Binding of divalent cations of dipalmitoylphosphatidylcholine bilayers and its effect on bilayer interaction

We have confirmed that CaCl2 swells the multilayer lattice formed by dipalmitolyphosphatidylcholine (DPPC) in an aqueous solution. Specifically, at room temperature 1 mM CaCl2 causes these lipid bilayers to increase their separation, dw, from 19 A in pure water to greater than 90 A. CaCl2 concentrations greater than 4 mM cause less swelling. We have measured the net repulsive force between the bilayers in 30 mM CaCl2 at T = 25 degrees C (below the acyl chain freezing temperature). For interbilayer separations between 30 and 90 A, the dominant repulsion between bilayers is probably electrostatic; Ca2+ binds to DPPc lecithin bilayers, imparting a charge to them. The addition of NaCl to CaCl2 solutions decreases this repulsion. For dw less than 20 A, the bilayer repulsion appears to be dominated by the "hydration forces" observed previously between both neutral and charged phospholipids. From the electrostatic repulsive force, we estimate the extent of Ca2+ binding to the bilayer surface. The desorption and bound Ca2+, apparent when bilayers are pushed together, is more rapid than one would expect if an association constant governed Ca2+ binding. The association affinity does not appear to be a fixed quantity but rather a sensitive function of ionic strength and bilayer separation.     

Pressure effects on dipalmitoylphosphatidylcholine bilayers measured by 2H nuclear magnetic resonance

The effects of pressure, up to 5 kbar, on multilamellar vesicles of 1,2-dipalmitoyl-sn-phosphatidylcholine perdeuterated in the acyl chains (DPPC-d62) were examined by using high-pressure NMR techniques. A deuterium probe was built, and the quadrupole splitting was measured against pressure at various temperatures. The experiments were performed on pure lipid bilayers in the liquid-crystalline state and on bilayers in the liquid-crystalline state containing the local anesthetic tetracaine. The results show that the order parameter of all segments of the acyl chains increases with pressure in the liquid-crystalline state. The more highly ordered regions of the chains are affected slightly more than the regions near the methyl ends. The addition of tetracaine increases the disorder of the chains, and pressure reverses the effect of anesthetic on the lipid as seen by the reversal of the changes in line shape and the measured order parameter.     

The interaction of cholesterol with bilayers of phosphatidylethanolamine

The interaction of cholesterol with the glycerol backbone segments of phospholipids was studied in bilayers of phosphatidylethanolamine containing equimolar amounts of cholesterol. Glycerol selectively deuterated at various positions was supplied to the growth medium of Escherichia coli strain 131 GP which is defective in endogeneous glycerol synthesis. The procedure enables the stereospecific labeling of the three glycerol backbone segments of the membrane phospholipids. Phosphatidylethanolamine with wild-type fatty acid composition was purified from E. coli cells and deuterium magnetic resonance spectra were obtained either from dispersions of pure phosphatidylethanolamine or from equimolar mixtures of phosphatidylethanolamine with cholesterol. For comparative purposes 1,2-di[9,10-²H₂]elaidoyl-sn-glycero-3-phosphoethanolamine and [3-α-²H]cholesterol were synthesized in order to monitor the behavior of the fatty acyl chains and of the cholesterol molecule itself. For all deuterated segments the deuterium quadrupole splittings as well as the deuterium spin-lattice (T₁) relaxation times were measured as a function of temperature. The glycerol backbone was found to be a remarkably stable structural element of the phospholipid molecule. The quadrupole splittings of the backbone segments changed only by at most 2 kHz upon incorporation of 50 mol % cholesterol. This was in contrast to the fatty acyl chains where the same amount of cholesterol increased the quadrupole splitting by more than 20 kHz. The glycerol segments exhibited the shortest T₁ relaxation times of all CH₂ segments indicating that the glycerol backbone is the slowest motional moiety of the lipid molecule. Addition of cholesterol has no effect on the backbone motion but the fast reorientation rate of the trans-double bonds in 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine is increased dramatically.     

Effects of alcohols on lipid bilayers with and without cholesterol: the dipalmitoylphosphatidylcholine system

Differential scanning calorimetry is a useful method to study the thermotropic phase transitions of a phospholipid bilayer. In the present study DSC is used to determine the effects of methanol and ethanol on DPPC and DPPC/2 mol% cholesterol bilayers. The biphasic effect of the main transition and the presence of an extra peak on the DSC cooling scans were observed above certain alcohol concentrations. In the presence of 2% cholesterol, the concentration at which the biphasic effect occurs is increased by both short-chain alcohols. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is used as a fluorescent probe to directly determine the onset of interdigitation in these systems as reflected by a drop in the DPH fluorescence intensity.     

The effect of ergosterol on dipalmitoylphosphatidylcholine bilayers: a deuterium NMR and calorimetric study

We have studied the effect of ergosterol, an important component of fungal plasma membranes, on the physical properties of dipalmitoylphosphatidylcholine (DPPC) multibilayers using deuterium nuclear magnetic resonance ((2)H NMR) and differential scanning calorimetry (DSC). For the (2)H NMR experiments the sn-1 chain of DPPC was perdeuterated and NMR spectra were taken as a function of temperature and ergosterol concentration. The phase diagram, constructed from the NMR spectra and the DSC thermograms, exhibits both solid-ordered (so) + liquid-ordered (lo) and liquid-disordered (ld) + lo phase coexistence regions with a clear three-phase line. This is the first demonstration that lo domains exist in liquid crystalline membranes containing ergosterol. The domain sizes in the ld+lo phase coexistence region were estimated by analyzing the exchange of labeled DPPC between the two regions, and depend on ergosterol concentration. The DPPC-ergosterol phase diagram is similar to that of the DPPC-cholesterol multibilayer system except that the so+lo and ld+lo phase coexistence regions are considerably broader.     

Lateral interaction of cholesterol in diacylphosphatidylcholesterol bilayers

Thermotropic phase-transition properties of the aqueous dispersions of several diacylphosphatidylcholesterol (DRCh) analogs are examined. The aqueous dispersions of their calcium salts exhibit characteristic endothermic thermotropic transitions due to a change in the conformation of acyl chains. These dispersions consist of osmotically intact liposomes that trap ions, and at the transition temperature there is anomalous increase in the ion leakage. Wide-angle electron diffraction studies of DPCh · Ca monolayers also exhibit a transition from a sharp 4.25 Å band to a broad one centering at 4.7 Å, reflecting an order-disorder transition in the acyl chains. The long-range order in the organization of acyl chains of DRCh molecules could arise from intermolecular interactions between the cholesterol moleties to form a functional dimer, and such dimers are apparently cross-linked by Ca²⁺ to form a long-range interacting lattice of acyl chains. Evidence for this model is adduced from the fluorescence properties of the dispersions of dimyristoylphosphatidylcholesta-5,7,9-trienol. The phase-transition properties of DRCh are an ideal illustration of calcium-induced isothermal phase transition.     

The condensing effect of cholesterol in lipid bilayers

The condensing effect of cholesterol on phospholipid bilayers was systematically investigated for saturated and unsaturated chains, as a function of cholesterol concentration. X-ray lamellar diffraction was used to measure the phosphate-to-phosphate distances, PtP, across the bilayers. The measured PtP increases nonlinearly with the cholesterol concentration until it reaches a maximum. With further increase of cholesterol concentration, the PtP remains at the maximum level until the cholesterol content reaches the solubility limit. The data in all cases can be quantitatively explained with a simple model that cholesterol forms complexes with phospholipids in the bilayers. The phospholipid molecules complexed with cholesterol are lengthened and this lengthening effect extends into the uncomplexed phospholipids surrounding the cholesterol complexes. This long-range thickening effect is similar to the effect of gramicidin on the thickness of lipid bilayers due to hydrophobic matching.     

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.     

Sphingomyelin--lecithin bilayers and their interaction with cholesterol.

Utilizing X-ray diffraction and differential scanning calorimetry (DSC), we have studied (1) the structure and thermotropic properties of hydrated N-palmitoylsphingomyelin, (2) the interaction of N-palmitoylsphingomyelin with dimyristoyllecithin, and (3) the interaction of cholesterol with N-palmitoylsphingomyelin and dimyristoyllecithin, both individually and in a 50:50 (mol/mol) mixture. N-Palmitoylsphingomyelin forms bilayers which undergo a thermotropic order--disorder (gel--liquid crystalline) transition at 40.5 degrees C (delta H = 5.8 kcal/mol). The bilayer repeat distance is 66.8 A at 10 degrees C and 61.6 A at 50 degrees C. N-Palmitoylsphingomyelin exhibits miscibility with dimyristoylecithin in both the gel and liquid-crystalline phases, and no lateral phase separation occurs. Scanning calorimetry indicates that interaction with cholesterol is similar for both N-palmitoylsphingomyelin and dimyristoyllecithin and that in a 50:50 (mol/mol) mixture cholesterol shows no preferential affinity for either phospholipid.     

Calorimetric,volumetric and structural studies of the interaction between chlorogenic acid and dipalmitoylphosphatidylcholine bilayers

Chlorogenic acid (CGA) is the main component of coffee and an antioxidant. CGA has been reported to bear various good health effects. At the same time, it has been found that the addition of CGA induces an undesirable deformation of red blood cells. This fact suggests that CGA may bind to the proteins or/and membrane lipids of red blood cells. This study aimed to examine how CGA binds the bilayers of phosphatidylcholine (PC), one of red blood cells' primary lipids. To this end, we investigated the effect of CGA on the phase behavior and the structure of dipalmitoyl-PC (DPPC) bilayers in the form of multi-lamellar vesicles. Calorimetry and dilatometry measurements showed that the DPPC chain melting transition cooperativity decreases as increasing CGA concentrations. In addition, X-ray diffraction results showed that the lamellar repeat periodicity becomes disordered, and the periodicity disappears completely at high CGA concentrations. Together with these findings, it can be inferred that the CGA molecules do not penetrate inside the DPPC bilayers but bind to their surface in a negatively charged form.     

The influence of cholesterol on the interaction between N-dodecyl-N,N-dimethyl-N-benzylammonium halides and phosphatidylcholine bilayers

Effects of N-dodecyl-N,N-dimethyl-N-benzylammonium halides (DBeAX) on thermotropic phase behavior of phosphatidylcholine/cholesterol bilayers as well as on 1H NMR spectra were studied. The surfactants were added either to the water phase or directly to the lipid phase (a mixed film was formed). The benzyl group, opposite to liposomes without cholesterol, is not incorporated into the bilayer in the gel state but only in the liquid state. All the halides DBeAX (particularly the chloride DBeAC) showed greater ability to destabilize the membrane structure in the presence than in the absence of cholesterol. The interaction of DBeAX with DPPC/cholesterol bilayers and subsequent changes in the phospholipid bilayer organization depended on the kind of counterion. The strongest effects were observed for chloride (most electronegative ion) and for iodide (largest ion). The effects of chloride and bromide on phase transition and 1H NMR spectra in the presence and absence of cholesterol were opposite. This is discussed in terms of the influence of counterions on the pair cholesterol-DPPC interactions.     

Effect of cholesterol on viscoelastic properties of dipalmitoylphosphatidylcholine multibilayers as measured by a laser-induced ultrasonic probe

Using a novel laser-induced ultrasonic probe, we have examined the bulk viscoelastic properties of fully hydrated dipalmitoylphosphatidylcholine (DPPC) aligned multibilayers in terms of the anisotropic in-plane elastic stiffness (C11) and viscosity (eta 11). Our measurements of C11 are in accord with those reported on Brillouin light scattering on a similar system. Our measurements on viscosity are the first of their kind and are, on the average, a factor of 10 lower than microviscosities estimated by spectroscopic techniques. We report the first comprehensive study of the effects of cholesterol on the bulk mechanical properties of DPPC multibilayers. At temperatures above the phase transition temperature of DPPC (Tc), an increase in both C11 and eta 11 is noticed when cholesterol is incorporated in the multibilayers. However, at temperatures below Tc, no measurable changes are detected in either C11 or eta 11. These results, reflecting changes in the bulk viscoelastic properties of the multibilayers, differ from the changes reported by local fluidity parameters in that the latter indicate a decrease in the bilayer fluidity in the presence of cholesterol above Tc and an increase below Tc ("dual effect" of cholesterol). Our data suggest that the "dual effect" of cholesterol is noticeable only on a molecular scale. Increasing cholesterol concentrations higher than 20 mol % cease to further affect C11 or eta 11 of the DPPC multibilayers. This agrees with various results reported in the literature, by techniques measuring the local effects of cholesterol, and supports the changes in molecular organization postulated to occur when cholesterol concentration reaches 20 mol % in the lipid bilayers.     

The effects of cholesterol on the time-resolved emission anisotropy of 12-(9-anthroyloxy)stearic acid in dipalmitoylphosphatidylcholine bilayers

The time-resolved fluorescence emission anisotropy of 12-(9-anthroyloxy)stearic acid (12-AS) and 1,6-diphenyl-1,3,5-hexatriene (DPH) have been measured in dipalmitoylphosphatidylcholine liposomes in the presence and absence of 40 mol% cholesterol at temperatures above and below the phase transition temperature (41°C). By using a synchronously-pumped mode-locked frequency-doubled dye laser and single photon counting detection with an excitation response function of 300 picosecond, rotational correlation times down to less than 1 nanosecond could be resolved. Whereas DPH showed only small changes in the limiting anisotropy on the addition of cholesterol, 12-AS showed significant increases in this parameter with the effect being potentiated at higher temperatures. This difference in behaviour has been attributed to a fluorophore-cholesterol interaction that resulted in a change in the fluorophore geometry. Not only do DPH and 12-AS sense different depolarizing rotations due to the different directions of their emission dipoles but also differ in their lipid interactions which alter their limiting anisotropies. The implication is that the comparison of steady-state anisotropy measurements between chemically identical fluorophores in different lipid environments may be complicated by molecular distortions that change the motions to which the steady-state fluorescence parameters will be sensitive.     

Effects of cholesterol on conformational disorder in dipalmitoylphosphatidylcholine bilayers. A quantitative IR study of the depth dependence

A method originally proposed by Snyder and Poore [(1973) Macromolecules 6, 708-715] as a specific probe of trans-gauche isomerization in hydrocarbon chains and recently applied [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939] to the quantitative determination of phospholipid acyl chain conformational order is utilized to monitor the effects of cholesterol at various depths in dipalmitoylphosphatidylcholine (DPPC) bilayers. The method is based on the observation that the CD2 rocking modes from the acyl chains of specifically deuterated phospholipids occur at frequencies in the Fourier transform infrared spectrum which depend upon the local geometry (trans or gauche) of the C-C-C skeleton surrounding a central CD2 group. Three specifically deuterated derivatives of DPPC, namely, 4,4,4',4'-d4 DPPC (4-d4 DPPC), 6,6,6',6'-d4 DPPC (6-d4 DPPC), and 12,12,12',12'-d4 DPPC (12-d4 DPPC), have been synthesized, and the effects of cholesterol addition at 2:1 DPPC/cholesterol (mol:mol) on acyl chain order at various temperatures have been determined. At 48 degrees C, cholesterol inhibits gauche rotamer formation by factors of approximately 9 and approximately 6 at positions 6 and 4, respectively, of the acyl chains, thus demonstrating a strong ordering effect in regions of the bilayer where the sterol rings are presumed to insert parallel to the DPPC acyl chains. In contrast, the ability of the sterol to order the acyl chains is much reduced at the 12-position. The sterol demonstrates only a slight disordering of phospholipid gel phases. Finally, the contributions of different classes of gauche conformers to the spectra have been determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative 2H- and 31P-NMR study on the properties of palmitoyllysophosphatidylcholine in bilayers with gramicidin, cholesterol and dipalmitoylphosphatidylcholine

The stoichiometric palmitoyllysophosphatidylcholine (lysoPC)/gramicidin (4:1, mol/mol) lamellar complex (Killian, J.A., De Kruijff, B., Van Echteld, C.J.A., Verkleij, A.J., Leunissen-Bijvelt, J. and De Gier, J. (1983) Biochim. Biophys. Acta 728, 141-144) is a useful model system to investigate the various aspects of lipid protein interactions. To study the effect of gramicidin on local order and motion of 1-palmitoyl-sn-glycero-3-phosphocholine (lysoPC) we employed 31P and 2H nuclear magnetic resonance (NMR) using selectively deuterated lysoPC's and we compared the results to those obtained for lysoPC in bilayers with cholesterol (1:1, mol/mol) and dipalmitoylphosphatidylcholine (DPPC) (1:4, mol/mol). 2H-NMR experiments on acyl chain deuterated lysoPC showed similar quadrupole splittings in the liquid crystalline state for the lysoPC/DPPC and the lysoPC/gramicidin samples. In the lysoPC/cholesterol sample an increase of the quadrupole splitting was found. T1 measurements showed that gramicidin decreases the lysoPC acyl chain motion, especially at the C12 position. In the lysoPC/cholesterol sample an increase of motion was observed as compared to lysoPC in fluid bilayers of DPPC. 31P-NMR and 2-H-NMR measurements of lysoPC, deuterated at the alpha- and beta-position of the choline moiety, indicated an increase in headgroup flexibility in all samples as compared to the parent compound DPPC. In addition, a change in headgroup conformation was observed. The alpha- and beta-segments in all samples exhibited concerted motion. It was found that also in the polar headgroup gramicidin induces a decrease of the rate of motion.     

The effect of cholesterol on the lateral diffusion of phospholipids in oriented bilayers

Pulsed field gradient NMR was utilized to directly determine the lipid lateral diffusion coefficient for the following macroscopically aligned bilayers: dimyristoylphosphatidylcholine (DMPC), sphingomyelin (SM), palmitoyloleoylphosphatidylcholine (POPC), and dioleoylphosphatidylcholine (DOPC) with addition of cholesterol (CHOL) up to approximately 40 mol %. The observed effect of cholesterol on the lipid lateral diffusion is interpreted in terms of the different diffusion coefficients obtained in the liquid ordered (l(o)) and the liquid disordered (l(d)) phases occurring in the phase diagrams. Generally, the lipid lateral diffusion coefficient decreases linearly with increasing CHOL concentration in the l(d) phase for the PC-systems, while it is almost independent of CHOL for the SM-system. In this region the temperature dependence of the diffusion was always of the Arrhenius type with apparent activation energies (E(A)) in the range of 28-40 kJ/mol. The l(o) phase was characterized by smaller diffusion coefficients and weak or no dependence on the CHOL content. The E(A) for this phase was significantly larger (55-65 kJ/mol) than for the l(d) phase. The diffusion coefficients in the two-phase regions were compatible with a fast exchange between the l(d) and l(o) regions in the bilayer on the timescale of the NMR experiment (100 ms). Thus, strong evidence has been obtained that fluid domains (with size of micro m or less) with high molecular ordering are formed within a single lipid bilayer. These domains may play an important role for proteins involved in membrane functioning frequently discussed in the recent literature. The phase diagrams obtained from the analysis of the diffusion data are in qualitative agreement with earlier published ones for the SM/CHOL and DMPC/CHOL systems. For the DOPC/CHOL and the POPC/CHOL systems no two-phase behavior were observed, and the obtained E(A):s indicate that these systems are in the l(d) phase at all CHOL contents for temperatures above 25 degrees C.     

Thermodynamical characteristics and volume compressibility of dipalmitoylphosphatidylcholine liposomes containing bacteriorhodopsin.

The effects of bacteriorhodopsin (BR) interaction with large dipalmitoylphosphatidylcholine (DPPC) liposomes (approx. 100 nm in diameter) were examined at various BR/DPPC ratios, using differential scanning calorimetry (DSC) and ultrasonic velocimetry (USV). On DSC, the lipid phase transition temperature, Tc, and the half-width of the phase transition peak, delta T1/2, showed significant non-monotonic changes with the increasing BR concentration. Two exponential segments could be distinguished in the dependence of the transition enthalpy change per mol of lipid (delta H/nL) on the BR/DPPC ratio: one corresponding to ratios between 0:1 and 1:64, and another corresponding to ratios between 1:44 and 1:16. A maximal value of delta H/nL was observed for BR/DPPC ratio 1:44, probably corresponding to maximal BR-lipid ordering with each BR molecule being surrounded by two layers of lipid molecules. The nonmonotonic changes of thermodynamical parameters suggest long-distance interactions between regions of altered bilayer structure which form around each BR molecule. The results obtained with USV provided support for the above conclusions. The dependence of ultrasound velocity increment A on BR concentration supplies information on relative changes of membrane volume compressibility. Decreasing volume compressibility is reflected in increasing values of parameter A. Within T less than Tc, the values of A increased with the increasing BR concentration; saturation was observed at BR/DPPC ratio 1:500 (A = A(BR/DPPC]. No significant BR-concentration dependent changes of A were observed at T greater than Tc. From these values the average diameter of the distorted region of lipid bilayer was estimated to be approximately 20 nm.     

High-pressure 31P NMR study of dipalmitoylphosphatidylcholine bilayers.

High-pressure 31P NMR was used for the first time to investigate the effects of pressure on the structure and dynamics of the phosphocholine headgroup in pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar aqueous dispersions and in DPPC bilayers containing the positively charged form of the local anesthetic tetracaine (TTC). The 31P chemical shift anisotropies, delta sigma, and the 31P spin-lattice relaxation times, T1, were measured as a function of pressure from 1 bar to 5 kbar at 50 degrees C for both pure DPPC and DPPC/TTC bilayers. This pressure range permitted us to explore the rich phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as gel I (P beta'), gel II (L beta'), gel III, gel IV, gel X, and the interdigitated, Gi, gel phase. For pure DPPC bilayers, pressure had an ordering effect on the phospholipid headgroup within the same phase and induced an interdigitated Gi gel phase which was formed between the gel I (P beta') and gel II (L beta') phases. The 31P spin-lattice relaxation time measurements showed that the main phase transition (LC to gel I) was accompanied by the transition between the fast and slow correlation time regimes. Axially symmetric 31P NMR lineshapes were observed at pressures up to approximately 3 kbar but changed to characteristic axially asymmetric rigid lattice lineshapes at higher pressures (3.1-5.1 kbar).(ABSTRACT TRUNCATED AT 250 WORDS)     

Magnetic alignment and orientational order of dipalmitoylphosphatidylcholine bilayers containing palmitoyllysophosphatidylcholine

Mixed bilayers of 1-palmitoyl-sn-glycero-3-phosphocholine (palmitoyllysophosphatidylcholine; PaLPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (dipalmitoyl phosphatidylcholine; DPPC) have been investigated by 2H-NMR and 31P-NMR spectroscopy. Binary phospholipid mixtures were studied in which the acyl chains of one or the other component were perdeuterated. At temperatures below the main order-disorder phase transition, the mixed PaLPC/DPPC bilayers appear to coexist with PaLPC micelles. The micelles disappear at temperatures above the phase transition, where mixed bilayers in the liquid-crystalline state are formed. The orientational order of the alkyl chains of the PaLPC component is essentially identical to that of the DPPC component in the mixed bilayers, both in the low temperature and liquid-crystalline phases. However, the presence of PaLPC perturbs the segmental ordering of DPPC as compared to the pure system. The order is increased in the low-temperature phase, where effective diffusion of the chains about their long axes occurs, but is decreased in the liquid-crystalline phase compared to pure DPPC bilayers. The mixed liquid-crystalline bilayers orient preferentially with their director axes perpendicular to the magnetic field. This alignment is easily observed in 31P- and 2H-NMR spectra, where the intensity of the perpendicular edges of the lineshapes is pronounced. One possible explanation of the magnetic alignment involves alteration of the curvature free energy of the DPPC bilayer due to incorporation of PaLPC in the mixed membranes.     

The interaction of cholesterol and cholest-4-en-3-one with dipalmitoylphosphatidylcholine. Comparison based on the use of three fluorophores

This study compares cholesterol-phospholipid and cholest-4-en-3-one-phospholipid interactions by their effect on thermotropic behavior of dipalmitoylphosphatidylcholine bilayers. This was approached by determining the temperature-dependent steady-state fluorescence anisotropy of three fluorophores; diphenylhexatriene (DPH), hydroxy-coumarin (HC) and trans-parinaric acid (TPA). The fluorophores monitor different lateral and vertical locations of the lipid bilayers; DPH and HC average laterally the properties of the hydrophobic and headgroup regions of the bilayer, respectively, while TPA distribution is determined by the lateral organization of the bilayer. The data show that the two steroids have similar qualitative but different quantitative effects. Both diminish the pretransition and behave as 'averagers', broadening the main gel to liquid crystalline phase transition through ordering of the acyl chains in the liquid crystalline state and disordering of them in the gel state. However, the mechanisms by which the two molecules operate are different. Cholesterol is more effective particularly on the hydrophobic region of the bilayer, and its effect is not linear with its mole fraction. A sharp increase of the steady-state fluorescence anisotropy occurs around 20 mol% cholesterol. The effect of cholestenone is proportional to its mole fraction. The difference between the effects of the two steroids is explained by the dissimilarity in their lateral distribution. Cholesterol forms cholesterol-rich domains. The size of the boundary regions which surround the cholesterol-rich domains changes drastically at about 20 mol% cholesterol. Cholestenone, on the other hand, is randomly distributed in the bilayer plane and therefore it does not cause the formation of such defined boundary regions. This study as well as reports by others suggests that the important structural differences between the two steroids are the molecular packing parameter and the presence of small polar group at the 3-beta position of the steroid.