the liquid-expanded state. For each of the two lipids, the presence of both PFOA and PFOS leads to the formation of a more fluidic layer at the air-water interface. Pulsed field gradient NMR measurements of the lateral diffusion coefficient (DL) of DMPC and PFOA in oriented bilayers reveal that, upon addition of PFOA to DMPC bilayers, DL of DMPC decreases for small amounts of PFOA, while larger additions produce an increased DL. The DL values of PFOA were found to be slightly larger than those for DMPC, probably as a consequence of the water solubility of PFOA. Furthermore, 31P and 2H NMR showed that the gel-liquid crystalline phase transition temperature decreased by the addition of PFOA for concentrations of 5 mol % and above, indicating a destabilizing effect of PFOA on the membranes. Deuterium order parameters of deuterated DMPC were found to increase slightly upon increasing the PFOA concentration. The monolayer experiments reveal that PFOS also penetrates slowly into already preformed lipid layers, leading to a change of their properties with time. These experimental observations are in qualitative agreement with the computational results obtained from the molecular dynamics simulations showing a slow migration of PFCs from the surrounding water phase into DPPC and DMPC bilayers.     

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.     

Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol % sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterols. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol % cholesterol), the average lifetime of trans-parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average lifetime of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial.     

Chain-melting phase transition and short-range molecular interactions in phospholipid foam bilayers

Occurrence of two-dimensional chain melting phase transition in foam bilayers was established for the first time. Microscopic horizontal foam bilayers [Newton black films (NBF)] were investigated by the microinterferometric method of Scheludko-Exerowa. The foam bilayers were formed from water-ethanol solutions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine (Egg PC) and samples of amniotic fluid (AF) at different temperatures. The influence of temperature on the foam bilayer thickness h(w) and on the critical concentration Cc for formation of foam bilayer was studied. It was shown that in the range of the main phase transition the temperature dependence of h(w) and C(c) changed specifically in the case of DMPC and DPPC foam bilayers. The thickness of the foam bilayers increased with decreasing temperature in the range of the main phase transition due to the melting of hydrocarbon tails of phospholipid molecules. These changes took place at the temperatures of the bulk chain-melting phase transitions, as determined by differential scanning calorimetry (DSC) for both aqueous, and water/ethanol DMPC, DPPC, and DPPC dispersions. An effect of the 'disperse medium' on h(w) was found for foam bilayers from DPPC. The results that foam bilayers could have different thickness at different temperatures disproved the current concept that NBF acquired constant thickness at concentrations higher than C(el,cr). The data for Cc were analysed on the basis of the hole-nucleation theory of bilayer stability of Kashchiev and Exerowa. This theory considered the amphiphile bilayer as a two-dimensional ordered system with short-range molecular interactions between the first neighbour molecules (as in a crystal). The short-range molecular interactions were presented by the parameter binding energy Q of an amphiphile molecule in the bilayer. The binding energy Q of two neighbouring phospholipids was calculated for the gel (30-60 kT) and liquid crystalline state (16-18 kT) of the bilayers from DMPC, DPPC, Egg PC, AF. Concentration/temperature phase diagram of DPPC foam bilayers that defined regions of gaseous (ruptured), gel and liquid crystalline foam bilayers were drawn. The values of Q obtained for various samples were very close and vary from 5.3 x 10(-20) to 9.4 x 10(-20) (approx. 13-22 kT) which indicated that in all cases the foam bilayers were in liquid-crystalline state. This is an important result since the parameter studied-threshold concentration (threshold dilution) is crucial for a very successful assessment of the risk for respiratory distress syndrome (RDS) in newborns and could be employed in medicine for assessment of other respiratory disturbances. It is to be expected that foam bilayers from phospholipids could be used as a model for investigation of short-range forces in biological structures, of interaction between membranes, etc.     

Monte Carlo study of lipid membranes: Simulation of diparmitoylphosphatidylcholine bilayers in gel and liquid-crystalline phases

The statistical properties of the bilayer membranes of diparmitoylphosphatidylcholine (DPPC) in the gel and liquid-crystal phases were studied by Monte Carlo (MC) simulation using potential functions of the Lennard-Jones, the simple Coulomb, and the bond torsion. The simulation was undertaken on a two-dimensional periodic condition imposed on the bilayer model consisting of faithfully described molecules. The structure and ordering of the model bilayers accorded well with experiments, and the segment order parameters were in agreement with those of the nuclear magnetic resonance (NMR) experiments. The two kinds of lipid chains of DPPC do not equivalently behave in the bilayers, and chain 2 has lower ordering than chain 1. The order parameters of the first eight segments of chain 2 in the liquid-crystal model are particularly small and are roughly constant. From electron density analysis, it has been observed that the liquid-crystal bilayer has about one excess water molecule per one lipid molecule in comparison with the gel bilayer. The energy difference between the two bilayer models, taking account of the water contribution, is consistent with the latent heat of the phase transition. © 1995 by John Wiley & Sons, Inc.     

Vesicle—vesicle interaction and forces between bilayers in phospholipid systems incorporating phosphatidylinositol

Sonicated vesicles have been prepared from mixtures of dipalmitoylphosphatidylcholine (DPPC) and phosphatidylinositol (PI) covering a range of composition. The effect of temperature on the rates of aggregation of the vesicles on addition of calcium and magnesium ions has been investigated. Apart from pure PI vesicles the rates of aggregation decrease dramatically as the temperature approaches the gel-to-liquid crystalline phase transition temperature of DPPC. Low angle X-ray analysis of lamellar phases of DPPC-PI (75:25 wt%) in the presence of Ca²⁺ ions shows that between 35 and 45°C the repeat distance goes from 136 to 68 Å. It is suggested that below the chain-melting temperature of DPPC Ca²⁺ ions induce lateral phase separation of PI giving a lamellar repeat distance corresponding to the thickness of two bilayers.The net repulsive pressure between DPPC-PI bilayers has been measured by a vapour pressure technique as a function of temperature. At close apposition (<15 Å) the pressure is characteristic of hydration repulsion and increases with temperature. The repulsive force between the bilayers, lateral pressure and compressibility of the bilayers have also been determined. On progressively removing water from between the bilayers 5–14% of the work done goes into bilayer deformation, the remaining 86–95% being required to bring the bilayers together.     

Condensing effect of palmitic acid on DPPC in mixed Langmuir monolayers

The interaction between deuterated dipalmitoylphosphatidylcholine (DPPC-d62) and palmitic acid (PA) in mixed Langmuir monolayers is studied using vibrational sum frequency generation (VSFG) spectroscopy. Palmitic acid is an additive in exogenous lung surfactant preparations such as Survanta and Surfaxin. The effect of PA on the chain conformation and orientation of DPPC in the liquid-expanded and condensed phases is explored. A condensing effect of PA on DPPC is observed with VSFG. At 12 mN/m, DPPC-d62 alone is in the liquid-expanded phase. Adding PA increases the conformational ordering of DPPC chains and causes DPPC to transition from the expanded phase into the condensed phase. At 42 mN/m, DPPC-d62 and PA form a mixed structure in the condensed phase. The presence of PA decreases the chain tilt angle of DPPC, increasing the orientational ordering of DPPC chains. At 42 mN/m, there is also evidence from the frequency red shift of the PO2- symmetric stretch that the carboxyl group of PA forms a hydrogen bond with the phosphate group of DPPC in the condensed phase. From this work the effect of PA on DPPC is 2-fold: (1) PA increases the chain ordering of DPPC and promotes the LE and TC phase separation and (2) due to the miscibility between DPPC and PA in the condensed phase, PA decreases the collapse pressure.     

Dynamic rheo-optical characterization of a low-density polyethylene/perdeuterated high-density polyethylene blend by two-dimensional step-scan FTIR spectroscopy

Dynamic mechanical analysis coupled with polarized step-scan FTIR transmission and two-dimensional correlation analysis (2D FTIR) has been used to monitor the submolecular orientational responses of the components of a semicrystalline 50 : 50 blend of low-density polyethylene (LDPE) and perdeuterated high-density polyethylene (d*-HDPE) to a small amplitude uniaxial 23.47 Hz sinusoidal mechanical strain. Perdeuteration of the HDPE component allowed the distinction of its response from that of the LDPE in the blend samples. The experiments were carried out at room temperature. Analysis of the data indicates that the crystalline parts of the two components reorient at different rates, with the functional groups of the high-density portion reorienting faster, in general, than those of the LDPE in response to the mechanical strain. © 1993 John Wiley & Sons, Inc.     

NSAIDs interactions with membranes: a biophysical approach

This work focuses on the interaction of four representative NSAIDs (nimesulide, indomethacin, meloxicam, and piroxicam) with different membrane models (liposomes, monolayers, and supported lipid bilayers), at different pH values, that mimic the pH conditions of normal (pH 7.4) and inflamed cells (pH 5.0). All models are composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) which is a representative phospholipid of most cellular membranes. Several biophysical techniques were employed: Fluorescence steady-state anisotropy to study the effects of NSAIDs in membrane microviscosity and thus to assess the main phase transition of DPPC, surface pressure-area isotherms to evaluate the adsorption and penetration of NSAIDs into the membrane, IRRAS to acquire structural information of DPPC monolayers upon interaction with the drugs, and AFM to study the changes in surface topography of the lipid bilayers caused by the interaction with NSAIDs. The NSAIDs show pronounced interactions with the lipid membranes at both physiological and inflammatory conditions. Liposomes, monolayers, and supported lipid bilayers experiments allow the conclusion that the pH of the medium is an essential parameter when evaluating drug-membrane interactions, because it conditions the structure of the membrane and the ionization state of NSAIDs, thereby influencing the interactions between these drugs and the lipid membranes. The applied models and techniques provided detailed information about different aspects of the drug-membrane interaction offering valuable information to understand the effect of these drugs on their target membrane-associated enzymes and their side effects at the gastrointestinal level.     

Large spontaneous deformation of uniaxially oriented main-chain liquid–crystalline elastomers composed of BB-n mesogens

We observed the spontaneous shape change of a uniaxially oriented liquid–crystalline elastomer composed of smectic main-chain liquid–crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracted by about 650% on heating up to the isotropic phase, the sample length recovered by about 450% on cooling to room temperature in the first heating–cooling process. In contrast, the elastomer exhibited completely reversible deformation in the second heating–cooling process. The shape change occurred with almost no change in the orientational order of mesogens in the temperature region of the liquid–crystalline phase. The mechanism of the spontaneous thermotropic shape change is discussed.     

The antimalarial agent halofantrine perturbs phosphatidylcholine and phosphatidylethanolamine bilayers: a differential scanning calorimetric study.

The interaction of halofantrine with phosphatidylcholine and phosphatidylethanolamine bilayers has been investigated by differential scanning calorimetry. Halofantrine caused a broadening of the gel to liquid crystalline phase transition endotherm of the phosphatidylcholines. A decrease in the transition temperature Tm and enthalpy (delta H) of transition was also observed. This varied with the chain length of the phospholipid and was more pronounced with short chain members. Halofantrine-induced changes to the thermotropic characteristics of dipalmitoylphosphatidylcholine (DPPC)/cholesterol bilayers suggested that the penetration of halofantrine into the bilayer was diminished in the presence of cholesterol. A more complex calorimetric profile was observed in the interaction of halofantrine with phosphatidylethanolamines and the results suggested that halofantrine did not disrupt the cooperativity of the phosphatidylethanolamine bilayers to the same extent as that observed with the phosphatidylcholines. Halofantrine caused significant perturbation of phospholipids and this property might have an important bearing on its pharmacodynamic effects.     

An investigation of solute-liquid crystal interaction in hydrogen bonded complexes of palmitic acid and p-octyloxybenzoic acid

The orientational order of a liquid crystalline phase which has a specific solute-liquid crystal interaction was investigated using nuclear magnetic resonance. Three isotopically substituted species of palmitic acid (palmitic acid-d₃₁, 1-¹³C-2.2-H₂-palmitic acid-d₂₉ and 2,2,3,3-H₄-palmitic acid-d₂₇) were dissolved in the liquid crystal p-octyloxybenzoic acid (p-OOBA) and the proton, deuteron and carbon 13 NMR spectra recorded as a function of temperature. ¹H-¹³H dipolar couplings were observed using a spin echo pulse sequence which removes heteronuclear dipolar couplings to the chain deuterons. In the case of the carbon 13 labelled compound, ¹H-¹³C dipolar couplings could be observed by applying an additional refocusing pulse to the ¹³C spins. The dipolar and quadrupolar couplings were used to calculate the complete orientational order matrix of the alpha methylene segment of palmitic acid in p-OOBA. The liquid crystal was shown to largely determine the orientational order of the head group and this was attributed to intermolecular hydrogen bonding. The dipolar and quadrupolar couplings for the rest of the chain were interpreted in terms of a mean field equilibrium statistical model, based on the Samulski Inertial Frame Model. Hydrogen bonding was shown to be of greater importance in the orientational ordering of the solutes in the liquid crystal than are electrostatic interactions in the ordering of the amphiphile in the potassium palmitate/water system.     

Cubatic liquid-crystalline behavior in a system of hard cuboids

The lyotropic phase behavior of cuboidal particles was investigated via Monte Carlo simulations. Hard cubes were approximated by suitably shaped clusters of hard spheres. Changes in concentration and structure of the system were monitored as a function of osmotic pressure P* (imposed in an isobaric ensemble). As expected, an isotropic phase prevailed at low concentrations (low P*) and a crystalline phase formed at high concentrations (high P*). A third distinct phase was also observed for an intermediate range of concentrations (approximately marked by breaks in the P* versus concentration curve). The structure of this mesophase was characterized both visually and analytically by calculating radial distribution functions and order parameters. It was found that such a mesophase exhibits orientational ordering along three axes (cubatic order) but significant translational disorder, thus having a structure clearly distinct from both isotropic and crystalline phases.     

Influence of La3+ on the Phase Behavior and the Pluidity of Phospholipid Bilayers

The influence of La³⁺ on the phase behavior and the fluidity of the negatively charged phospholipid dipalmitoylphosphatidylglycerol(DPPG) bilayers was studied by differential scanning calorimetry (DSC) and FT-Raman spectroscopy. La³⁺, induced a phase separation of DPPG bilayers, and formed three peaks. La³⁺ increased the phase transition temperature of the new peaks, broadened the half width of the DSC signal, and stabilized a gel phase relative to a crystalline phase of DPPG bilayers. La³⁺ was shown to increase interchain order and intermolecular ordering of the lipid lattice, and decreased the fluidity of DPPG bilayers.     

Mixing of perfluorooctanesulfonic acid (PFOS) potassium salt with dipalmitoyl phosphatidylcholine (DPPC)

Perfluorooctane-1-sulfonic acid (PFOS) is emerging as an important persistent environmental pollutant. To gain insight into the interaction of PFOS with biological systems, the mixing behavior of dipalmitoylphosphatidylcholine (DPPC) with PFOS was studied using differential scanning calorimetry (DSC) and fluorescence anisotropy measurements. In the DSC experiments the onset temperature of the DPPC pretransition (Tp) decreased with increasing PFOS concentration, disappearing at XDPPC < or = 0.97. The main DPPC phase transition temperature showed a depression and peak broadening with increasing mole fraction of PFOS in both the DSC and the fluorescence anisotropy studies. From the melting point depression in the fluorescence anisotropy studies, which was observed at a concentration as low as 10 mg/L, an apparent partition coefficient of K = 5.7 x 10(4) (mole fraction basis) was calculated. These results suggest that PFOS has a high tendency to partition into lipid bilayers. These direct PFOS-DPPC interactions are one possible mechanism by which PFOS may contribute to adverse effects, for example neonatal mortality, in laboratory studies and possibly in humans.     

Discrete and heterogeneous rotational dynamics of single membrane probe dyes in gel phase supported lipid bilayer

In order to probe the local dynamics of lipid bilayers in the gel phase, we measured the rotational time trajectories of a membrane probe, diI(3), in supported bilayers of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) using single molecule fluorescence polarization imaging. diI(3) has two hydrocarbon tails that mimic phospholipid tails and has its transition dipole moment lying mostly on the plane of the membrane; hence it is an excellent probe for rotational dynamics in membranes. Above the transition temperature, the probes are laterally mobile and do not display polarized emission. In the gel phase below the transition temperature, lateral mobility is severely reduced and the emission becomes polarized with its polarization direction changing in the milliseconds time scale. Molecule by molecule analysis of the rotational time scales revealed significant heterogeneities among molecules, much larger than would be due to statistical noise. Control experiments using small unilamellar vesicles suggest that the heterogeneities are not caused by surface interactions and are intrinsic to the gel phase membrane. The rotational dynamics is strongly temperature dependent and the thermally activated state for the rotational motion has a large entropic barrier (> 30kB), indicating that relatively large local disorder is required for the rotational motion to occur. Rotational hopping between discrete angles has been observed at the lowest temperatures (approximately 10 degrees C). Our results suggest that the gel phase membrane is not uniform at the microscopic level but is highly dynamic with the rigidity of local environments constantly changing.     

Spontaneous pattern of linear molecules in strongly confined spaces

In this work, we study the tight packing of short linear molecules in confined space by performing molecular dynamic simulations. The short chain-like molecules spontaneously arrange within single-walled carbon nanotubes (SWNTs) and exhibit a variety of chiral and achiral structures, depending on the pore size and molecule length. Simulation results show that the packing structures for these confined short linear molecules are controlled by the competition between positional order and orientational order. For linear molecules with short molecular length, such as the two-site Lennard-Jones molecules, the orientational order gradually decreases as temperature increases, and then the positional order begins to disappear. While for longer molecules, such as four-site Lennard-Jones molecules, the positional order decreases more rapidly than the orientational order as temperature increases. We also investigated the effect of molecular rigidity. For linear molecules with higher rigidity, part of packing structures may slowly rotate as a whole, and the rotation of packing arrangements is found to be induced by the preexisting defects.     

Effects of high electrolyte concentration on DPPC-multilayers: an ESR and DSC investigation

The effects of salinity on the lateral headgroup interactions of dipalmitoylphosphatidylcholine (DPPC) molecules in fully hydrated multilayers have been investigated by spin label electron spin resonance (ESR) spectroscopy and differential scanning calorimetry (DSC).By increasing the NaCl concentration from 0 to 3 M in the multilayers' dispersion medium, the ESR measurements performed with the 5-stearic acid spin label and di-tert-butyl-nitroxide show an increase in the orientational degree of order of the lipid molecules, mainly in the gel phase, and a decrease of the membrane permeability. An upward shift from 31.5° to 36.5°C and from 40.5° to 41.9°C of the pre- and main DPPC phase transition temperatures, respectively, is observed with 5-SASL, while slightly higher values are detected with DTBN. Small effects are evident on the properties of the liquid crystalline phase of the DPPC multilayers.The DSC measurements also reveal an upward shift of the pre- and main transition temperatures. The shifts, however, are more marked if compared to the ones observed with the ESR technique.The findings suggest an increase in the packing density of the DPPC molecules in the multilayers in presence of high salt concentration. Dehydration of the DPPC interfacial region with a variation of the lateral electrostatic interactions between phospholipid polar heads trigger the phenomena observed.