Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C(n)=12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.     

Thermotropic and barotropic phase transitions in bilayer membranes of ether-linked phospholipids with varying alkyl chain lengths

The bilayer phase transitions of a series of ether-linked phospholipids, 1,2-dialkylphosphatidylcholines containing linear saturated alkyl chain (C_n = 12, 14, 16 and 18), were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the pre- and main-transitions for the ether-linked PC bilayer membranes were calculated and compared with those of a series of ester-linked PCs, 1,2-diacylphosphatidylcholines. The thermodynamic quantities of the main transition for the ether-linked PC bilayers showed distinct dependence on alkyl-chain length and were slightly different from those of the ester-linked PC bilayers. From the comparison of thermodynamic quantities for the main transition between both PC bilayers, we revealed that the attractive interaction in the gel phase for the ether-linked PC bilayers is weaker than that for the ester-linked PC bilayers. Regarding the pretransition, although changes in enthalpy and entropy for both PC bilayers were comparable to each other, the volume changes of the ether-linked PC bilayers roughly doubled those of the ester-linked PC bilayers. The larger volume change results from the smallest partial molar volume of the ether-linked PC molecule in the interdigitated gel phase. Further, we constructed the temperature-pressure phase diagrams for the ether-linked PC bilayers by using the phase-transition data. The region of the interdigitated gel phase in the phase diagrams was extended by applying pressure and by increasing the alkyl-chain length of the molecule. Comparing the phase diagrams with those for the ester-linked PC bilayers, it was proved that the phase behavior of the ester-linked PC bilayers under high temperature and pressure is almost equivalent to that of the ether-linked PC bilayers in the vicinity of ambient pressure.     

Interbilayer interactions between sphingomyelin and sphingomyelin/cholesterol bilayers.

Pressure versus fluid spacing relations have been obtained for sphingomyelin bilayers in the gel phase and equimolar sphingomyelin/cholesterol in the liquid-crystalline phase by the use of X-ray diffraction analysis of osmotically stressed aqueous dispersions and oriented multilayers. For interbilayer separations in the range of 5-20 A, the repulsive hydration pressure decays exponentially with increasing fluid spacing. The decay length (lambda) of this repulsive pressure is about 2 A for both bovine brain and N-tetracosanoylsphingomyelin, similar to that previously found for phosphatidylcholine bilayers. However, both the magnitude of the hydration pressure and the magnitude of the dipole potential (V) measured for monolayers in equilibrium with liposomes are considerably smaller for sphingomyelin than for either gel or liquid-crystalline phosphatidylcholine bilayers. Addition of equimolar cholesterol increases both the magnitude of the hydration pressure and the dipole potential. These data suggest that the magnitude of the hydration pressure depends on the electric field at the interface as given by (V/lambda)2. For sphingomyelin bilayers, there is a sharp upward break in the pressure-fluid spacing relation at an interbilayer spacing of about 5 A, indicating the onset of steric hindrance between the head groups of apposing bilayers.     

Repulsive interactions between uncharged bilayers. Hydration and fluctuation pressures for monoglycerides.

Pressure versus distance relations have been obtained for solid (gel) and neat (liquid-crystalline) phase uncharged lipid bilayers by the use of x-ray diffraction analysis of osmotically stressed monoglyceride aqueous dispersions and multilayers. For solid phase monoelaidin bilayers, the interbilayer repulsive pressure decays exponentially from a bilayer separation of approximately 7 A at an applied pressure of 3 x 10(7) dyn/cm2 to a separation of approximately 11 A at zero applied pressure, where an excess water phase forms. The decay length is approximately 1.3 A, which is similar to the value previously measured for gel phase phosphatidylcholine bilayers. This implies that the decay length of the hydration pressure does not depend critically on the presence of zwitterionic head groups in the bilayer surface. For liquid-crystalline monocaprylin, the repulsive pressure versus distance curve has two distinct regions. In the first region, for bilayer separations of approximately 3-8 A and applied pressures of 3 x 10(8) to 4 x 10(6) dyn/cm2, the pressure decays exponentially with a decay length of approximately 1.3 A. In the second region, for bilayer separations of approximately 8-22 A and applied pressures of 4 x 10(6) to 1 x 10(5) dyn/cm2, the pressure decays much more gradually and is inversely proportional to the cube of the distance between bilayers. These data imply that two repulsive pressures operate between liquid-crystalline monocaprylin bilayers, the hydration pressure, which dominates at small (3-8 A) bilayer separations, and the fluctuation pressure, which dominates at larger bilayer separations (greater than 8 A) and strongly influences the hydration properties of the liquid-crystalline bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Steric repulsion between phosphatidylcholine bilayers

The change in pressure needed to bring egg phosphatidylcholine bilayers into contact from their equilibrium separation in excess water has been determined as a function of both distance between the bilayers and water content. A distinct upward break in the pressure-distance relation appears at an interbilayer separation of about 5 A, whereas no such deviation is present in the pressure-water content relation. Thus, this break is not a property of the dehydration process per se, but instead is attributed to steric repulsion between the mobile lipid head groups that extend 2-3 A into the fluid space between bilayers. That is, electron density profiles of these bilayers indicate that the observed break in the pressure-spacing relation occurs at a bilayer separation where extended head groups from apposing bilayers come into steric hindrance. The pressure-spacing data are used to separate steric pressure from the repulsive hydration pressure, as well as to quantitate the range and magnitude of the steric interaction. An appreciable fraction of the measured steric energy can be ascribed to a decrease in configurational entropy due to restricted head-group motion as adjacent bilayers come together.     

Range of the solvation pressure between lipid membranes: dependence on the packing density of solvent molecules

Well-ordered multilamellar arrays of liquid-crystalline phosphatidylcholine and equimolar phosphatidylcholine-cholesterol bilayers have been formed in the nonaqueous solvents formamide and 1,3-propanediol. The organization of these bilayers and the interactions between apposing bilayer surfaces have been investigated by X-ray diffraction analysis of liposomes compressed by applied osmotic pressures up to 6 X 10(7) dyn/cm2 (60 atm). The structure of egg phosphatidylcholine (EPC) bilayers in these solvents is quite different than in water, with the bilayer thickness being largest in water, 3 A narrower in formamide, and 6 A narrower in 1,3-propanediol. The incorporation of equimolar cholesterol increases the thickness of EPC bilayers immersed in each solvent, by over 10 A in the case of 1,3-propanediol. The osmotic pressures of various concentrations of the neutral polymer poly(vinylpyrrolidone) dissolved in formamide or 1,3-propanediol have been measured with a custom-built membrane osmometer. These measurements are used to obtain the distance dependence of the repulsive solvation pressure between apposing bilayer surfaces. For each solvent, the solvation pressure decreases exponentially with distance between bilayer surfaces. However, for both EPC and EPC-cholesterol bilayers, the decay length and magnitude of this repulsive pressure strongly depend on the solvent. The decay length for EPC bilayers in water, formamide, and 1,3-propanediol is found to be 1.7, 2.4, and 2.6 A, respectively, whereas the decay length for equimolar EPC-cholesterol bilayers in water, formamide, and 1,3-propanediol is found to be 2.1, 2.9, and 3.1 A, respectively. These data indicate that the decay length is inversely proportional to the cube root of the number of solvent molecules per unit volume.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactions between charged, uncharged, and zwitterionic bilayers containing phosphatidylglycerol.

Pressure vs. distance relationships have been obtained for phosphatidylglycerol bilayers, in both charged and uncharged states. Water was removed from the lipid multilayers by the application of osmotic pressures in the range of 0-2.7 x 10(9) dyn/cm2, and the distance between adjacent bilayers was obtained from Fourier analysis of lamellar x-ray diffraction data. For phosphatidylglycerol bilayers made electrically neutral either by lowering the pH or by adding equimolar concentrations of the positively charged lipid stearylamine, the pressure-distance data could be fit with a single exponential. The measured decay lengths were 1.1 A at low pH and 1.5 A with stearylamine, which are similar to decay lengths of the hydration pressure found for gel phases of other neutral bilayers. In addition, the magnitude of this repulsive pressure was proportional to the square of the Volta potential (equivalent to the dipole potential for electrically neutral bilayers) measured in monolayers in equilibrium with bilayers, in agreement with results previously found for the hydration pressure between phosphatidylcholine bilayers. For charged phosphatidylglycerol bilayers, the pressure-distance relation had two distinct regions. For bilayer separations greater than 10 A, the pressure-distance data had an exponential decay length (11 A) and a magnitude consistent with that expected for electrostatic repulsion from double-layer theory. For bilayer separations of 2-10 A, the pressure decayed much more rapidly with increasing bilayer separation (decay length less than 1 A). We interpret these data at low bilayer separations in terms of a combination of hydration repulsion and steric hindrance between the lipid head groups and the sodium ions trapped between apposing bilayers.     

Range and magnitude of the steric pressure between bilayers containing phospholipids with covalently attached poly(ethylene glycol).

The interactive properties of liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-lipids) are of interest because such liposomes are being developed as drug delivery vehicles and also are ideal model systems for measuring the properties of surface-grafted polymers. For bilayers containing PEG-lipids with PEG molecular weights of 350, 750, 2000, and 5000, pressure-distance relations have been measured by X-ray diffraction analysis of liposomes subjected to known applied osmotic pressures. The distance between apposing bilayers decreased monotonically with increasing applied pressure for each concentration of a given PEG-lipid. Although for bilayers containing PEG-350 and PEG-750 the contribution of electrostatic repulsion to interbilayer interactions was significant, for bilayers containing PEG-2000 and PEG-5000 the major repulsive pressure between bilayers was a steric pressure due to the attached PEG. The range and magnitude of this steric pressure increased both with increasing PEG-lipid concentration and PEG size, and the extension length of the PEG from the bilayer surface at maximum PEG-lipid concentration depended strongly on the size of the PEG, being less than 35 A for PEG-750, and about 65 A for PEG-2000 and 115 A for PEG-5000. The measured pressure-distance relations have been modeled in terms of current theories (deGennes, 1987; Milner et al., 1988b) for the steric pressure produced by surface-grafted polymers, as modified by us to take into account the effects of polymer polydispersity and the possibility that, at low grafting densities, polymers from apposing bilayers surfaces can interpenetrate or interdigitate. No one theoretical scheme is sufficient to account for all the experimental results. However, for a given pressure regime, PEG-lipid size, and PEG-lipid surface density, the appropriately modified theoretical treatment gives a reasonable fit to the pressure-distance data.     

Neutron and X-ray diffraction structural analysis of phosphatidylinositol bilayers

Phosphatidylinositol (PI) bilayers, squeezed together by applied osmotic pressures, were studied by both neutron diffraction and X-ray diffraction. The lamellar repeat period for PI bilayers decreased from 9.5 nm at an applied pressure of 1.1.10(6) dyn/cm2 (1.1 atm) to 5.4 nm at an applied pressure of 1.6.10(7) dyn/cm2 (16 atm). Further increases in applied pressure, up to 2.7.10(9) dyn/cm2 (2700 atm) reduced the repeat period by only about 0.3 nm, to 5.1 nm. Thus, a plot of applied pressure versus repeat period shows a sharp upward break for repeat periods less than about 5.4 nm. For repeat periods of less than 5.4 nm, analysis of neutron-scattering density profiles and electron-density profiles indicates that the structure of the PI bilayers changes as the bilayers are dehydrated, even though there are only small changes in the repeat period. These structural changes are most likely due to removal of water from the headgroup regions of the bilayer. D2O/H2O exchange experiments show that, at an applied pressure of 2.8.10(7) dyn/cm2, water is located between adjacent PI headgroups in the plane of the bilayer. We conclude that, although electrostatics provide the dominant long-range repulsive interaction, hydration repulsion and steric hindrance between PI headgroups from apposing bilayers provide the major barriers for the close approach of adjacent PI bilayers for repeat periods less than 5.4 nm. This structural analysis also indicates that the phosphoinositol group extends from the plane of the bilayer into the fluid space between adjacent bilayers. This extended orientation for the headgroup is consistent with electrophoretic measurements on PI vesicles.     

Lateral pressure profiles in cholesterol–DPPC bilayers

By means of atomistic molecular dynamics simulations, we study cholesterol–DPPC (dipalmitoyl phosphatidylcholine) bilayers of different composition, from pure DPPC bilayers to a 1:1 mixture of DPPC and cholesterol. The lateral pressure profiles through the bilayers are computed and separated into contributions from the different components. We find that the pressure inside the bilayer changes qualitatively for cholesterol concentrations of about 20% or higher. The pressure profile in the inside of the bilayer then turns from a rather flat shape into an alternating sequence of regions with large positive and negative lateral pressure. The changes in the lateral pressure profile are so characteristic that specific interaction between cholesterol and molecules such as membrane proteins mediated solely via the lateral pressure profile might become possible.     

Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations

It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.     

Interaction of saponin and digitonin with black lipid membranes and lipid monolayers

The effects of the plant glycosides saponin as well as digitonin on the electrical conductance of black lipid membranes and the effect of these agents on the surface pressure of lipid monofilms was investigated. Both saponin and digitonin induced channel-like fluctuations in planar bilayers made either of diphytanoylphosphatidylcholine ( DPhPC ) or of DPhPC and cholesterol 2: 1 (w/w). In cholesterol-free bilayers the amount needed to induce an increase in conductance was 0.3-1 mg/ml for saponin and about 0.2 mg/ml for digitonin. In contrast, in cholesterol-containing bilayers the concentration needed to induce pores was about 10 micrograms/ml for both saponin and digitonin. In cholesterol-containing membranes the fluctuating pores induced by saponin were about 3-times more permeable to K+ than to Cl- and the macroscopic current showed an ohmic behaviour. Surface pressure experiments demonstrate that both glycosides could penetrate into lipid monofilms of pure DPhPC spread at the air/water interface with an initial surface pressure of 30 mN/m. The increase in surface pressure was considerably enhanced in cholesterol-containing films. It is assumed that the channel-like fluctuations induced by saponin as well as digitonin, in both cholesterol-free and cholesterol-rich bilayers are due to the formation of micellar structures within the lipid lattice. Probably the penetration of the glycosides into the lipid bilayer is considerably enhanced by the presence of cholesterol.     

Cholesterol modifies the short-range repulsive interactions between phosphatidylcholine membranes

Pressure versus distance relationships have been obtained for egg phosphatidylcholine bilayers containing a range of cholesterol concentrations. Water was removed from between adjacent bilayers by the application of osmotic pressures in the range of 0.4-2600 atm (4 x 10(5)-2.6 x 10(9) dyn/cm2), and the distance between adjacent bilayers was obtained by Fourier analysis of X-ray diffraction data. For applied pressures up to about 50 atm and bilayer surface separations of 15-5 A, the incorporation of up to equimolar cholesterol has little influence on plots of pressure versus bilayer separation. However, for the higher applied pressures, cholesterol reduces the interbilayer separation distance by an amount that depends on the cholesterol concentration in the bilayer. For example, the incorporation of equimolar cholesterol reduces the distance between bilayers by as much as 6 A at an applied pressure of 2600 atm. At this applied pressure, electron density profiles show that the high-density head-group peaks from apposing bilayers have merged. This indicates that equimolar concentrations of cholesterol spread the lipid molecules apart in the plane of the bilayer enough to allow the phosphatidylcholine head groups from apposing bilayers to interpenetrate as the bilayers are squeezed together. All of these X-ray and pressure-distance data indicate that, by reducing the volume fraction of phospholipid head groups, cholesterol markedly reduces the steric repulsion between apposing bilayers but has a much smaller effect on the sum of the longer ranged repulsive hydration and fluctuation pressures. Increasing concentrations of cholesterol monotonically increase the dipole potential of egg phosphatidylcholine monolayers, from 415 mV with no cholesterol to 493 mV with equimolar cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adhesion between cerebroside bilayers

The structure, hydration properties, and adhesion energy of the membrane glycolipid galactosylceramide (GalCer) were studied by osmotic stress/X-ray diffraction analysis.(1) Fully hydrated GalCer gave a repeat period of 67 A, which decreased less than 2 A with application of applied osmotic pressures as large as 1.6 x 10(9) dyn/cm(2). These results, along with the invariance of GalCer structure obtained by a Fourier analysis of the X-ray data, indicated that there was an extremely narrow fluid space (less than the diameter of a single water molecule) between fully hydrated cerebroside bilayers. Electron density profiles showed that the hydrocarbon chains from apposing GalCer monolayers partially interdigitated in the center of the bilayer. To obtain information on the adhesive properties of GalCer bilayers, we incorporated into the bilayer various mole ratios of the negatively charged lipid dipalmitoylphosphatidylglycerol (DPPG) to provide known electrostatic repulsion between the bilayers. Although 17 and 20 mol % DPPG swelled (disjoined) the GalCer bilayers by an amount predictable from electrostatic double-layer theory, 5, 10, 13, and 15 mol % DPPG did not disjoin the bilayers. By calculating the magnitude of the electrostatic pressure necessary to disjoin the bilayers, we estimated the adhesion energy for GalCer bilayers to be about -1.5 erg/cm(2), a much larger value than that previously measured for phosphatidylcholine bilayers. The observed discontinuous disjoining with increased electrostatic pressure and this relatively large value for adhesion energy indicated the presence of an attractive interaction, in addition to van der Waals attraction, between cerebroside bilayers. Possible attractive interactions are hydrogen bond formation and hydrophobic interactions between the galactose headgroups of apposing GalCer bilayers.     

Hydration force and bilayer deformation: a reevaluation

The hydration repulsive force between lipid bilayers and the deformability of both gel and liquid-crystalline bilayers have been quantitated by an X-ray diffraction analysis of osmotically stressed liposomes. Both sampling theorem reconstructions and electron density distributions were calculated from diffraction data obtained from multilayers with applied osmotic pressures of 0-50 atm. The bilayer thickness and area per lipid molecule remain nearly constant (to within about 4%) in this pressure range, as adjacent bilayers move from their equilibrium separation in excess water to within 2-4 A of each other. This analysis indicates that the bilayers are relatively incompressible. This results differs from previously published X-ray diffraction studies of bilayer compressibility but agrees with direct mechanical measurements of the bilayer compressibility modulus. It is also found that the hydration repulsive force decays exponentially with separation between bilayers with a decay constant of 1.4 A for gel-state dipalmitoylphosphatidylcholine and 1.7 A for liquid-crystalline egg phosphatidylcholine bilayers. This implies that the exponential decay constant is not necessarily equal to the diameter of a water molecule, as has been previously suggested on experimental and theoretical grounds.     

High-pressure infrared study of phosphatidylserine bilayers and their interactions with the local anesthetic tetracaine

High-pressure Fourier-transform infrared (FT-IR) spectroscopy was used to study the barotropic behavior of phosphatidylserine bilayers and their interactions with the local anesthetic tetracaine. The model membrane systems studied were multilamellar aqueous dispersions of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) in the absence and the presence of tetracaine at pH 5.5 and 9.5. The infrared spectra were measured at 28 degrees C in a diamond anvil cell as a function of pressure up to 25 kbar. The results show that the barotropic behavior of the negatively charged phosphatidylserine bilayers is very similar to that observed for zwitterionic phospholipids, such as phosphatidylcholine and phosphatidylethanolamine, with corresponding acyl chains. The results also indicate that the local anesthetic partitions into phosphatidylserine bilayers in an environment close to the membrane-water interface and interacts electrostatically with the lipid head group. Application of high hydrostatic pressure on the lipid-anesthetic systems results in the pressure-induced expulsion of the anesthetic from a membrane to an aqueous environment. The pressures required for expulsion of anesthetic from bilayers are much higher for the unsaturated lipid (DOPS) than for the saturated lipid (DMPS) (approximately 6 kbar vs approximately 2 kbar, respectively). Whereas incorporation of the anesthetic into DOPS bilayers does not affect significantly the structural and dynamic properties of the disordered acyl chains in the liquid-crystalline phase, it orders the DMPS acyl chains in the gel phase.     

The effect of pressure on the phase diagram of mixed dipalmitoyl-dymyristoylphosphatidylcholine bilayers

The application of 136 atm of helium pressure to suspensions of mixed dipalmitoyl-dimyristoylphosphatidylcholine bilayers caused a 3–5°C elevation of points on the envelope of the binary phase diagram.Membrane bilayers containing lateral phase separations are able to respond to external pressure by converting fluid phase phospholipids to the more compact gel phase.     

Lipid bilayer pressure profiles and mechanosensitive channel gating

The function of membrane proteins often depends on the proteins' interaction with their lipid environment, spectacularly so in the case of mechanosensitive channels, which are gated through tension mediated by the surrounding lipids. Lipid bilayer tension is distributed quite inhomogeneously, but neither the scale at which relevant variation takes place nor the effect of varying lipid composition or tension has yet been investigated in atomic detail. We calculated lateral pressure profile distributions in lipid bilayers of various composition from all-atom molecular dynamics simulations totaling 110.5 ns in length. Reproducible pressure profile features at the 1 A length scale were determined. Lipids with phosphatidylcholine headgroups were found to shift the lateral pressure out of the hydrophobic core and into the headgroup region by an amount that is independent of area per lipid. POPE bilayers simulated at areas smaller than optimal exerted dramatically higher lateral pressure in a narrow region at the start of the aliphatic chain. Stretching of POPC bilayers increased tension predominantly in the same region. A simple geometric analysis for the gating of the mechanosensitive channel MscL suggests that pressure profiles affect its gating through the second moment of the profile in a tension-independent manner.     

Effects of hydrostatic pressure on the location of PRODAN in lipid bilayers: a FT-IR study

The effects of hydrostatic pressure on the location of 6-propionyl-2-(dimethylamino)naphthalene (PRODAN), an environmentally sensitive fluorescent probe, in phosphatidylcholine lipid bilayers have been studied by Fourier-transform infrared spectroscopy (FT-IR) over the pressure range of 0.001-25 kbar. The results derived from the PRODAN C = O stretching band, the correlation field splitting of the methylene scissoring mode, and the methylene symmetric stretching mode as well as the absorption of the naphthalene ring show that in the sample of 4% (w/w) PRODAN in dimyristoyl-L-alpha-phosphatidylcholine (DMPC) at pH 6.8, most of the PRODAN molecules are embedded in the bilayers. In contrast, at pH 3.0, PRODAN was found to reside either on the membrane surface or dispersed in water. Compared to DMPC, egg yolk phosphatidylcholine (egg PC), which contains a substantial amount of unsaturated fatty acyl chains, is more susceptible to PRODAN permeation. The present study shows that the pressure dependence of the location of PRODAN in lipid membranes is different from that of tetracaine, a local anesthetic, in lipid bilayers. The model regarding the PRODAN location in lipid bilayers derived from the present infrared data has been compared with that obtained with previous fluorescence studies.     

A high-pressure, infrared spectroscopic study of the solvation of bilirubin in lipid bilayers

The location of bilirubin IXa in lipid bilayers of dimyristoylphosphatidylcholine or dioleoylphosphatidylcholine was studied by determining the effects of bilirubin on the infrared spectra of the lipids as a function of pressure. It was found for both bilayers that bilirubin intercalated into the polymethylene chain region of the bilayer, being located between the carbonyl region and the methylene group two carbons from the methyl terminus. Small amounts of bilirubin interacted with the choline region of dioleoylphosphatidylcholine. Lesser amounts interacted with the choline region of dimyristoylphosphatidylcholine. This difference between the two types of bilayers was attributed to the degradation of small amounts of bilirubin IXa to more polar isomers in the presence of dioleoylphosphatidylcholine. In dioleoyl- but not dimyristoylphosphatidylcholine, bilirubin interacted with the C = O region, probably indicating that bilirubin in the latter type of bilayer was intercalated into the polymethylene chains above and below the double bond. Bilirubin decreased the pressure required for the liquid-crystal to gel-phase transition in both bilayers at 28 degrees C. Bilirubin was not forced out of either bilayer at pressures as high as 20 kbar.