Changes in phase transitions of phosphatidylethanolamine- and phosphatidylcholine-water dispersions induced by small modifications in the headgroup and backbone regions

The effect of methylated phosphatidylethanolamine derivatives on the ionization properties of signaling phosphatidic acid

Phosphatidylethanolamine (PE) and Phosphatidylcholine (PC) are the most abundant glycerophospholipids in eukaryotic membranes. The differences in the physicochemical properties of their headgroups have contrasting modulatory effects on their interaction with intracellular macromolecules. As such, their overall impact on membrane structure and function differs significantly. Enzymatic methylation of PE's amine headgroup produces two methylated derivatives namely monomethyl PE (MMPE) and dimethyl PE (DMPE) which have physicochemical properties that generally range between that of PE and PC. Additionally, their influence on membrane properties differs from both PE and PC. Although variations in headgroup methylation have been reported to affect signaling pathways, the direct influence that these differences exert on the ionization properties of signaling phospholipids have not been investigated. Here, we briefly review membrane function and structure that are mediated by the differences in headgroup methylation between PE, MMPE, DMPE and PC. In addition, using ³¹P MAS NMR, we investigate the effect of these four phospholipids on the ionization properties of the ubiquitous signaling anionic lipid phosphatidic acid (PA). Our results show that PA's ionization properties are differentially affected by changes in phospholipid headgroup methylation. This could have important implications for PA-protein binding and hence physiological functions in cells where signaling events lead to changes in abundance of methylated PE derivatives in the membrane.

Molar ratios of therapeutic water-soluble phenothiazine·water-insoluble phospholipid adducts reveal a Fibonacci correlation and a putative link for structure–activity relationships

The fact that non-antibiotics can sensitise microorganisms for antibiotic treatment suggests that these molecules have valuable potential to treat multiple drug resistance.

Structure of the ripple phase of phospholipid multibilayers

We present electron density maps (EDMs) of the ripple phase formed by phosphorylcholine lipids such as dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dihexadecyl phosphatidylcholine, and dilauroyl phosphatidylcholine (DLPC). With the exception of DLPC, the rippled bilayers have a sawtooth shape in all the systems, with one arm being almost twice as long as the other. For DMPC and POPC bilayers, EDMs have been obtained at different temperatures at a fixed relative humidity, and the overall shape of the ripples and the ratio of the lengths of the two arms are found to be insensitive to temperature. EDMs of all the systems with saturated hydrocarbon chains suggest the existence of a mean chain tilt along the ripple wave vector. In the literature it is generally assumed that the asymmetry of the rippled bilayers (absence of a mirror plane normal to the ripple wave vector) arises from a sawtoothlike height profile. However, in the case of DLPC, the height profile is found to be almost symmetric and the asymmetry results mainly from different bilayer thicknesses in the two arms of the ripple. We also present EDMs of the metastable ripple phase of dipalmitoyl phosphatidylcholine, formed on cooling from the L(alpha) phase.

Phospholipids chiral at phosphorus. Properties of small unilamellar vesicles of chiral thiophosphatidylcholine

The recent observation of the differences in the biophysical properties between 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the Rp, Sp, and Rp + Sp isomers of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DPPsC) in the multilamellar phase [Tsai, M.-D., Jiang, R.-T., & Bruzik, K. (1983) J. Am. Chem. Soc. 105, 2478-2480] prompted us to investigate the biophysical properties of the small unilamellar vesicles (SUV) of the above phospholipids. It was found that DPPC and DPPsC isomers showed approximately the same critical micelle concentrations and formed spherical SUV upon injection of their ethanolic solutions into an aqueous solution. However, the average sizes of the SUV of DPPsC isomers were significantly greater than that of DPPC prepared under the same conditions, as shown by their electron micrographs. The results of both 31P NMR line widths and the ratios of the entrapped solute to the total phospholipids further supported the following order in the average radius of the SUV: (Sp)-DPPsC greater than (Rp + Sp)-DPPsC greater than (Rp)-DPPsC greater than DPPC. Complete lysis of the SUV by melittin was demonstrated in all four cases. The DPPsC isomers showed gel-liquid-crystal transition temperatures of 43.8 +/- 0.1 degrees C, which are considerably higher than that of DPPC (37.9 degrees C) under the same conditions. In the SUV of an equimolar mixture of DPPC and (Rp + Sp)-DPPsC, DPPsC preferred to stay in the inner layer on the basis of 31P NMR studies by use of a shift reagent PrCl3.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase properties of liquid-crystalline Phosphatidylcholine/Phosphatidylethanolamine bilayers revealed by fluorescent probes

The mixing properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were examined in liquid-crystalline phase using fluorescent probes incorporated into lipid bilayers. The excimer to monomer (E/M) fluorescence ratio of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (PPC) versus PPC concentration was higher for binary mixtures containing phosphatidylcholine (PC)/phosphatidylethanolamine (PE) (1:1) compared to PC matrix. When POPC was gradually replaced with POPE, the E/M ratio also increased suggesting the enhanced lateral mobility or the lateral enrichment of PPC into domains or both. Evidences for the PE-induced domain formation were further provided by resonance energy transfer between 2-(4, 4-difluoro-5-methyl-4-boro-3a, 4a-diaza-s-indacene-3-dodecanoyl)-1-hexadecanoyl-sn-glycero- 3-phospho choline and PPC, which was enhanced as a function of PE concentration, and by the polarization of 1,6-diphenyl-1,3, 5-hexatriene. In addition, PE reduced free volume and polarity of lipid bilayers as measured by the emission fluorescence of 1,2-bis PPC and 6-lauroyl-2-dimethylaminonaphthalene. When POPE analogs with a methylated head group instead of normal POPE were used, the diminished effect on the domain formation was shown in the order N-methyl PE > N,N-dimethyl PE. The results suggest that the mixing properties of POPE and POPC are not random but that lipid domains of phospholipids are formed.

Structural requirements for cationic lipid mediated phosphorothioate oligonucleotides delivery to cells in culture

A series of 2,3-dialkyloxypropyl quaternary ammonium lipids containing hydroxyalkyl chains on the quaternary amine were synthesized, formulated with dioleoylphosphatidylethanolamine (DOPE) and assayed for their ability to enhance the activity of an intercellular adhesion molecule 1 (ICAM-1) antisense oligonucleotide, ISIS 1570. Cationic liposomes prepared with hydroxyethyl, hydroxypropyl and hydroxybutyl substituted cationic lipid all enhanced the activity of the ICAM-1 antisense oligonucleotide. Cationic lipids containing hydroxypentyl quaternary amines only marginally enhanced the activity of ISIS 1570. Hydroxyethyl cationic lipids synthesized with dimyristyl (Cl4:0) and dioleyl (C18:1) alkyl chains were equally effective. Activity of cationic lipids containing saturated alkyl groups decreased as the chain length increased, i.e. the dimyristyl (C14:0) was more effective than dipalmityl (C16:0) lipid, which was more effective than distearyl (C18:0). The phase transition temperature of cationic lipids containing saturated aliphatic chains was 56 degrees C for the distearyl lipid, 42 degrees C for the dipalmityl lipid and 24 degrees C for the dimyristyl lipid. Cationic lipids with dioleyl alkyl chains required DOPE for activity, with optimal activity occurring at 50 mole%. In contrast, a dimyristyl containing cationic lipid did not require DOPE to enhance the activity of ISIS 1570. Formulation with different phosphatidylethanolamine derivatives, revealed that optimal activity was obtained with DOPE. These studies demonstrate that several cationic lipid species enhance the activity of phosphorothioate antisense oligonucleotides and provide further information on the mechanism by which cationic lipids enhance the activity of phosphorothioate oligodeoxynucleotides.

Phase behavior of fluorocarbon di-O-alkyl-glycerophosphocholines and glycerophosphoethanolamines and long-term shelf stability of fluorinated liposomes

This paper is devoted to the morphological characterization, by freeze-fracture electron microscopy, and to the thermotropic phase behavior, by differential scanning calorimetry and X-ray diffraction of the aqueous dispersions of various fluorocarbon/fluorocarbon or mixed fluorocarbon/hydrocarbon 1,2- or 1,3-di-O-alkyl-glycerophosphocholines (PC) and 1,2-di-O-alkyl-glycerophosphoethanolamines (PE). The fluorinated PCs form classical lamellar phases and liposomes while an interdigitated lamellar phase has been evidenced for a hydrocarbon 1,3-analog. The fluorinated PEs display a lamellar to hexagonal phase transition which occurs almost simultaneously with the gel-to-fluid lamellar phase transition. The impact of each of the structural features [ether vs ester chemical junction connecting the hydrophobic chains on glycerol, their position (1,2- vs 1,3 isomers), number and length of the perfluoroalkylated chains, length of the fluorinated tail and hydrocarbon spacer, PC vs PE polar head] of the fluorinated phospholipids on the phase transition thermodynamic parameters (Tc, delta H, delta S) is discussed. Most of the liposomes formed from the fluorinated ether-PCs display a remarkable long-term shelf stability: they can be thermally sterilized and stored at room temperature for several months without any significant modification of their size and size distribution.

Thermotropic and barotropic phase behavior of dihexadecylphosphatidylcholine bilayer membrane

The temperature (T)-pressure (P) phase diagram of the ether-linked dihexadecylphosphatidylcholine (DHPC) multilamellar vesicles was constructed by the method of high-pressure optical density. The DHPC membrane at ambient pressure undergoes the pretransition (at 33.6 degrees C) from the interdigitated gel (L beta I) phase to the ripple gel (P' beta) phase, and succeedingly the main transition (at 44.4 degrees C) from the P' beta phase to the liquid crystal (L alpha) phase. Since the slope of the T-P diagram for the pretransition, 0.316 K MPa-1, is larger than that for the main transition, 0.242 K MPa-1, the phase boundary between P' beta and L beta I phases disappeared at high pressure above 130 MPa. A triple point among L beta I, P' beta and L alpha phases was found at 130 MPa and 74.5 degrees C. Difference in phase diagrams between the ether-linked and ester-linked phospholipid bilayer membranes has been elucidated.

Molecular order and hydration property of amine group in phosphatidylethanolamine and its N-methyl derivatives at subzero temperatures

The molecular order and hydration properties of the amine group in phosphatidylethanolamine and its N-methyl derivatives were studied by 2H-NMR at subzero temperatures. Three coexisting signals with 2H-NMR quadrupolar splittings of 146, 106, and 28.8 KHz were detected from the fully hydrated phosphatidylethanolamine/D2O at the lowest studied temperature of -120 degrees C by using short recycle time in the applied NMR pulse sequence. These signals have been assigned to originate from frozen D2O in the interbilayer space and the deuterated amine group, i.e., -ND, with and without threefold symmetric motions. Comparative 2H-NMR studies of phosphatidylethanolamine/D2O with different degrees of methylation over a temperature range between -40 and -120 degrees C lead to the following conclusions. First, the bond angle of -D attached to the nitrogen atom of the amine group may be determined by the 2H-NMR quadrupolar splittings, i.e., 106 and 28.8 KHz, of the two coexisting signals of the deuterated amine group and found to be 112.9 for the gel-state phosphatidylethanolamine. Second, assuming the applicability of the empirical equation for the hydrogen bond distance of N+D--O with deuteron quadrupole coupling constants and using the intermolecular hydrogen bond distance of the amine group determined in single crystals of phosphatidylethanolamine bilayers, the largest measured quadrupolar splitting (delta nu Q) of N-D in this study, i.e., 106 KHz, is close to the static value. This interpretation is also consistent with the fact that the delta nu Q value determined remains constant in the temperature range between -70 and -120 degrees C. Third, the molecular order parameter of the amine group, as calculated from the ratio of the libration-averaged and static delta nu Q value for the lipid with different degrees of methylation, suggests that the perturbation of the headgroup interaction is most significant for the final methylation step. Finally, measurement of the spectral intensity of isotropic unfrozen D2O signals in D2O/phospholipid dispersions at temperatures below the homogeneous nucleation temperature of ice formation for D2O, i.e., below -34 degrees C, suggests that the first methylation step perturbs the neighboring water most significantly. Assuming that the molecular order of the amine group and the amount of unfrozen water detected under the present experimental condition can be taken as a measure of the hydrogen-bonding ability and the extent of perturbation caused by the methyl group, respectively, the gradual methylation of the amine group perturbs the interactions of the N-methylated headgroups in a nonlinear fashion. The results provide a molecular explanation for the phase behavior of phospholipids with different degrees of methylation.

Stimulation of phosphatidylethanolamine synthesis during the last stages of the G1 phase in concanavalin A-activated human peripheral lymphocytes

Phospholipid synthesis was investigated in concanavalin A-activated human peripheral lymphocytes up until 72 h following cell activation, i.e., during the G1 and S phases of the cell cycle. Using [32P]phosphate pulse experiments (5 h), striking differences were observed between phosphatidylethanolamine (PE) and phosphatidylcholine (PC) synthesis. Both the incorporation of [32P]phosphate into PE and the PE/PC incorporation ratio were greatly enhanced, 16-fold and 8-fold, respectively, after 48 h of incubation with the mitogen. This increase in PE synthesis was still observed when cell entry into the S phase was inhibited by an excess of concanavalin A; thereby it must be related to the late stages of the G1 phase. The stimulation of the incorporation into PE was the same for both [14C]ethanolamine and [32P]phosphate, therefore suggesting the involvement of the phosphoethanolamine synthesis pathway. Kinetics of continuous incorporation of [32P]phosphate into PE and PC indicated that the PE/PC net synthesis ratio was enhanced in activated cells, which corresponds to PE enrichment in lymphocyte membranes. The stimulation of PE synthesis in late G1 may be of importance for cell progression through the cell cycle by changing the membrane physical properties. Furthermore, it may serve as a test for checking lymphocyte reactivity to mitogens.

Expression of liver fatty acid binding protein alters plasma membrane lipid composition and structure in transfected L-cell fibroblasts

Liver fatty acid binding protein, L-FABP, is an abundant protein that binds fatty acids in vitro. The effects of L-FABP on plasma membrane lipid composition, distribution, and physical structure were determined in intact L-cell fibroblasts transfected with cDNA encoding L-FABP. L-FABP expression altered plasma membrane phospholipids by decreasing both phosphatidylethanolamine and esterified oleic acid content, and increasing sphingomyelin. L-FABP also binds sterols and stimulates sterol uptake and esterification. The fluorescent sterol dehydroergosterol was used to examine sterol distribution in the transfected cell plasma membrane. Dehydroergosterol codistributed equally with the cholesterol in both the bulk membrane and the individual bilayer leaflets. The sterol/phospholipid ratio was decreased in the inner leaflet due to sterol depletion. Concomitantly, intermembrane sterol transfer from the rapidly exchangeable lateral sterol domains as measured by exchange of dehydroergosterol, was reduced. The fluidity of the plasma membrane was measured with the fluorescent molecule diphenylhexatriene by multifrequency (1-250 MHz) phase and modulation fluorometry. Both the bulk plasma membrane and the plasma membrane outer leaflet lipids were fluidized in transfected cells. These alterations of plasma membrane structure and composition are consistent with a role for L-FABP in regulating intracellular sterol and fatty acid distribution and thereby membrane lipid domain structure.

Influence of Ca2+ and ethanol on the aggregation and thermal phase behaviour of L-dihexadecylphosphatidylcholine liposomes

The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid] ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca(2+)-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8 degrees in the pretransition temperature Tp in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in Tp and finally the disappearance of the pretransition. Concomitantly a slight elevation in Tm occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.

Chain substituted polymerizable ether lipids: synthesis of sorbyl and diacetylenic ether glycerophosphocholine

Three novel polymerizable ether lipids, 1,2-O-bis[10(2',4'-hexadienoyloxy)decyl]-rac, 1,2-O-bis(10,12-tricosadiynyl)-rac, and (-)-2,3-O-bis(10,12-tricosadiynyl)-sn-glycero-1-phosphocholine, were synthesized from 3-O-benzyl-rac, 3-O-trityl-rac and (-)-1-O-trityl-sn-glycerol as starting materials, respectively. All the reactions employed in these multi-step syntheses are straightforward giving an overall yield of 21% for the sorbyl, 42% for the racemic diacetylenic and 44% for the chiral diacetylenic lipid. All the lipids form bilayer assemblies on hydration and show transitions from gel to liquid-crystalline phases at 11.4 degrees, 27.6 degrees and 30.0 degrees C, respectively. Bilayer assemblies of each are photoreactive and are readily polymerized by irradiation with 254 nm light. Tubules of the chiral diacetylenic ether lipid were observed.

Calorimetric studies on the influence of N-methylated headgroups on the mixing behavior of diheptadecanoyl phosphatidylcholine with 1-behenoyl-2-lauroylphosphatidylcholine

Recent studies of five different phosphatidylcholine/phosphatidylcholine (PC/PC) systems indicate that binary mixtures of phosphatidylcholines in which one component has a normalized chain length difference (delta C/CL) in the range of 0.09-0.40 and the other a delta C/CL in the range of 0.42-0.57 exhibit the phase behavior of a eutectic system. Here, delta C is the effective chain-length difference between the two acyl chains, and CL is the effective length of the longer of the two acyl chains for the same lipid molecule in the gel state. In each mixture, gel phase immiscibility occurs over a wide compositional range due to the difference in the gel phase acyl chain packing properties of each component. Although the mixtures differ in the location of their eutectic horizontal, with respect to temperature, all have a similar eutectic point that occurs at a composition of approximately 40 mol percent of the component with the delta C/CL value in the range of 0.42-0.57. Here, we extend these studies by systematically modifying the headgroup of C(17):C(17)PC and then analyzing the mixing behavior of the modified lipid with C(22):C(12)PC using DSC. Progressive demethylation of the C(17):C(17)PC headgroup leads to an increase in gel phase immiscibility and a decrease in the amount of C(22):C(12)PC that comprises the eutectic composition. The temperature defining the location of the eutectic horizontal, however, remains virtually unchanged in all three phase diagrams. Our results suggest that the eutectic composition is influenced by changes in gel phase acyl chain packing that are dependent on headgroup-headgroup interactions. In contrast, the eutectic nature of the phase diagram and the location of its solidus line are properties of acyl chain interactions that are independent of phospholipid headgroup-headgroup interactions.

Structural requirements of lysophospholipid-regulated mitochondrial Ca2+ transport

Analogues of lysophosphatidylcholine, including PAF (platelet-activating-factor) and HePC (an experimental anticancer drug), were studied for their influence on mitochondrial Ca2+ transport and membrane potential. Lysophospholipids released Ca2+ from mitochondria and reduced the maximal Ca2+ uptake. The structure-activity relations indicate that deprotonated head groups like phosphocholines yield active compounds while partially protonated head groups like phosphoethanolamines are essentially inactive. Structural requirements for the apolar part of the molecules were acyl or alkyl chain lengths of less than 18 carbon atoms at the C1-position of the glycerol backbone and residues of small size and/or low polarity at the C2-position. Choline lysophospholipids, but not ethanolamine lysophospholipids, may therefore induce mitochondrial Ca2+ efflux and become mediators of ischaemic tissue damage where dysregulated phospholipase A2 activity and an impairment of mitochondrial function are supposed to play a crucial role.

Effect of inorganic cations on phase transitions

The effect of protons and cations on the crystal (gel)-to-liquid crystal transition temperature Tm of isoelectric and negatively charged phospholipids are summarized. The general trends emerging are as follows: Tm depends on the state of ionization of the phospholipid in that Tm-vs-pH-curves parallel the titration curve of the phospholipid. Protonation of phospholipids causes Tm to increase, deprotonation or ionization has the opposite effect. The effects of cations on the Tm of phospholipids may be grouped into non-specific and specific effects. Unspecific effects of cations such as the screening of negative charges of the phospholipid polar group are qualitatively similar to protonation: Tm increases, in the order monovalent less than divalent less than trivalent cations and the effects on negatively charged phospholipids are larger than those on isoelectric phospholipids. Unspecific, electrostatic effects on Tm are reasonably well accounted for by the Gouy-Chapman theory. If, however, specific binding comes into play and/or electrostatic effects are accompanied by changes in phospholipid structure, simple, electrostatic theories fail to explain the observed changes in Tm. The crystal (gel)-to-liquid crystal transition is also a function of the degree of hydration: Tm generally decreases with increasing hydration reaching a plateau in excess H2O. In addition to screening of electric charges, ions may exert yet another non-specific effect: ions may affect Tm indirectly by competing with the phospholipid polar group for water of hydration. This indirect effect plays a role at high ionic strength and/or at low hydration of the phospholipid. Specific binding of cations to negatively charged phospholipids can lead to tight associations of the metal ion with the lipid polar group. Isothermal crystallization of the phospholipid bilayer is induced that is accompanied by a total or partial loss of water of hydration resulting in a marked increase in Tm. For instance, in crystalline Ca2(+)-phosphatidylserine complexes Tm is increased by more than 100 degrees C.

Thermotropic properties of phospholipid analogues

To understand the structural bases for the polymorphism of phospholipids, it is often essential to study the properties of "unnatural" phospholipid analogues with modified polar headgroups and or backbone structures. While the thermodynamic characteristics of the "classical" hydrated-gel-to-liquid-crystalline phase transition often appear surprisingly insensitive to these aspects of phospholipid structure, the rich and diverse solid-phase polymorphism of phospholipids is in fact exquisitely sensitive to the nature of both the polar headgroup and the backbone moieties. The tendencies of different phospholipids to form nonlamellar phases at higher temperatures also depend strongly (and in a sometimes surprising manner) on fine details of the headgroup and backbone structures. These points are illustrated by discussions of how the structures of headgroup- and backbone-modified phospholipid analogues influence their proclivities to form distinct types of hydrated solid phases, dehydrated "crystralline" phases and nonlamellar phases.

The hydration pressure between lipid bilayers. Comparison of measurements using x-ray diffraction and calorimetry

The hydration pressure between dipalmitoyl phosphatidyl-N,N-dimethylethanolamine (DPPE-Me2) bilayers has been analyzed by both x-ray diffraction measurements of osmotically stressed liposomes and by differential scanning calorimetry. By the x-ray method, we obtain a magnitude (Po) and decay length (lambda) for the hydration pressure which are both quite similar to those found for bilayers of other zwitterionic lipids, such as phosphatidylcholines. That is, x-ray analysis of DPPE-Me2 in the gel phase gives lambda = 1.3 A, the same as that previously measured for the analogous gel phase lipid dipalmitoylphosphatidylcholine (DPPC), and Po = 3.9 x 10(9) dyn/cm2, which is in excellent agreement with the value of 3.6 x 10(9) dyn/cm2 calculated from the measured Volta potential of DPPE-Me2 monolayers in equilibrium with liposomes. These results indicate that the removal of one methyl group to convert DPPC to DPPE-Me2 does not markedly alter the range or magnitude of the hydration pressure. Calorimetry shows that the main gel to liquid-crystalline phase transition temperature of DPPE-Me2 is approximately constant for water contents ranging from 80 to 10 water molecules per lipid molecule, but increases monotonically with decreasing water content below 10 waters per lipid. A theoretical fit to these temperature vs. water content data predicts lambda = 6.7 A. The difference in observed values of lambda for x-ray and calorimetry measurements can be explained by effects on the thermograms of additional intra- and intermolecular interactions which occur at low water contents where apposing bilayers are in contact. We conclude that, although calorimetry provides important data on the energetics of bilayer hydration, it is difficult to obtain quantitative information on the hydration pressure from this technique.

Single and multiple desipramine exposures of cultured cells. Changes in cellular anisotropy and in lipid composition of whole cells and of plasma membranes

Effects of the antidepressant drug desipramine (DMI) on fluorescence anisotropy were studied in living cultured human fibroblasts, rat brain astrocytes and rat ROC-1 hybridoma cells (oligodendrocytes x C6). Fluorescence anisotropy, a measure for fluidity, was measured by means of a fluorescence polarization technique using a set of n-(9-anthroyloxy) fatty acids as markers. Apparent fluorescence anisotropies were determined in cells following single or multiple dose exposures to 5 microM DMI at 37 degrees and compared to control cells. In all three cell types single doses of DMI led to significant decreases in anisotropies of the deeper layers (12-AS) of the membranes only, suggesting increases in fluidity. Repeated exposures to 5 microM DMI led to cell specific, significant changes in anisotropies of the superficial membrane layers, as determined by 2-AP, 6-, 7- and 9-AS. The resulting anisotropy values of the three different cell types became more alike than prior to DMI exposure. Alterations in anisotropies were accompanied with changes in the phospholipid patterns of whole cells and isolated plasma membrane vesicles. The changes of PC/PE ratios were consistent with changes observed in fluorescence anisotropies. Such alterations may be individual regulatory responses of the cells to the chronic presence of the drug within the membranes.

Effect of chain-linkage on the structure of phosphatidyl choline bilayers. Hydration studies of 1-hexadecyl 2-palmitoyl-sn-glycero-3-phosphocholine

While hydrated dipalmitoyl phosphatidylcholine (DPPC) forms tilted chain L beta' bilayers in the gel phase, the ether-linked analogue dihexadecyl phosphatidylcholine (DHPC) exhibits gel phase polymorphism. At low hydration DHPC forms L beta' phases but at greater than 30% H2O a chain-interdigitated gel phase is observed (Ruocco, M. J., D. S. Siminovitch, and R. G. Griffin. 1985. Biochemistry. 24:2406-2411; Kim, J.T., J. Mattai, and G.G. Shipley. 1987. Biochemistry. 26:6599-6603). In this study we report the behavior of a phosphatidylcholine (PC) with both types of chain linkage, 1-hexadecyl-2-palmitoyl-sn-glycero-3-phosphocholine (HPPC). HPPC has been investigated as a function of hydration using differential scanning calorimetry (DSC) and x-ray diffraction. By DSC, over the hydration range 5. 1-70.3 wt% H2O, HPPC exhibits two reversible transitions. The reversible main chain-melting transition decreases from 69 degrees C, reaching a limiting value of 40 degrees C at full hydration. X-ray diffraction patterns of hydrated HPPC have been recorded as a function of hydration at 20 degrees and 50 degrees C. At 50 degrees C, melted-chain L alpha bilayer phases are observed at all hydrations. At 20 degrees C, at low hydrations (less than 34 wt% H2O) HPPC exhibits diffraction patterns characteristic of bilayer gel phases similar to those of the gel phase of DPPC. In contrast, at greater than or equal to 34 wt% H2O, HPPC shows a much reduced bilayer periodicity, d = 47 A, and a single sharp reflection at 4.0 A in the wide angle region. This diffraction pattern is identical to that exhibited by the interdigitated phase of DHPC. Therefore, in the gel phase HPPC undergoes a hydration-dependent conversion from a regular bilayer structure to an interdigitated bilayer arrangement. Clearly, the presence of a single ether linkage (at the sn-i position) is sufficient to allow formation of the chain-interdigitated phase in a hydration-dependent way essentially identical to that of DHPC.

Cross-linking of phosphatidylethanolamine neighbors with dimethylsuberimidate is sensitive to the lipid phase

Dimethylsuberimidate was reacted with aqueous dispersions of dipalmitoylphosphatidylethanolamine, dimyristoylphosphatidylethanolamine, dilauroylphosphatidylethanolamine, and dielaidoylphosphatidylethanolamine at pH 10 and at pH 8. The amount of amidine dimer formation was about four times greater above the gel-to-fluid phase transition of each lipid than below the transition. The transition temperature of each phosphatidylethanolamine, measured by steady-state fluorescence anisotropy of cis-parinaric acid, was lower at pH 10 than at pH 8 or in water. The ability of dimethylsuberimidate to discriminate between phosphatidylethanolamines in the fluid and gel phases should allow use of this reagent to identify phosphatidylethanolamine species within the gel or fluid lipid phase.

Molecular dynamics of antitumor ether-linked phospholipids in model membranes: a spin-label study

We have synthesized a spin-labeled derivative of ET-18-OCH3, a known antitumor ether-linked phospholipid. The spin-labeled analog was shown to be as potent as ET-18-OCH3 in inhibiting 3H-thymidine uptake of HL60 leukemic cells. Electron spin resonance (ESR) studies showed that the mobility of this ether-linked phospholipid in the membrane is more restricted when compared to its ester-linked counterparts. It is probable that the absence of the bulky carbonyl oxygens allows closer packing of the two alkyl chains in the ether-linked phospholipid, thereby reducing the angular amplitude of the motion of the alkyl chains. These findings may be of importance in elucidating mechanisms by which the antitumor ether-linked phospholipids perturb the structure of cellular membranes.

Temperature-sensitive release of adriamycin, an amphiphilic antitumor agent, from dipalmitoylphosphatidylcholine-cholesterol liposomes

Drugs for temperature-sensitive liposomes have been limited to the hydrophilic drugs, such as methotrexate and cis-dichlorodiammineplatinum, with a low affinity for the lipid bilayer. It was, however, of importance to investigate whether the concept of temperature-sensitive liposomes can be extended to amphiphilic or lipophilic compounds, because some useful drugs are amphiphilic or lipophilic. In this study we tried to use adriamycin, an amphiphilic antitumor agent, as a drug for temperature-sensitive liposomes. In the absence of serum, the liposomes prepared from dipalmitoylphosphatidylcholine released adriamycin in a temperature-sensitive manner, i.e., they retained the major portion of entrapped adriamycin at a lower temperature 32 degrees C, and released around 70% of the drug at 42 degrees C, a temperature higher than the phase-transition temperature of the phospholipid. However, when serum was present, the liposomes were leaky even at 32 degrees C. To raise the stability of the liposomes, we included various mol% of cholesterol in the liposomal membrane and examined temperature sensitivity and stability of the liposomes in the presence of serum. Our results indicated that the liposomes including 20 mol% cholesterol were considerably stable and exhibited the maximal temperature-sensitive release of adriamycin in the presence of serum.

Effect of stereoconfiguration on ripple phases (P beta') of dipalmitoylphosphatidylcholine

Mixtures of sn-1 (D) and sn-3 (L) enantiomers of fully hydrated dipalmitoylphosphatidylcholine (DPPC) were studied with differential scanning calorimetry and freeze-fracture microscopy. The pretransition temperature of racemic mixtures of DPPC was 1.8 C degrees below that of either pure sn-1 or sn-3 enantiomers, which had similar pretransition temperatures. The main transition temperature of racemic mixtures was also depressed, but to a lesser extent, 0.8 C degrees. Freeze-fracture images of liposomes of sn-1, sn-3, and racemic mixtures of DPPC frozen from the P beta' phase showed well-defined ripples of wavelength 13 nm. Lipid stereoconfiguration had no effect on ripple wavelength, configuration or amplitude, or on the number and nature of surface defects.

Modelling the phase equilibria in two-component membranes of phospholipids with different acyl-chain lengths

A phenomenological model is proposed to describe the membrane phase equilibria in binary mixtures of saturated phospholipids with different acyl-chain lengths. The model is formulated in terms of thermodynamic and thermomechanic properties of the pure lipid bilayers, specifically the chain-melting transition temperature and enthalpy, the hydrophobic bilayer thickness, and the lateral area compressibility modulus. The model is studied using a regular solution theory made up of a set of interaction parameters which directly identify that part of the lipid-lipid interaction which is due to hydrophobic mismatch of saturated chains of different lengths. It is then found that there is effectively a single universal interaction parameter which, in the full composition range, describes the phase equilibria in mixtures of DMPC/DPPC, DPPC/DSPC, DMPC/DSPC, and DLPC/DSPC, in excellent agreement with experimental measurements. The model is used to predict the variation with temperature and composition of the specific heat, as well as of the average membrane thickness and area in each of the phases. Given the value of the universal interaction parameter, the model is then used to predict the phase diagrams of binary mixtures of phospholipids with different polar head groups, e.g., DPPC/DPPE, DMPC/DPPE and DMPE/DSPC. By comparison with experimental results for these mixtures, it is shown that difference in acyl-chain lengths gives the major contribution to deviation from ideal mixing. Application of the model to mixtures with non-saturated lipids is also discussed.

Effects of platelet activating factor and related lipids on phase transition of dipalmitoylphosphatidylcholine

Recent evidence localizing the inflammatory mediator, platelet activating factor, (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to the membranes of stimulated neutrophils raises the possibility that PAF may, in addition to its activities as a mediator, alter the physical properties of membranes. Accordingly, the effects of PAF and related alkyl ether and acyl analogs on phase transition thermodynamics of dipalmitoylphosphatidylcholine (DPPC) were studied using fluorescence polarization of the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). PAF, its ester analog (1-palmitoyl-2-acetylphosphatidylcholine) and both the corresponding alkyl and acyl lysophospholipid analogs (each at a concentration of 10 mol%) significantly decreased the phase transition temperature and broadened the phase transition of DPPC (P less than 0.05). The relative potency of the lipids in causing this effect was ester-PAF greater than or equal to PAF greater than or equal to lyso-PAF greater than lyso-PC suggesting that the fluidization of the synthetic membranes was attributable to both the 2-position acetyl group and the 1-position alkyl linkage. Furthermore, using various related compounds, increases in chain length and degree of unsaturation in the 2-position were shown to enhance the depression in transition temperature and broadening of the phase transition. Phase transition thermodynamics were also assessed using differential scanning calorimetry. Similar depression in the phase transition temperature was measured for PAF and both the alkyl and acyl lysophospholipids. Broadening of the phase transition for DPPC by the various analogs was assessed by calculation of transition peak width and cooperative unit. Data from fluorescence polarization and differential scanning calorimetry provide similar though not identical results and support the hypothesis that the unique features of PAF may alter membrane physical properties and could ultimately explain some of its biologic actions.

How membrane chain melting properties are regulated by the polar surface of the lipid bilayer

The principle of regulation of various membrane properties by the hydrocarbon membrane interior is now well understood. The mechanism by which the interfacial membrane region including aqueous solution affects the state of the lipid bilayer matrix, however, is as yet unclear, despite its great biological and physiological significance. Data and analysis presented in this paper show that apart from the lipid chain type, length, and degree of unsaturation the main factors determining the characteristics of lipid membranes are surface polarity and interfacial hydration. These incorporate the effects of head group dipole and multipole moments as well as the head group ability for hydrogen bonding and can account for most of the changes in the physicochemical membrane state caused by the lipid head group structure, bulk pH value, salt content, solute adsorption, etc. The effects of membrane potential are much less, only 10-30% of the former. Variations in hydration thus not only govern the short- and medium-range intermolecular and intermembrane interactions but also provide a fast and energetically inexpensive regulatory mechanism for lipid membranes to adapt their characteristics, at least locally or transiently, to new requirements.

Lipid intermolecular hydrogen bonding: influence on structural organization and membrane function

The great variety of different lipids in membranes, with modifications to the hydrocarbon chains, polar groups and backbone structure suggests that many of these lipids may have unique roles in membrane structure and function. Acidic groups on lipids are clearly important, since they allow interaction with basic groups on proteins and with divalent cations. Another important property of certain lipids is their ability to interact intermolecularly with other lipids via hydrogen bonds. This interaction occurs through acidic and basic moieties in the polar head groups of phospholipids, and the amide moiety and hydroxyl groups on the acyl chain, sphingosine base and sugar groups of sphingo- and glycolipids. The putative ability of different classes of lipids to interact by intermolecular hydrogen bonding, the molecular groups which may participate and the effect of these interactions on some of their physical properties are summarized in Table IX. It is frequently questioned whether intermolecular hydrogen bonding could occur between lipids in the presence of water. Correlations of their properties with their molecular structures, however, suggest that it can. Participation in intermolecular hydrogen bonding increases the lipid phase transition temperature by approx. 8-16 Cdeg relative to the electrostatically shielded state and by 20-30 Cdeg relative to the repulsively charged state, while having variable effects on the enthalpy. It increases the packing density in monolayers, possibly also in the liquid-crystalline phase in bilayers, and decreases the lipid hydration. These effects can probably be accounted for by transient, fluctuating hydrogen bonds involving only a small percentage of the lipid at any one time. Thus, rotational and lateral diffusion of the lipids may take place but at a slower rate, and the lateral expansion is limited. Intermolecular hydrogen bonding between lipids in bilayers may be significantly stabilized, despite the presence of water, by the fact that the lipids are already intermolecularly associated as a result of the hydrophobic effect and the Van der Waals' interactions between their chains. The tendency of certain lipids to self-associate, their asymmetric distribution in SUVs, their preferential association with cholesterol in non-cocrystallizing mixtures, their temperature-induced transitions to the hexagonal phase and their inhibitory effect on penetration of hydrophobic residues of proteins partway into the bilayer can all be explained by their participation in intermolecular hydrogen bonding interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Deuterium nuclear magnetic resonance studies on the plasmalogens and the glycerol acetals of plasmalogens of Clostridium butyricum and Clostridium beijerinckii

Deuterium nuclear magnetic resonance was used to investigate the structure of different lipid fractions isolated from the anaerobic bacteria Clostridium butyricum and Clostridium beijerinckii. The fractions isolated from C. butyricum were (1) phosphatidylethanolamine/plasmenylethanolamine and (2) the glycerol acetal of plasmenylethanolamine, and from C. beijerinckii similar fractions containing principally (1) phosphatidyl-N-monomethylethanolamine, along with its plasmalogen, and (2) the glycerol acetal of this plasmalogen were isolated. The third fraction from both species consisted largely of the acidic lipids phosphatidylglycerol and cardiolipin along with plasmalogen forms of these lipids. Palmitic acid with deuterium labels at C-2, C-3, or C-4 or oleic acid with deuterium labels at C-2 and C-9,10 was added to the growth medium and incorporated to various extents in the lipid fractions. Biochemical analysis showed that palmitic acid and oleic acid were preferentially bound to the sn-2 and sn-1 positions, respectively, of the glycerol backbone when both fatty acids were added to the medium. From the 2H NMR spectra, the hydrocarbon chain ordering near the lipid-water interface could be determined and appeared to be similar for all three lipid fractions. The deuterium quadrupole splitting and order parameter were low at the C-2 segment and increased by almost a factor of 2 at positions C-3 and C-4 for cells fed with deuterated palmitic acid along with unlabeled oleic acid. These results agree with previous findings on pure diacyl lipids in which the sn-2 chain was found to adopt a bent conformation at the carbon segment C-2. However, two unusual quadrupole splittings could be detected for the plasmalogens.(ABSTRACT TRUNCATED AT 250 WORDS)