Permeability and morphology of low temperature phases in bilayers of single and of mixtures of phosphatidylcholines

The properties of subtransitions were studied in aqueous dispersions of saturated phosphatidylcholines (PC) by means of permeability measurements, freeze-fracture electron microscopy, and differential scanning calorimetry (DSC). For dispersions of C16PC, a C16PC analog (2,3-dipalmitoyl-cyclopentano-1-phosphocholine with four methylene residues between the nitrogen and the phosphorus atoms) and C17PC, there was good agreement between phase properties (including subtransitions) as observed by DSC and temperature-related permeability. C16PC and C17PC dispersions also displayed a 'crinkled' surface morphology in the subgel state. The phase diagram for mixtures of C14PC and C16PC was consistent with ideal mixing of these two components in the subgel state and also illustrated the relative independence of the subtransition on acyl chain length as compared to the pre- and main transitions. Together, these results indicate that (i) permeability, DSC and freeze-fracture electron microscopy measurements do correlate reasonably well with the existence of a subgel state, (ii) mixtures of lipids with similar acyl chain lengths can be used to investigate subtransitions, (iii) the development of a subtransition appears to be mainly a function of the non-acyl chain moiety of the phospholipid.     

Comparative study of the gel phases of ether- and ester-linked phosphatidylcholines

Calorimetric, X-ray diffraction, and 31P nuclear magnetic resonance (NMR) studies of aqueous dispersions of 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) gel phases at low temperatures (-60 to 22 degrees C) show thermal, structural, and dynamic differences when compared to aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) gel phases at corresponding temperatures. Differential scanning calorimetry of DHPC dispersions demonstrates a reversible, low-enthalpy "subtransition" at 4 degrees C in contrast to the conditionally reversible, high-enthalpy subtransition observed at 17 degrees C for annealed DPPC bilayers. X-ray diffraction studies indicate that DHPC dispersions form a lamellar gel phase with dav congruent to 46 A both above and below the "subtransition". It is suggested that the reduced dav observed for DHPC (46 A as compared to 64 A in DPPC) is due to an interdigitated lamellar gel phase which exists at all temperatures below the pretransition at 35 degrees C. 31P NMR spectra of DHPC gel-phase bilayers show an axially symmetric chemical shift anisotropy powder pattern which remains sharp down to -20 degrees C, suggesting the presence of fast axial diffusion. In contrast, 31P spectra of DPPC bilayers indicate this type of motion is frozen out at approximately 0 degrees C.     

Mixed-chain phosphatidylcholine bilayers: structure and properties

Calorimetric and X-ray diffraction data are reported for two series of saturated mixed-chain phosphatidylcholines (PCs), 18:0/n:0-PC and n:0/18:0-PC, where the sn-1 and sn-2 fatty acyl chains on the glycerol backbone are systematically varied by two methylene groups from 18:0 to 10:0 (n = 18, 16, 14, 12, or 10). Fully hydrated PCs were annealed at -4 degrees C and their multilamellar dispersions characterized by differential scanning calorimetry and X-ray diffraction. All mixed-chain PCs form low-temperature "crystalline" bilayer phases following low-temperature incubation, except 18:0/10:0-PC. The subtransition temperature (Ts) shifts toward the main (chain melting) transition temperature (Tm) as the sn-1 or sn-2 fatty acyl chain is reduced in length; for the shorter chain PCs (18:0/12:0-PC, 12:0/18:0-PC, and 10:0/18:0-PC), Ts is 1-2 degrees C greater than Tm, and the subtransition enthalpy (delta Hs) is much greater than for the longer acyl chain PCs. Tm decreases with acyl chain length for both series of PCs except 18:0/10:0-PC, while for the positional isomers, n:0/18:0-PC and 18:0/n:0-PC, Tm is higher for the isomer with the longer acyl chain in the sn-2 position of the glycerol backbone. The conversion from the crystalline bilayer Lc phase to the liquid-crystalline L alpha phase with melted hydrocarbon chains occurs through a series of phase changes which are chain length dependent. For example, 18:0/18:0-PC undergoes the phase changes Lc----L beta'----P beta'----L alpha, while the shorter chain PC, 10:0/18:0-PC, is directly transformed from the Lc phase to the L alpha phase. However, normalized enthalpy and entropy data suggest that the overall thermodynamic change, Lc----L alpha, is essentially chain length independent. On cooling, the conversion to the Lc phases occurs via bilayer gel phases, L beta', for the longer chain PCs or through triple-chain interdigitated bilayer gel phases, L beta, for the shorter chain PC 18:0/12:0-PC and possibly 10:0/18:0-PC. Molecular models indicate that the bilayer gel phases for the more asymmetric PC series, 18:0/n:0-PC, must undergo progressive interdigitation with chain length reduction to maintain maximum chain-chain interaction. The L beta phase of 18:0/10:0-PC is the most stable structure for this PC below Tm. The formation and stability of the triple-chain structures can be rationalized from molecular models.     

Differential scanning calorimetry study of mixed-chain phosphatidylcholines with a common molecular weight identical with diheptadecanoylphosphatidylcholine

To examine the thermotropic phase behavior of various mixed-chain phosphatidylcholines in excess water and to compare it with the known behavior of identical-chain phosphatidylcholines, we have carried out high-resolution differential scanning calorimetric (DSC) studies on aqueous dispersions of 10 different mixed-chain phosphatidylcholines. These lipids, C(16):C(18)PC, C(18):C(16)PC, C(15):C(19)PC, C(19):C(15)PC, C(14):C(20)PC, C(20):C(14)PC, C(13):C(21)PC, C(21):C(13)PC, C(12):C(22)PC, and C(22):C(12)PC, have a common molecular weight which is the same as that of C(17):C(17)PC, an identical-chain phosphatidylcholine with a molecular weight of 762.2. When the values of any of the thermodynamic parameters (Tm, delta H, and delta S) of the mixed-chain phosphatidylcholines and C(17):C(17)PC are plotted against the normalized chain-length difference (delta C/CL), a linear function with negative slope is obtained provided that the value of delta C/CL is within the range of 0.09-0.4. The linear relationship suggests that these mixed-chain phospholipids are packed in the gel-state bilayer similar to the bilayer structure of C(17):C(17)PC at T less than Tm; however, the negative slope suggests that the conformational statistics of the hydrocarbon chain and the lateral lipid-lipid interactions of these phosphatidylcholines in the gel-state bilayer are perturbed proportionally by a progressive increase in the chain-length inequivalence between the two acyl chains within each lipid molecule. When the value of delta C/CL for mixed-chain phosphatidylcholines reaches the range of 0.44-0.55, the thermotropic phase behavior deviates markedly from that of less asymmetric phosphatidylcholines, suggesting that these highly asymmetric lipids are packed into mixed interdigitated bilayers at T less than Tm. The heating and cooling pathways of aqueous dispersions prepared from the 10 mixed-chain phospholipids are also discussed.     

Binary mixtures of saturated and unsaturated mixed-chain phosphatidylcholine. A differential scanning calorimetry study

High-resolution differential scanning calorimetry (DSC) has been used to study the aqueous dispersions of mixed-chain phosphatidylcholines prepared from colyophilized mixtures of C(18):C(11:1 delta 10) PC/C(18):C(10)PC and C(18):C(11:1 delta 10) PC/C(18):C(11)PC of various molar ratios. These mixed-chain phospholipids are characterized by a marked disparity in their acyl-chain lengths; however, the sn-1 acyl chain in the fully extended conformation is about twice as long as the sn-2 acyl chain. Their thermotropic behavior was determined, and the phase diagrams of these two mixtures were constructed from the calorimetric data. Results indicate that C(18):C(11:1 delta 10)PC/C(18):C(10)PC and C(18):C-(11:1 delta 10)PC/C(18):C(11)PC are miscible in all proportions with a near-ideal behavior of mixing in the gel and liquid-crystalline phases. Equimolar mixtures of diC(14)PC/C(18):C(11:1 delta 10)PC, diC(14)PC/C(18):C(10)PC, and diC(14)PC/C(18):C(11)PC have also been studied by DSC. These phosphatidylcholines in the 1:1 mixture differ in Tm by less than 11 degrees C; however, they exhibit gel-phase immiscibility in the plane of the bilayer. Taken together, these studies suggest that C(18):C(11)PC and C(18):C(11:1 delta 10)PC are packed similarly to C(18):C(10)PC in excess water as mixed interdigitated bilayers, at T less than Tm, which transform into partially interdigitated bilayers when heated above Tm.     

Differential scanning calorimetric study of a homologous series of fully hydrated saturated mixed-chain C(X):C(X + 6) phosphatidylcholines

The successive high-resolution differential scanning calorimetric (DSC) thermograms for aqueous dispersions of a homologous series of mixed-chain phosphatidylcholines, C(X):C(X + 6)PC, have been recorded and analyzed. In this series of saturated mixed-chain phosphatidylcholines, the total number of carbon atoms in the sn-1 acyl chain increases from 11 to 20, and the sn-2 acyl chain is always 6 methylene units longer than the sn-1 acyl chain. In the initial heating DSC thermograms, two prominent endothermic transitions are detected for all the samples prepared from the various C(X):C(X + 6)PCs except C(12):C(18)PC. In contrast, a single exothermic transition is observed on cooling for all the samples except C(13):C(19)PC. The temperature difference between the two endothermic transitions increases linearly as the acyl chain length of C(X):C(X + 6)PC becomes progressively longer. Interestingly, the main phase transition occurs before the subtransition for C(11):C(17)PC dispersions. Our DSC data further demonstrate that the thermodynamic parameters (Tm, delta H, and delta S) associated with the main phase transition for fully hydrated C(13):C(19)PC and other identical MW phosphatidylcholines are inversely related to the corresponding values of the chain-length inequivalence (delta C/CL) for these lipids. This linear relationship can be employed to map the Tm values for aqueous dispersions prepared from a large number of mixed-chain phosphatidylcholines whose values of delta C/CL are within the range of 0.1-0.4.     

Effect of lipid acyl chain length on activity of sodium-dependent leucine transport system in Pseudomonas aeruginosa

The sodium-dependent leucine transport system of Pseudomonas aeruginosa was reconstituted into liposomes of binary lipid mixtures of dilauroylphosphatidylethanolamine (di(12:0)PE)/phosphatidylcholine (PC) with cis-monounsaturated fatty acid chains (di(n:1)PC) (n = 14-22) or dioleoylphosphatidylethanolamine (di(18:1)PE)/di(n:1)PC (n = 14-22). Leucine carrier proteins can be activated with phosphatidylethanolamine, whereas activation does not occur in PC-reconstituted vesicles (Uratani, Y., and Aiyama, A. (1986) J. Biol. Chem. 261, 5450-5454). Na+-dependent counterflow was measured at 30 degrees C as reconstituted transport activity. Proteoliposomes containing di(12:0)PE exhibited high counterflow activity at the PC acyl carbon number (n) of 18 and 20 but no or low activity at n = 14, 16, and 22. On the other hand, proteoliposomes containing di(18:1)PE exhibited higher transport activity than those vesicles with di(12:0)PE and corresponding di(n:1)PC. A lipid mixture of di(18:1)PE and di(16:1)PC supported maximal activity. These results show that the leucine transport system of P. aeruginosa is dependent on the lipid acyl chain length and suggest that there exists optimal bilayer thickness for maximal carrier activity.     

Acyl chain-length asymmetry alters the interfacial elastic interactions of phosphatidylcholines.

Phosphatidylcholines (PCs) with stearoyl (18:0) sn-1 chains and variable-length, saturated sn-2 acyl chains were synthesized and investigated using a Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at various constant temperatures between 10 degrees C and 30 degrees C. Over this temperature range, 18:0-10:0 PC displayed only liquid-expanded behavior. In contrast, di-14:0 PC displayed liquid-expanded behavior at 24 degrees C and 30 degrees C, but two-dimensional phase transitions were evident at 20 degrees C, 15 degrees C, and 10 degrees C. The average molecular area of 18:0-10:0 PC was larger than that of liquid-expanded di-14:0 PC at equivalent surface pressures, and the shapes of their liquid expanded isotherms were somewhat dissimilar. Analysis of the elastic moduli of area compressibility (Cs(-1)) as a function of molecular area revealed shallower slopes in the semilog plots of 18:0-10:0 PC compared to di-14:0 PC. At membrane-like surface pressures (e.g., 30 mN/m), 18:0-10:0 PC was 20-25% more elastic (in an in-plane sense) than di-14:0 PC. Other PCs with varying degrees of chain-length asymmetry (18:0-8:0 PC, 18:0-12:0 PC, 18:0-14:0 PC, 18:0-16:0 PC) were also investigated to determine whether the higher in-plane elasticity of fluid-phase 18:0-10:0 PC is a common feature of PCs with asymmetrical chain lengths. Two-dimensional phase transitions in 18:0-14:0 PC and 18:0-16:0 PC prevented meaningful comparison with other fluid-phase PCs at 30 mN/m. However, the Cs(-1) values for fluid-phase 18:0-8:0 PC and 18:0-12:0 PC were similar to that of 18:0-10:0 PC (85-90 mN/m). These values showed chain-length asymmetrical PCs to have 20-25% greater in-plane elasticity than fluid-phase PCs with mono- or diunsaturated acyl chains.     

Eutectic phase behavior of 1-stearoyl-2-caprylphosphatidylcholine and dimyristoylphosphatidylcholine mixtures

The thermotropic behavior of aqueous dispersions of C(18):C(10)PC/diC(14)PC mixtures with different molar ratios has been investigated by high-resolution differential scanning calorimetry. C(18):C(10)PC is a highly asymmetric lipid molecule, whereas diC(14)PC is a symmetric species with the same molecular weight. Their packing properties in the bilayer are known to be similar at T greater than Tm, but very dissimilar at T less than Tm. Calorimetric results indicate that C(18):C(10)PC and diC(14)PC are completely miscible in the liquid-crystalline state. In the gel state, however, C(18):C(10)PC and diC(14)PC are only partially miscible. The temperature-composition phase diagram for C(18):C(10)PC/diC(14)PC mixtures has the shape characteristic of a typical eutectic system.     

Electron spin resonance characterization of liquid ordered phase of detergent-resistant membranes from RBL-2H3 cells.

The dynamic structure of detergent-resistant membranes (DRMs) isolated from RBL-2H3 cells was characterized using two different acyl chain spin-labeled phospholipids (5PC and 16PC), a headgroup labeled sphingomyelin (SM) analog (SD-Tempo) and a spin-labeled cholestane (CSL). It was shown, by comparison to dispersions of SM, dipalmitoylphosphatidylcholine (DPPC), and DPPC/cholesterol of molar ratio 1, that DRM contains a substantial amount of liquid ordered phase: 1) The rotational diffusion rates (R( perpendicular)) of 16PC in DRM between -5 degrees C and 45 degrees C are nearly the same as those in molar ratio DPPC/Chol = 1 dispersions, and they are substantially greater than R( perpendicular) in pure DPPC dispersions in the gel phase studied above 20 degrees C; 2) The order parameters (S) of 16PC in DRM at temperatures above 4 degrees C are comparable to those in DPPC/Chol = 1 dispersions, but are greater than those in DPPC dispersions in both the gel and liquid crystalline phases. 3) Similarly, R( perpendicular) for 5PC and CSL in DRM is greater than in pure SM dispersions in the gel phase, and S for these labels in DRM is greater than in the SM dispersions in both the gel and liquid crystalline phases. 4) R( perpendicular) of SD-Tempo in DRM is greater than in dispersions of SM in both gel and liquid phases, consistent with the liquid-like mobility in the acyl chain region in DRM. However, S of SD-Tempo in DRM is substantially less than that of this spin label in SM in gel and liquid crystalline phases (in absolute values), indicating that the headgroup region in DRMs is less ordered than in pure SM. These results support the hypothesis that plasma membranes contain DRM domains with a liquid ordered phase that may coexist with a liquid crystalline phase. There also appears to be a coexisting region in DRMs in which the chain labels 16PC and 5PC are found to cluster. We suggest that other biological membranes containing high concentrations of cholesterol also contain a liquid ordered phase.     

IR spectroscopic determination of gel state miscibility in long-chain phosphatidylcholine mixtures.

We report on the gel-state microaggregation in binary mixtures of diacylphosphatidylcholines over temperatures ranging from -19 degrees C to near the gel-to-liquid crystal transition. Microaggregates with lateral dimensions in the range 1-100 chains were detected and measured with an isotope infrared method that relates the splitting or the shape of the methylene scissors band to aggregate size. Measurements were made on fully hydrated dispersions of diC18DPC/diC20HPC, diC18DPC/diC22HPC, and diC18DPC/diC24HPC at molar ratios of 4:1. Low levels of aggregation were determined with reference to the spectrum of the random mixture diC18DPC/diC18HPC. For diC18DPC/diC20HPC at -19 degrees C, which previous calorimetric measurements have indicated is a nearly ideal, we found about 4% of the minority component chains to be involved in aggregates. For diC18DPC/diC22HPC, the value increased to about 11%. DiC18DPC/diC24HPC was found to be highly fractionated, in agreement with the earlier studies. The unit subcell, which defines the type of acyl-chain packing, was determined for the components of the mixtures. The temperature behavior of the phases and the temperatures at which the minority component domains undergo dissolution were determined.     

Mechanism and kinetics of the subtransition in hydrated L-dipalmitoylphosphatidylcholine

The mechanism of the subtransitions (Lc to L beta') in L-dipalmitoylphosphatidylcholine bilayers in excess water has been investigated by time-resolved X-ray diffraction using synchrotron radiation. The temperature dependence of the diffraction patterns closely correlate with the asymmetric excess specific heat variation recorded by differential scanning calorimetry. During the subtransition two prominent wide-angle reflections, characteristic of the low-temperature crystalline phase, Lc, gradually change such that a sharp peak at a spacing of 0.430 nm decreases in intensity and ultimately disappears while a broader peak initially located at 0.375 nm progressively shifts to an eventual spacing of 0.410 nm. This behaviour is interpreted as a lateral deformation of the acyl chain packing subcell as the chains begin to rotate until a state is reached where the chains pack on a regular hexagonal array characteristic of the L beta phase. An increase in lamellar repeat distance from 6.0 to 6.4 nm takes place simultaneously with the acyl chain rearrangement at relatively low (5 K/min) as well as high (6 K/s) heating rates. As judged from the shape of the wide-angle peak, transformation to L beta' phase occurs some minutes after transition to the L beta phase. The X-ray data characterise the subtransition as a continuous (second order) phase transition in which a presumably orthorhombic subcell is transformed into a hexagonal subcell in a gradual process. In temperature jump experiments at 6 K/s between 0 degree C and 80 degrees C the relaxation time of the subtransition was found to be about 5 s while the relaxation time of the main gel to liquid-crystalline transition was about 2 s.     

Adaptation of biological membranes to temperature: molecular species compositions of phosphatidylcholine and phosphatidylethanolamine in mitochondrial and microsomal membranes of liver from thermally-acclimated rainbow trout

Summary Molecular species profiles were determined for both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of mitochondrial and microsomal membrane fractions from liver tissue of thermally-acclimated rainbow trout,Salmo gairdneri. The predominant molecular species of PC were 16:0/22:6, 16:0/18:1, 16:0/20:3 and 16:0/22:5, whereas predominant molecular species of PE were 18:1/20:4, 14:0/16:0, 18:0/22:6 and 18:1/22:6. PE possessed short chain saturates (primarily 14:0/16:0) and monoenes (primarily 14:0/16:1) not present in PC and larger proportions of polyunsaturated (18:0/22:6, 18:0/22:5 and 18:1/22:6. and diunsaturated molecular species than PC. Differences between membrane fractions were most evident in warm (20°C)-acclimated trout. Mitochondria contained higher proportions of long-chain, polyunsaturated molecular species of PE, but less of the corresponding species of PC than other membrane fractions. Rankings based on unsaturation index were accordingly: mitochondria heavy microsomes>light microsomes for PE, but heavy microsomes>light microsomes>-mitochondria for PC. Mitochondria were notable for high proportions of diunsaturated molecular species of both phosphatides. Growth at cold temperatures (5°C) was generally associated with a replacement of shorter chain mono- and dienoic molecular species (16:0/18:1, 16:1/18:1, 14:0/16:2 and 18:1/18:1 in the case of PC and 14:0/16:1, 14:0/16:2 and 16:1/18:1 for PE), and occasionally saturates, with long-chain, polyunsaturated molecular species (for PC, C_36–38: 16:0/22:6, 16:1/22:6, 16:0/20:3 and 16:0/20:5; for PE, C_38–40: 18:1/20:4, 16:1/22:6, 18:0/20:5, 18:2/20:4, 18:0/22:5 and 18:0/22:6). However, compositions of mitochondrial PE and PC from heavy microsomes were not significantly influenced by acclimation temperature. The role of phospholipase A₂, in addition to other metabolic processes, in mediating these changes is discussed.Abbreviations ACL average chain length - UI unsaturation index     

Controlling membrane cholesterol content. A role for polyunsaturated (docosahexaenoate) phospholipids

The molecular organization of cholesterol in 1,2-didocosahexaenoylphosphatidylcholine (22:6-22:6PC) and 1-stearoyl-2-docosahexaenoylphosphatidylcholine (18:0-22:6PC) bilayers was investigated. Using low- and wide-angle X-ray diffraction (XRD), we determined that the solubility of the sterol at 20 degrees C was 11 +/- 3 mol % in 22:6-22:6PC vs 55 +/- 3 mol % in 18:0-22:6PC bilayers. Solubility in the dipolyunsaturated membrane rose to 17 +/- 3 mol % at 40 degrees C, while in the saturated-polyunsaturated membrane there was no change within experimental uncertainty. We compared the molecular orientation of [3alpha-(2)H(1)]cholesterol incorporated into 22:6-22:6PC bilayers to its solubility limit and into 18:0-22:6PC bilayers to a comparable concentration (10 mol %) in solid-state (2)H NMR experiments. The sterol possessed a tilt angle alpha(0) = 24 degrees +/- 1 degrees in 22:6-22:6PC that was independent of temperature over a range from 20 to 40 degrees C. In contrast, the value was alpha(0) = 21 degrees +/- 1 degrees in 18:0-22:6 bilayers at 20 degrees C and increased to alpha(0) = 24 degrees +/- 1 degrees at 40 degrees C. We attribute the low solubility of cholesterol in 22:6-22:6PC membranes to steric incompatibility between the rigid steroid moiety and the highly disordered docosahexaenoic acid (DHA) chain, which has the potential to promote lateral heterogeneity within DHA-rich membranes. Considering 22:6-22:6PC to be the most unsaturated phospholipid found in vivo, this model membrane study provides a point of reference for elucidating the role of sterol-lipid interactions in controlling local compositional organization. Our results form the basis for a model that is consistent with cholesterol's ability to modulate the activity of certain neural transmembrane proteins.     

Scanning calorimetric study of fully hydrated asymmetric phosphatidylcholines with one acyl chain twice as long as the other

The asymmetric C(18):C(10)PC molecules are known by X-ray diffraction to self-assemble, in excess water, into a lamellar structure known as the mixed interdigitated bilayer at T less than Tm. In this structure, the long C(18)-acyl chain is interdigitated fully across the entire hydrocarbon width of the bilayer, while the shorter C(10)-acyl chain, which is about half as long as the C(18)-acyl chain, packs end to end with a C(10)-acyl chain of another lipid molecule in the opposing bilayer leaflet. We have synthesized the following asymmetric phosphatidylcholines (PC's): C(16):C(9)PC, C(16):C(10)PC, C(18):C(10)PC, C(18):C(11)PC, C(20):C(11)PC, C(20):C(12)PC, C(22):C(12)PC, C(22):C(13)PC, C(8):C(18)PC, and C(10):C(22)PC. These 10 asymmetric phosphatidylcholines have a common characteristic; i.e., the length of the longer extended acyl chain is about twice as long as that of the shorter acyl chain. On the basis of the known lamellar structure of C(18):C(10)PC, we anticipate that these asymmetric phosphatidylcholines will also form mixed interdigitated bilayers. We have employed high-resolution differential scanning calorimetry (DSC) to investigate the thermotropic behavior of liposomes prepared from these asymmetric phosphatidylcholines. If our anticipation is correct, one would find that the thermodynamic data (Tm, delta H, or delta S) associated with the main thermal phase transitions of these asymmetric phosphatidylcholine dispersions will fit into a continuous curve as they are plotted as a function of the hydrocarbon width of the putative mixed interdigitated bilayer. Experimental data presented in this paper indeed bear this out. For comparison, a DSC study of multilamellar dispersions prepared from a series of saturated symmetric phosphatidylcholines has also been carried out.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cholesterol-multilipid interactions in bilayers

To extend our knowledge of model membrane systems based upon one lipid component, multi-lamellar bilayers were made of cholesterol with two phospholipids in equimolar ratio, and the enthalpy change delta H of the main phase transition of the temary mixture was measured by differential scanning calorimetry (DSC) as a function of increasing cholesterol concentration c. The lipids were saturated phosphatidylcholines CnPC of acyl chain length n, and as the n of the two lipids became more different (from C13PC/C14PC to C14PC/C15PC to C14PC/C18PC to C14PC/C19PC) distinct breaks in the delta H versus c plots were observed. These mixtures displayed only one broad DSC endotherm. Mixtures of an unsaturated lipid C18: 1PC (dioleoyl) with C16PC or with C18PC showed two peaks, with each peak being associated with its parent lipid. However, the delta H versus c plots for each of these peaks showed an initial independence of cholesterol concentration followed by a dependence on cholesterol concentration. These results indicate that, in lipid mixtures, the type of interaction of cholesterol with each lipid component depends on the concentration of cholesterol present.     

Interdigitated bilayer packing motifs: Raman spectroscopic studies of the eutectic phase behavior of the 1-stearoyl-2-caprylphosphatidylcholine/dimyristoylphosphatidylcholine binary mixture.

The thermotropic properties and acyl chain packing characteristics of multilamellar dispersions of binary mixtures of 1-stearoyl-2-caprylphosphatidylcholine (C(18):C(10)PC), an asymmetric chain species, and dimyristoylphosphatidylcholine (C(14):C(14)PC), a symmetric chain lipid, were monitored by vibrational Raman spectroscopy. In order to examine each component of the binary mixture separately, the acyl chains of the symmetric chain species were perdeuterated. As shown by differential scanning calorimetry, the mismatch in the gel phase bilayer thickness between the two lipid components generates a lateral phase separation resulting in two distinct gel phases, G(I) and G(II), which coexist over much of the composition range. The Raman data demonstrate that the mixed interdigitated phase (three chains per headgroup), analogous to single component phase behavior, is retained when the C(18):C(10)PC component act as a host for the G(I) gel phase. In contrast, the C(18):C(10)PC molecules exhibit partial interdigitation (two chains per headgroup) when they are included as guests within the C(14):C(14)PC host matrix to form the G(II) gel phase. Compared to pure C(14):C(14)PC bilayers at equivalent reduced temperatures, the host G(II) gel phase C(14):C(14)PC molecules exhibit an increased acyl chain order, while for the host G(I) gel phase the C(14):C(14)PC lipid species show increased intrachain disorder.     

Mixing behavior of identical molecular weight phosphatidylcholines with various chain-length differences in two-component lamellae.

It has recently been suggested that mixed-chain phosphatidylcholines with normalized chain length differences (deltaC/CL) in the range of 0.10-0.40 undergo spontaneous self-assembly in excess water at T less than Tm into the partially interdigitated bilayer and those with delta C/CL values in the range of 0.44-0.57 form, in excess water, mixed interdigitated bilayers at T less than Tm. The mixing behavior of binary mixtures of C(22):C(12)PC/C(17):C(17)PC, C(22):C(12)/C(15):C(19)PC, and C(15):C(19)PC/C(13):C(21)PC reported in this work is used to support this view. The values of delta C/CL for C(17):C(17)PC, C(15):C(19)PC, C(13):C(21)PC, and C(22):C(12)PC are 0.10, 0.15, 0.35, and 0.55, respectively. The binary mixture of C(15):C(19)PC/C(13):C(21)PC exhibits a lens-shaped phase diagram, indicating that these two identical molecular weight (MW) lipids with delta C/CL values less than 0.4 are completely miscible over the entire compositional range in both gel and liquid-crystalline phases. In contrast, the phase diagrams of C(22):C(12)PC/C(17):C(17)PC and C(22):C(12)PC/C(15):C(19)PC are eutectic, indicating immiscibility of the component lipids over a wide compositional range in the gel phase. This immiscibility of identical MW lipids in the bilayer plane can be attributed to the different packing properties of the component lipids in the bilayer at T less than Tm.     

Hydrophobic thickness, lipid surface area and polar region hydration in monounsaturated diacylphosphatidylcholine bilayers: SANS study of effects of cholesterol and beta-sitosterol in unilamellar vesicles

The influence of a mammalian sterol cholesterol and a plant sterol beta-sitosterol on the structural parameters and hydration of bilayers in unilamellar vesicles made of monounsaturated diacylphosphatidylcholines (diCn:1PC, n=14-22 is the even number of acyl chain carbons) was studied at 30 degrees C using small-angle neutron scattering (SANS). Recently published advanced model of lipid bilayer as a three-strip structure was used with a triangular shape of polar head group probability distribution (Kucerka et al., Models to analyze small-angle neutron scattering from unilamellar lipid vesicles, Physical Review E 69 (2004) Art. No. 051903). It was found that 33 mol% of both sterols increased the thickness of diCn:1PC bilayers with n=18-22 similarly. beta-sitosterol increased the thickness of diC14:1PC and diC16:1PC bilayers a little more than cholesterol. Both sterols increased the surface area per unit cell by cca 12 A(2) and the number of water molecules located in the head group region by cca 4 molecules, irrespective to the acyl chain length of diCn:1PC. The structural difference in the side chain between cholesterol and beta-sitosterol plays a negligible role in influencing the structural parameters of bilayers studied.     

A statistical mechanical model of the pre- and subtransitions of lecithin membranes

A statistical mechanical model with experimentally proved facts as starting points is presented. This model explains on molecular level, the pre- and subtransitions appearing in lipid membranes. The model describes the main features of the transitions, the hysteresis of the subtransition and the mobility changes of the heads and chains at these transitions. The model was expanded for phosphatidylcholine homologues with arbitrary chain lengths, and a qualitative agreement in the case of pretransition as far as a quantitative one for the subtransition were found.