Thermotropic phase behavior of mixed-chain phosphatidylglycerols: implications for acyl chain packing in fully hydrated bilayers

In this communication we report the first systematic investigation of the thermodynamic properties of fully hydrated mixed-chain phosphatidylglycerols (PG) using high-resolution differential scanning calorimetry (DSC). The crystal structure of dimyristoylphosphatidylglycerol shows an acyl chain conformation that is nearly opposite to that of phosphatidylcholine (PC). In PC, the sn-1 chain is straight while the sn-2 chain contains a bend; for PG, the sn-1 contains a bend while the sn-2 chain is in the all-trans conformation (R.H. Pearson, I. Pascher, The molecular structure of lecithin dihydrate, Nature, 281 (1978) 499-501; I. Pascher, S. Sundell, K. Harlos, H. Eibl, Conformational and packing properties of membrane lipids: the crystal structure of sodium dimyristoylphosphatidylglycerol, Biochim. Biophys. Acta, 896 (1987) 77-88). If the structure of PG found in the single crystal can be extrapolated to that in the fully hydrated gel-state bilayer, the observed difference in acyl chain conformations implies that modulation of the acyl chain asymmetry will have an opposite effect on the thermotropic phase behavior of PG and PC. For example, it is expected, based on the crystal structures, that C(15):C(13)PG should have a higher main phase transition temperature (Tm) than C(14):C(14)PG, and C(13):C(15)PG should have a lower Tm than C(14):C(14)PG. However, our DSC studies show clearly that the expectation is not borne out by experimental data. Rather, the Tm values of C(15):C(13)PG, C(14):C(14)PG, and C(13):C(15)PG are 18.2 degrees C, 23.1 degrees C, and 24.4 degrees C, respectively. Several other PGs, each with a unique acyl chain composition, have also been studied in this laboratory using high-resolution DSC. It is shown that the acyl chain conformation of fully hydrated PG in general is nearly opposite to that seen in the PG crystal structure.     

Hydrocarbon chain packing and the effect of ethanol on the thermotropic phase behavior of mixed-chain phosphatidylglycerols

Previous studies in this laboratory have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines and the effect of ethanol concentration ([EtOH]) on their melting behavior (Li et al., Biophys J., 70 (1996) 2784-2794). This present investigation extends these findings to another phospholipid family by using high-resolution differential scanning calorimetry (DSC) to characterize the effect of ethanol concentration on the main phase transition temperature (Tm) of five molecular species of mixed-chain phosphatidylglycerol (PG). For C(14):C(18)PG, C(15):C(17)PG, C(16):C(16)PG, and C(17):C(15)PG, a biphasic profile in the Tm versus [EtOH] plot was observed, and the minimum in the plot for each PG occurred at 33, 15, 19, and 36 mg/ml, respectively. This biphasic behavior is typical of phospholipids whose acyl chain asymmetry is fairly small. For C(18):C(14)PG, only a linear decrease in the Tm was observed as a function of ethanol concentration; this effect is characteristic of highly asymmetric phospholipids. Our DSC results obtained with mixed-chain PG in the presence of ethanol demonstrate that the acyl chain asymmetry of the five lipids studied can be ranked as follows: C(15):C(17)PG相似文献   

The gel phase of dipalmitoyl phosphatidylcholine. An infrared characterization of the acyl chain packing

It is shown, by infrared spectroscopy, that the packing in the gel phase of fully-hydrated dipalmitoyl phosphatidylcholine is not uniform over a large temperature range. With decreasing temperature, starting at that of the pretransition, there is a gradual change in the molecular packing of the acyl chains, from near hexagonal to orthorhombic of monoclinic.     

Sterol structure and sphingomyelin acyl chain length modulate lateral packing elasticity and detergent solubility in model membranes

Membrane microdomains, such as caveolae and rafts, are enriched in cholesterol and sphingomyelin, display liquid-ordered phase properties, and putatively function as protein organizing platforms. The goal of this investigation was to identify sterol and sphingomyelin structural features that modulate surface compression and solubilization by detergent because liquid-ordered phase displays low lateral elasticity and resists solubilization by Triton X-100. Compared to cholesterol, sterol structural changes involved either altering the polar headgroup (e.g., 6-ketocholestanol) or eliminating the isooctyl hydrocarbon tail (e.g., 5-androsten-3beta-ol). Synthetic changes to sphingomyelin resulted in homogeneous acyl chains of differing length but of biological relevance. Using a Langmuir surface balance, surface compressional moduli were assessed at various surface pressures including those (pi > or =30 mN/m) that mimic biomembrane conditions. Sphingomyelin-sterol mixtures generally were less elastic in a lateral sense than chain-matched phosphatidylcholine-sterol mixtures at equivalent high sterol mole fractions. Increasing content of 6-ketocholestanol or 5-androsten-3beta-ol in sphingomyelin decreased lateral elasticity but much less effectively than cholesterol. Our results indicate that cholesterol is ideally structured for maximally reducing the lateral elasticity of membrane sphingolipids, for enabling resistance to Triton X-100 solubilization, and for interacting with sphingomyelins that contain saturated acyl chains similar in length to their sphingoid bases.     

Polymer chain packing analysis using molecular modeling

This article describes the methodology used in applying molecular modeling to investigate the chain packing of polymers. Models for polyethylene and Uans-polyacetylene were developed in order to study the cell packing energy as a function of the chain setting angles. This approach was found to yield chain setting angle values that corresponded to those determined experimentally by other authors. The limitations of the method are also discussed.     

Transbilayer diffusion of phospholipids: dependence on headgroup structure and acyl chain length

The kinetics and thermodynamics of the transmembrane movement (flip-flop) of fluorescent analogs of phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylcholine (PC), and phosphatidylethanolamine (PE) were investigated to determine the contributions of headgroup composition and acyl chain length to phospholipid flip-flop. The phospholipid derivatives containing n-octanoic, n-decanoic or n-dodecanoic acid in the sn-1 position and 9-(1-pyrenyl)nonanoic acid in the sn-2 position were incorporated at 3 mol% into sonicated single-bilayer vesicles of 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (POPC). The kinetics of diffusion of the pyrene-labeled phospholipids from the outer and inner monolayers of the host vesicles to a large pool of POPC acceptor vesicles were monitored by the time-dependent decrease of pyrene excimer fluorescence. The observed kinetics of transfer were biexponential, with a fast component due to the spontaneous transfer of pyrenyl phospholipids in the outer monolayer of labeled vesicles and a slower component due to diffusion of pyrenyl phospholipid from the inner monolayer of the same vesicles. Intervesicular transfer rates decreased approx. 8-fold for every two carbons added to the first acyl chain. Correspondingly, the free energy of activation for transfer increased approx. 1.3 kcal/mol. With the exception of PE, the intervesicular transfer rates for the different headgroups within a homologous series were nearly the same, with the PC derivative being the fastest. Transfer rates for the PE derivatives were 5-to 7-fold slower than the rates observed for PC. Phospholipid flip-flop, in contrast, was strongly dependent on headgroup composition with a smaller dependence on acyl chain length. At pH 7.4, flip-flop rates increased in the order PC less than PG less than PA less than PE, where the rates for PE were at least 10-times greater than those of the homologous PC derivative. Activation energies for flip-flop were large, and ranged from 38 kcal/mol for the longest acyl chain derivative of PC to 25 kcal/mol for the PE derivatives. Titration of the PA headgroup at pH 4.0 produced an approx. 500-fold increase in the flip-flop rate of PA, while the activation energy decreased 10 kcal/mol. Increasing acyl chain length reduced phospholipid flip-flop rates, with the greatest change observed for the PC analogs, which exhibited an approx. 2-fold decrease in flip-flop rate for every two methylene carbons added to the acyl chain at the sn-1 position.(ABSTRACT TRUNCATED AT 400 WORDS)     

The influence of headgroup structure and fatty acyl chain saturation of phospholipids on monolayer behavior: a comparative rheological study

This paper compares six phospholipidic monolayers at the water/chloroform interface by performing dilational rheological measurements with a drop tensiometer apparatus. The chosen lipids differ both in their headgroup structure and fatty acyl chain saturation or symmetry. The study concentrated on monolayers formed with DPPC, DPPE, DOPC, DOPE, POPC and POPE. Using a generalized Maxwell rheological model, transposed at the interface, the intimate intermolecular interactions between amphiphilic molecules are studied on and off the monolayer plane. The equilibrium and nonequilibrium phenomena are analyzed and, respectively, correlated with monolayer cohesion and with monolayer/sub-surface interactions. The purpose of this work is to gain further insights into the influences (as slight as they are) of the weak changes in phospholipid structure and on the behavior of the monolayers. The results, widely described, provide further details on nuances existing between very similar molecules, and likewise, on the synergies created between the different effects.     

Sphingomyelin interfacial behavior: the impact of changing acyl chain composition

Sphingomyelins (SMs) containing homogeneous acyl chains with 12, 14, 16, 18, 24, or 26 carbons were synthesized and characterized using an automated Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at constant temperatures between 10 degrees C and 30 degrees C. SM containing lauroyl (12:0) acyl chains displayed only liquid-expanded behavior. Increasing the length of the saturated acyl chain (e.g., 14:0, 16:0, or 18:0) resulted in liquid-expanded to condensed two-dimensional phase transitions at many temperatures in the 10-30 degrees C range. Similar behavior was observed for SMs with lignoceroyl (24:0) or (cerotoyl) 26:0 acyl chains, but isotherms showed only condensed behavior at 10 and 15 degrees C. Insights into the physico-mechanical in-plane interactions occurring within the different SM phases and accompanying changes in SM phase state were provided by analyzing the interfacial area compressibility moduli. At similar surface pressures, SM fluid phases were less compressible than those of phosphatidylcholines with similar chain structures. The area per molecule and compressibility of SM condensed phases depended upon the length of the saturated acyl chain and upon spreading temperature. Spreading of SMs with very long saturated acyl chains at temperatures 30-35 degrees below T(m) resulted in condensed films with lower in-plane compressibilities, but consistently larger cross-sectional molecular areas than the condensed phases achieved by spreading at temperatures only 10-20 degrees below T(m). This behavior is discussed in terms of the enhancement of SM lateral aggregation by temperature reduction, a common approach used during domain isolation from biomembranes.     

Effect of ceramide acyl chain length on skin permeability and thermotropic phase behavior of model stratum corneum lipid membranes

Stratum corneum ceramides play an essential role in the barrier properties of skin. However, their structure-activity relationships are poorly understood. We investigated the effects of acyl chain length in the non-hydroxy acyl sphingosine type (NS) ceramides on the skin permeability and their thermotropic phase behavior. Neither the long- to medium-chain ceramides (8-24 C) nor free sphingosine produced any changes of the skin barrier function. In contrast, the short-chain ceramides decreased skin electrical impedance and increased skin permeability for two marker drugs, theophylline and indomethacin, with maxima in the 4-6C acyl ceramides. The thermotropic phase behavior of pure ceramides and model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesterol sulfate was studied by differential scanning calorimetry and infrared spectroscopy. Differences in thermotropic phase behavior of these lipids were found: those ceramides that had the greatest impact on the skin barrier properties displayed the lowest phase transitions and formed the least dense model stratum corneum lipid membranes at 32°C. In conclusion, the long hydrophobic chains in the NS-type ceramides are essential for maintaining the skin barrier function. However, this ability is not shared by their short-chain counterparts despite their having the same polar head structure and hydrogen bonding ability.     

Effect of acyl chain unsaturation on the packing of model diacylglycerols in simulated monolayers.

In a companion study of the effects of acyl chain unsaturation on a series of model sn-1,2-diacylglycerols (DGs) we showed that individual DGs could adopt one of three energy-minimized conformations depending on the number and location of cis double bonds in the sn-2 chain. Here we show that each of these conformations promotes a distinct type of packing arrangement in a simulated DG monolayer. One conformation, shown by sn-1-18:0 DGs containing an sn-2 22:6(n-3)-, 20:4(n-6)-, or 20:3(n-9)- group, determines a regular packing that resembles a known hybrid subcell, HS2, of crystalline hydrocarbon chains. The second conformation, shown by DGs containing an sn-2 18:0-, 18:2(n-6)-, or 18:3(n-3)- group, determines a regular packing that resembles a second known, distinct hydrocarbon subcell, HS1. The third conformation, that of 18:0/18:1(n-9) DG, determines a much looser, less energetically favorable packing. Stable heterogeneous packings are possible for DGs that have similar conformations, but mixed packings of DGs that have dissimilar conformations are less stable. These results raise the possibility that differences in sn-2 acyl chain unsaturation among membrane sn-1,2-diacylglycerophospholipids may promote the formation of different domains.     

Differential scanning calorimetric study of the effect of sterol side chain length and structure on dipalmitoylphosphatidylcholine thermotropic phase behavior.

We have investigated the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers containing a series of cholesterol analogues varying in the length and structure of their alkyl side chains. We find that upon the incorporation of up to approximately 25 mol % of any of the side chain analogues, the DPPC main transition endotherm consists of superimposed sharp and broad components representing the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. Moreover, the behavior of these components is dependent on sterol side chain length. Specifically, for all sterol/DPPC mixtures, the sharp component enthalpy decreases linearly to zero by 25 mol % sterol while the cooperativity is only moderately reduced from that observed in the pure phospholipid. In addition, the sharp component transition temperature decreases for all sterol/DPPC mixtures; however, the magnitude of the decrease is dependent on the sterol side chain length. With respect to the broad component, the enthalpy initially increases to a maximum around 25 mol % sterol, thereafter decreasing toward zero by 50 mol % sterol with the exception of the sterols with very short alkyl side chains. Both the transition temperature and cooperativity of the broad component clearly exhibit alkyl chain length-dependent effects, with both the transition temperature and cooperativity decreasing more dramatically for sterols with progressively shorter side chains. We ascribe the chain length-dependent effects on transition temperature and cooperativity to the hydrophobic mismatch between the sterol and the host DPPC bilayer (see McMullen, T. P. W., Lewis, R. N. A. H., and McElhaney, R. N. (1993) Biochemistry 32:516-522).(ABSTRACT TRUNCATED AT 250 WORDS)     

The enthalpy of acyl chain packing and the apparent water-accessible apolar surface area of phospholipids

The energetics of phospholipid aggregation depend on the apparent water-accessible apolar surface area (ASAap), ordering effects of the chains, and headgroup interactions. We quantify the enthalpy and entropy of these interactions separately. For that purpose, the thermodynamics of micelle formation of lysophosphatidylcholines (LPCs, chains C10, C12, C14, and C16) and diacylphosphatidylcholines (DAPCs, chains C5, C6) and C7) are studied using isothermal titration calorimetry. The critical micelle concentration (CMC) values are 90, 15, and 1.9 mM (C5-C7-DAPC) and 6.8, 0.71, 0.045, and 0.005 mM (LPCs). The group contributions per methylene of DeltaDeltaG(0) = -3.1 kJ/mol and DeltaDeltaC(P) = -57 J/(mol. K) for LPCs agree with literature data on hydrocarbons and amphiphiles. An apparent deviation of DAPCs (-2.5 kJ/mol, 45 J/(mol. K)) is due to an intramolecular interaction between the two chains, burying 20% of the surface. The chain/chain interaction enthalpies in a micelle core are by approximately -2 kJ/(mol) per methylene group more favorable than in bulk hydrocarbons. We conclude that the impact of the chain conformation and packing on the interaction enthalpy is very pronounced. It serves to explain a variety of effects reported on membrane binding. Interactions within the water-accessible region show considerable DeltaH, but almost no DeltaG(0). The heat capacity changes suggest about three methylene groups (ASAap approximately 100 A2) per LPC remain exposed to water in a micelle (DAPC: 2 CH2/70 A2).     

Glycosphingolipids: 2H NMR study of the influence of carbohydrate headgroup structure on ceramide acyl chain behavior in glycolipid-phospholipid bilayers

Galactosyl- and glucosylceramide, globoside, and dihydrolactosylceramide, bearing [2,2-2H2]stearic acid, have been studied at a concentration of 10 mol% in bilayers of dimyristoylphosphatidylcholine by 2H NMR. The quadrupolar splitting delta vQ of the C2 deuterons were measured at several temperatures in the range of 30-60 degrees C. Spin-lattice relaxation times T1 of C2 deuterons were determined in the same temperature range for all lipids but globoside. T1 values at 30 and 50 degrees C were unexpectedly short (6-8 ms), indicating reduced mobility of the ceramide acyl chains compared to that of the host phospholipid. At all temperatures, both delta vQ and T1 were essentially identical for the monoglycosylated species, GalCer and GlcCer, indicating that the order and dynamics of the upper portion of the fatty acyl chain are insensitive to this small change in the headgroup structure. In the case of globoside, where the glycolipid headgroup is equivalent to that of GlcCer extended by three sugar residues, values for the quadrupolar splittings associated with the acyl chain C2-position were very close to those obtained for Gal- and GlcCer. In contrast, the delta vQ values obtained for the diglycosyl species, LacCer, were significantly different at all temperatures. This different behavior of LacCer relative to that of the other glycolipids most likely originates from an orientational change of the acyl chain at the C2-position due to the absence of a 4,5 double bond in dihydrosphingosine. T1 values for the GlcCer and GalCer systems increased with temperature, indicating that the motions responsible for relaxation were in the short correlation time regime.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydrocarbon chain packing modes in lipids: effect of altered sub-cell dimensions and chain rotation

The lateral hydrocarbon chain packing modes of lipids have been described in terms of specific hydrocarbon sub-cells as deduced from single crystal structural studies. To understand the changes in hydrocarbon chain packing in lipid bilayers induced by variations in temperature, hydration, ion-binding, etc., we have examined the effect on the calculated X-ray diffraction pattern of (a) systematic variations in the dimensions of the hydrocarbon sub-cell and (b) the effect of chain rotation at fixed lattice sites. For the O perpendicular (orthorhombic) sub-cell, the a and b sub-cell parameters were varied from as = 4.96 to 4.85 A and bs = 7.42 to 8.40 A in six steps and the positions (s = 2 sin theta/lambda) and intensities (Icalc = F2) of the strong sub-cell reflections calculated. In this way, the conversion of the O perpendicular sub-cell (with either fixed chain orientations or simulated chain rotation) to the hexagonal (H) sub-cell (with chain rotation) was followed. Notably, the two strong reflections characteristic of the O perpendicular sub-cell at 4.12 A (110) and 3.71 A (020) show progressive shifts in position and intensity, finally merging to give the strong (O1O) reflection at 4.2 A characteristic of the hexagonal sub-cell. Similar calculations were performed for the orthorhombic (O' perpendicular) and monoclinic (M parallel) sub-cells. This approach can be used to analyze changes in the X-ray diffraction data due to modifications of the hydrocarbon chain packing modes characteristic of simple and complex lipids.     

Effect of acyl chain structure and bilayer phase state on binding and penetration of a supported lipid bilayer by HPA3

The effect of acyl chain structure and bilayer phase state on binding and penetration by the peptide HPA3 was studied using dual polarisation interferometry. This peptide is an analogue of Hp(2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1 (RpL1) which has been shown to have antimicrobial and cell-penetrating properties. The binding of HPA3 to zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitolyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and negatively charged membranes composed of DMPC and 1,2-dimyristoyl-sn-glycero-3-(phosphor-rac-(1-glycerol)) (DMPG) or POPC and 1-palmitolyl-2-oleyl-sn-glycero-3-(phosphor-rac-(1-glycerol)) (POPG) was determined using dual polarisation interferometry (DPI). Mass and birefringence were measured in real time, enabling the creation of birefringence–mass plots for detailed analysis of the changes in lipid bilayer order during the peptide-binding process. HPA3 bound to all four lipids and the binding progressed as a single phase for the saturated gel phase bilayers DMPC and DMPC–DMPG. However, the binding process involved two or more phases, with penetration of the unsaturated fluid phase POPC and POPC–POPG bilayers. Structural changes in the saturated bilayer were partially reversible whereas binding to the unsaturated bilayer resulted in irreversible changes in membrane structure. These results demonstrate that more disordered unsaturated bilayers are more susceptible to further disorganisation and have a lower capacity to recover from peptide-induced structural changes than saturated ordered bilayers. In addition, this study further establishes DPI as powerful tool for analysis of multiphase peptide-insertion processes associated with complex structural changes in the liquid-crystalline membrane.     

Effects of phospholipid acyl chain structure on physical properties: I. Isobranched phosphatidylcholines

All of the isobranched fatty acids of 12 to 18 carbons have been synthesized in gram quantities by a convenient acetylene coupling reaction followed by catalytic hydrogenation. The corresponding phosphatidylcholines (PCs) have been synthesized and their thermotropic phase behavior investigated by differential thermal analysis. The isobranched acyl phosphatidylcholines show gel-to-liquid-crystalline phase transition temperature (T_cs) some 20°C below those of the corresponding straight-chain PCs and appear to exhibit two slowly interconverting low-temperature phases below T_c. The observed strong alternation of T_cs between isobranched PCs with odd- and even-carbon number acyl chains contrasts with the behavior of the straight-chain PCs and suggests that the acyl chains of the branched-chain PCs are strongly tilted with respect to the bilayer normal below and/or above T_c while those of the straight-chain PCs are not. These results clearly indicate significant differences in the overall packing of branched-and straight-chain PCs in the gel and possibly the liquid-crystalline state.     

The dependence of lipid asymmetry upon phosphatidylcholine acyl chain structure

Lipid asymmetry, the difference in inner and outer leaflet lipid composition, is an important feature of biomembranes. By utilizing our recently developed MβCD-catalyzed exchange method, the effect of lipid acyl chain structure upon the ability to form asymmetric membranes was investigated. Using this approach, SM was efficiently introduced into the outer leaflet of vesicles containing various phosphatidylcholines (PC), but whether the resulting vesicles were asymmetric (SM outside/PC inside) depended upon PC acyl chain structure. Vesicles exhibited asymmetry using PC with two monounsaturated chains of >14 carbons; PC with one saturated and one unsaturated chain; and PC with phytanoyl chains. Vesicles were most weakly asymmetric using PC with two 14 carbon monounsaturated chains or with two polyunsaturated chains. To define the origin of this behavior, transverse diffusion (flip-flop) of lipids in vesicles containing various PCs was compared. A correlation between asymmetry and transverse diffusion was observed, with slower transverse diffusion in vesicles containing PCs that supported lipid asymmetry. Thus, asymmetric vesicles can be prepared using a wide range of acyl chain structures, but fast transverse diffusion destroys lipid asymmetry. These properties may constrain acyl chain structure in asymmetric natural membranes to avoid short or overly polyunsaturated acyl chains.     

Lactosylceramide molecular species specificity of rat liver CMP-N-acetylneuraminate:lactosylceramide sialyltransferase

Six naturally occurring and three synthetic molecular species of lactosylceramide (LacCer) were used to examine the molecular species specificity of CMP-N-acetylneuraminate:lactosylceramide alpha 2,3-sialyltransferase in a Golgi-rich fraction of rat liver. The enzyme molecular species specificity was determined either in the presence of nonspecific lipid transfer protein or in the presence of detergents. Assays performed in the presence of transfer protein showed that for those lactosylceramide molecular species with either d18:1 or d18:0 long chain base the enzyme activity decreased linearly as the effective carbon number of the fatty acid increased. An increase in the carbon number of the long chain base decreased the activity of the enzyme twice as much as a corresponding increase in the carbon number of the fatty acid. On the other hand, when the enzyme activity was assayed in the presence of detergents, there was no significant difference in activity among the various molecular species of lactosylceramide based upon the carbon number of the fatty acid or on the presence of a double bond in the long chain base. However, the decrease in enzyme activity with an increase in the carbon number of the long chain base persisted. These results demonstrate that sialyltransferase has binding specificity with respect to the long chain base, but not the fatty acid. The apparent molecular species towards the fatty acid is related to the aqueous solubility of the various LacCer molecular species.     

Effect of ceramide structure on membrane biophysical properties: the role of acyl chain length and unsaturation

Ceramide is an important bioactive sphingolipid involved in a variety of biological processes. The mechanisms by which ceramide regulates biological events are not fully understood, but may involve alterations in the biophysical properties of membranes. We now examine the properties of ceramide with different acyl chains including long chain (C16- and C18-), very long chain (C24-) and unsaturated (C18:1- and C24:1-) ceramides, in phosphatidylcholine model membranes. Our results show that i) saturated ceramides have a stronger impact on the fluid membrane, increasing its order and promoting gel/fluid phase separation, while their unsaturated counterparts have a lower (C24:1-) or no (C18:1-) ability to form gel domains at 37°C; ii) differences between saturated species are smaller and are mainly related to the morphology and size of the gel domains, and iii) very long chain ceramides form tubular structures likely due to their ability to form interdigitated phases. These results suggest that generation of different ceramide species in cell membranes has a distinct biophysical impact with acyl chain saturation dictating membrane lateral organization, and chain asymmetry governing interdigitation and membrane morphology.     

Dependence of lipid chain and head group packing of the inverted hexagonal phase on hydration

A model which positions the hydrophobic/hydrophilic boundary in phosphatidylethanolamine lipids at the first CH₂ group in the acyl or alkyl chain is used to calculate the surface area per lipid, the mean chain and head-group dimensions and diameters of the hydrophilic tubes of the inverted hexagonal phase of didodecylphosphatidylethanolamine. The calculated surface areas compare favorably with areas obtained for the lamellar liquid crystal phase of the same lipid using the same boundary. Placement of the boundary within the lipid structure permits a determination of the maximum headgroup packing at hydration levels down to complete dehydration. The headgroup dimensions are consistent with a 5 Å diam void at the center of a hydrophilic tube at zero hydration. The calculated mean fluid chain length is ~2 Å smaller than the mean chain length of the lamellar phase at comparable levels of hydration. Comparison of the calculated mean fluid chain length and distances between hydrophobic boundaries shows that the fluid chains are interdigitated between adjacent tubes, and not interdigitated in the central space between three tubes. At low hydration the chains interdigitate in both spaces. The number of lipids packed around a tube at low hydration is only a function of the headgroup geometry, whereas at high hydration, it is a function of the number of carbon atoms in the chains.