Studies of the structure and organization of cationic lipid bilayer membranes: calorimetric, spectroscopic, and x-ray diffraction studies of linear saturated P-O-ethyl phosphatidylcholines

Differential scanning calorimetry, x-ray diffraction, and infrared and (31)P-nuclear magnetic resonance ((31)P-NMR) spectroscopy were used to examine the thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2-diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry studies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures slightly below the single endotherm observed upon heating. These cooling exotherms have both been assigned to transitions between the liquid-crystalline and gel phases of these lipids by x-ray diffraction. The x-ray diffraction data also show that unlike the parent PCs, the chain-melting phase transition of these Et-PCs involves a direct transformation of a chain-interdigitated gel phase to the lamellar liquid-crystalline phase for the homologous series of n > or = 14. Our (31)P-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel and liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the (31)P-NMR spectra observed in the interdigitated gel phase indicates that phosphate headgroup reorientation is subject to constraints that are not encountered in the non-interdigitated gel phases of parent PCs. The infrared spectroscopic data indicate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regions of these bilayers are less hydrated than those of corresponding PC bilayers in both the gel and liquid-crystalline phases. Our results indicate that esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charged surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing cis-monounsaturated acyl chain homologues of oleic acid: differential scanning calorimetric and 31P NMR spectroscopic studies

The thermotropic phase behavior of dioleoylphosphatidylcholine and six of its longer chain homologues was studied by differential scanning calorimetry and 31P nuclear magnetic resonance (NMR) spectroscopy. Aqueous dispersions of these compounds all exhibit a single endotherm upon heating but upon cooling exhibit at least two exotherms, both of which occur at temperatures lower than those of their heating endotherm. The single transition observed upon heating was shown by 31P NMR spectroscopy to be a net conversion from a condensed, subgel-like phase (Lc phase) to the liquid-crystalline state. Aqueous ethylene glycol dispersions of these compounds also exhibit single endotherms upon heating and cooling exotherms centered at temperatures lower than those of their corresponding heating endotherm. However, the behavior of the aqueous ethylene glycol dispersions differs with respect to their transition temperatures and enthalpies as well as the extent of "undercooling" observed, and there is some evidence of discontinuities in the cooling behavior of the odd- and even-numbered members of the homologous series. Like the aqueous dispersions, 31P NMR spectroscopy also shows that the calorimetric events observed in aqueous ethylene glycol involve net interconversions between an Lc-like phase and the liquid-crystalline state. However, the Lc phase formed in aqueous ethylene glycol dispersions exhibits a considerably broader powder pattern than that observed in water. This, together with the fact that the transition enthalpies of the aqueous ethylene glycol dispersions are considerably higher than those of the aqueous dispersions, indicates that these lipids form more ordered Lc phases in aqueous ethylene glycol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing omega-cyclohexyl fatty acids. Differential scanning calorimetric and 31P NMR spectroscopic studies

The thermotropic phase behavior of aqueous dispersions of 10 phosphatidylcholines containing omega-cyclohexyl-substituted acyl chains was studied by differential scanning calorimetry and 31P nuclear magnetic resonance spectroscopy. The presence of the omega-cyclohexyl group has a profound effect on the thermotropic phase behavior of these compounds in a manner dependent on whether the fatty acyl chains have odd- or even-numbered linear carbon segments. The thermotropic phase behavior of the odd-numbered phosphatidylcholines is characterized by a single heating endotherm that was shown to be a superposition of at least two structural events by calorimetric cooling experiments. 31P NMR spectroscopy also showed that the single endotherm of the odd-chain compounds is the structural equivalent of a concomitant gel-gel and gel to liquid-crystalline phase transition. The calorimetric behavior of the even-numbered phosphatidylcholines is characterized by a complex array of gel-state phenomena, in addition to the chain-melting transition, in both the heating and cooling modes. The gel states of these even-numbered compounds are characterized by a relatively greater mobility of the phosphate head group as seen by 31P NMR spectroscopy. The differences between the odd-numbered and even-numbered compounds are reflected in a pronounced odd-even alternation in the characteristic transition temperatures and enthalpies and in differences in their responses to changes in the composition of the bulk aqueous phase. Moreover, both the odd-numbered and even-numbered omega-cyclohexylphosphatidylcholines exhibit significantly lower chain-melting transition temperatures and enthalpies than do linear saturated phosphatidylcholines of comparable chain length.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermotropic phase behavior of phosphatidylcholines with omega-tertiary-butyl fatty acyl chains.

The thermotropic phase behavior of a homologous series of phosphatidylcholines containing acyl chains with omega-tertiary butyl groups was studied by differential scanning calorimetry, Fourier transform infrared spectroscopy, and 31P-nuclear magnetic resonance spectroscopy (31P-NMR). Upon heating, aqueous dispersions of these lipids exhibit single transitions which have been identified as direct conversions from Lc-like gel phases to the liquid-crystalline state by both infrared and 31P-NMR spectroscopy. The calorimetric data indicate that the thermodynamic properties of the observed transition are strongly dependent upon whether the acyl chains contain an odd- or an even-number of carbon atoms. This property is manifest by a pronounced odd/even alternation in the transition temperatures and transition enthalpies of this homologous series of lipids, attributable to the fact that the odd-numbered compounds form gel phases that are more stable than those of their even-numbered counterparts. The spectroscopic data also suggest that unlike other lipids which exhibit the so-called odd/even effect, major odd/even discontinuities in the packing of the polymethylene chains are probably not the dominant factors responsible for the odd/even discontinuities exhibited by these lipids, because only subtle differences in the appropriate spectroscopic parameters were detected. Instead, the odd/even alternation in the physical properties of these lipids may be attributable to significant differences in the organization of the carbonyl ester interfacial regions of the lipid bilayer and to differences in the intermolecular interactions between the terminal t-butyl groups of the odd- and even-numbered homologues. Our results also suggest that the presence of the bulky t-butyl groups in the center of the lipid bilayer reduces the conformational disorder of the liquid-crystalline polymethylene chains, and promotes the formation of Lc-like gel phases. However, these Lc-like gel phases are considerably less ordered than those formed by saturated, straight-chain lipids.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing dl-methyl anteisobranched fatty acids. 2. An infrared spectroscopy study

The thermotropic phase behavior of four members of the homologous series of dl-methyl anteisobranched phosphatidylcholines was investigated by Fourier transform infrared spectroscopy. The odd-numbered phosphatidylcholines exhibit spectral changes in two distinct temperature ranges, while their even-numbered counterparts exhibit spectral changes within only a single temperature range. The high-temperature transition observed in the odd-numbered phosphatidylcholines and the single thermotropic event characteristic of the phase behavior of their even-numbered counterparts are both identified as gel/liquid-crystalline phase transitions. The low-temperature event exhibited only by the odd-numbered phospholipids is identified as a gel/gel phase transition that involves changes in the packing mode of the acyl chain methylene groups, as well as changes in the conformation of the glycerol ester interface. These infrared spectroscopic data thus suggest that at low temperatures the odd-numbered methyl anteisobranched phosphatidylcholines form a highly ordered condensed phase similar to the Lc phases of the linear saturated n-acyl-phosphatidylcholines. A comparable condensed phase was not formed by the even-numbered anteisobranched phosphatidylcholines under similar conditions. The properties of the gel states of the even-numbered anteisoacylphosphatidylcholines were generally similar to those of the high-temperature gel states of their odd-numbered counterparts. Those gel states exhibit spectral characteristics indicative of hexagonally packed but relatively mobile acyl chains. The temperature-dependent changes in the spectral characteristics of these gel states were continuous and were not resolved into the discrete but overlapping transitions observed by differential scanning calorimetry.     

An infrared spectroscopic study of the thermotropic phase behavior of phosphatidylcholines containing omega-cyclohexyl fatty acyl chains

The thermotropic phase behavior of an odd- and an even-numbered member of the homologous series of 1,2-di-omega-cyclohexylphosphatidylcholines was studied using Fourier transform infrared spectroscopy. The results obtained indicate that the pronounced discontinuities in the behavior of the odd- and even-numbered homologues observed by differential scanning calorimetry can be attributed to differences in the organization of their respective gel states. The single phase transition exhibited by the odd-numbered compounds upon heating is shown by infrared spectroscopy to be a direct transition from a condensed, subgel-like phase (Lc phase) to the liquid-crystalline state (L alpha phase). In contrast, the multiple transitions exhibited by the even-numbered homologues are shown to be due to the initial conversion of an L beta-like phase to a more loosely packed gel phase, followed by the acyl chain-melting transition. Moreover, the major changes in the interaction between the acyl chains, and in the organization of the interfacial region of the bilayers formed by the even-numbered homologue, occur at temperatures below that of the onset of the chain-melting phase transition. The infrared spectroscopic changes observed also suggest that above the chain-melting transition, the odd- and even-numbered homologues form similar liquid-crystalline phases that are more 'ordered' than those of normal saturated straight-chain phosphatidylcholines. Most likely this is because the large size and the intrinsic rigidity of the omega-cyclohexyl group reduces the conformational disorder of the liquid-crystalline state by 'dampening' all acyl chain motions. The formation of a relatively ordered liquid-crystalline state may be the critical property exploited by the thermoacidophylic organisms in which omega-cyclohexyl fatty acids naturally occur.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 1. Differential scanning calorimetric studies

The thermotropic phase behavior of aqueous dispersions of phosphatidylcholines containing one of a series of methyl iso-branched fatty acyl chains was studied by differential scanning calorimetry. These compounds exhibit a complex phase behavior on heating which includes two endothermic events, a gel/gel transition, involving a molecular packing rearrangement between two gel-state forms, and a gel/liquid-crystalline phase transition, involving the melting of the hydrocarbon chains. The gel to liquid-crystalline transition is a relatively fast, highly cooperative process which exhibits a lower transition temperature and enthalpy than do the chain-melting transitions of saturated straight-chain phosphatidylcholines of similar acyl chain length. In addition, the gel to liquid-crystalline phase transition temperature is relatively insensitive to the composition of the aqueous phase. In contrast, the gel/gel transition is a slow process of lower cooperativity than the gel/liquid-crystalline phase transition and is sensitive to the composition of the bulk aqueous phase. The gel/gel transitions of the methyl iso-branched phosphatidylcholines have very different thermodynamic properties and depend in a different way on hydrocarbon chain length than do either the "subtransitions" or the "pretransitions" observed with linear saturated phosphatidylcholines. The gel/gel and gel/liquid-crystalline transitions are apparently concomitant for the shorter chain iso-branched phosphatidylcholines but diverge on the temperature scale with increasing chain length, with a pronounced odd/even alternation of the characteristic temperatures of the gel/gel transition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing dl-methyl anteisobranched fatty acids. 1. Differential scanning calorimetric and 31P NMR spectroscopic studies

The thermotropic phase behavior of aqueous dispersions of nine dl-methyl branched anteisoacylphosphatidylcholines was studied by differential scanning calorimetry and 31P nuclear magnetic resonance spectroscopy. The calorimetric studies demonstrate that these compounds all exhibit a complex phase behavior, consisting of at least two minor, low-enthalpy, gel-state transitions which occur at temperatures just prior to the onset of the gel/liquid-crystalline phase transition. In addition, at still lower temperatures, anteisobranched phosphatidylcholines containing fatty acyl chains with an odd number of carbon atoms show a major, higher enthalpy, gel-state transition, which was assigned to a conversion from a condensed to a more loosely packed gel phase. No such transition was observed for the even-numbered compounds in aqueous dispersion, but when dispersed in aqueous ethylene glycol, a major gel-state transition is clearly discernible for two of the even-numbered phospholipids. The major gel-state transition exhibits heating and cooling hysteresis and is fairly sensitive to the composition of the bulk aqueous phase. 31P NMR spectroscopic studies indicate that the major gel-state transition is accompanied by a considerable change in the mobility of the phosphate head group and that, at temperatures just prior to the onset of the gel/liquid-crystalline phase transition, the mobility of the phosphate head group is comparable to that normally exhibited by the liquid-crystalline state of most other phospholipids. The temperatures at which the gel/liquid-crystalline phase transition occurs and the enthalpy change associated with this process are considerably lower than those of the saturated n-acyl-PC's of comparable acyl chain length.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains

The thermotropic phase behavior of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chains of 10-22 carbons was studied by differential scanning calorimetry. When fully hydrated and thoroughly equilibrated by prolonged incubation at appropriate low temperatures, all of the compounds studied form an apparently stable subgel phase (the Lc phase). The formation of the stable Lc phase is a complex process which apparently proceeds via a number of metastable intermediates after being nucleated by incubation at appropriate low temperatures. The process of Lc phase formation is subject to considerable hysteresis, and our observations indicate that the kinetic limitations become more severe as the length of the acyl chain increases. The kinetics of Lc phase formation also depend upon whether the acyl chains contain an odd or an even number of carbon atoms. The Lc phase is unstable at higher temperatures and upon heating converts to the so-called liquid-crystalline state (the L alpha phase). The conversion from the stable Lc to the L alpha phase can be a direct, albeit a multistage process, as observed with very short chain phosphatidylcholines, or one or more stable gel states may exist between the Lc and L alpha states. For the longer chain compounds, conversions from one stable gel phase to another become separated on the temperature scale, so that discrete subtransition, pretransition, and gel/liquid-crystalline phase transition events are observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of hydrostatic pressure on bilayer phase behavior and dynamics of dilauroylphosphatidylcholine.

Deuterium nuclear magnetic resonance spectroscopy was used to study the thermotropic phase behavior of dilauroylphosphatidylcholine (DLPC) bilayers at pressures up to 221 MPa. Pressure was found to separate the liquid crystal to gel transition from the gel to ordered crystalline phase transition. The jump in chain order observed on cooling through the transition into the gel phase was found to be small and thus consistent with the trend in longer chain saturated diacyl phosphatidylcholines. On cooling, DLPC was observed to enter an unusual state above the transition into the gel phase. This unusual state displayed fluid-like conformational order but short transverse relaxation times. It was found to be much better pronounced and to span a broader temperature range at elevated pressure than at lower pressures. Transverse relaxation measurements of deuterons on the chain alpha-carbons revealed a substantial slowing of molecular motions within the temperature range of the unusual fluid phase. The observation of such a phase at high pressure appears to be consistent with recent reports of an unusual fluid phase, Lx, in DLPC at ambient pressure.     

Thermotropic properties of saturated mixed acyl phosphatidylethanolamines

The mixed acyl phosphatidylethanolamine (PE) series C(18)C(18)PE, C(18)C(16)PE, C(18)C(14)PE, C(18)C(12)PE, and C(18)C(10)PE has been prepared from the corresponding phosphatidylcholines by phospholipase D mediated transphosphatidylation. The thermotropic behavior of unhydrated and hydrated preparations of these PEs has been investigated by differential scanning calorimetry and 31P NMR spectroscopy. Unhydrated preparations of the PEs undergo crystalline to liquid-crystalline transitions (Tm+h), which correspond to the simultaneous hydration and acyl chain melting of poorly hydrated crystalline samples. Hydrated preparations of the PEs undergo gel to liquid-crystalline transitions (Tm) when scanned immediately subsequent to cooling from temperatures above their respective Tm+hs. Multilamellar bilayers of C(18)C(18)PE, C(18)C(16)PE, and C(18)C(14)PE pack without significant interdigitation of the phospholipid acyl chains across the bilayer center in the gel phase. C(18)C(10)PE multilamellar preparations exhibit a mixed-interdigitated gel phase packing of the phospholipid acyl chains. Hydrated bilayers of C(18)C(12)PE adopt a mixed-interdigitated gel phase packing at temperatures below 13.9 degrees C. Between 13.9 degrees C and the gel to liquid-crystalline transition temperature of 36.9 degrees C, the C(18)C(12)PE bilayer adopts a noninterdigitated gel phase packing. The metastable behavior of fully hydrated and partially hydrated preparations of the mixed acyl PEs has been investigated. Bilayers of C(18)C(18)PE, C(18)C(16)PE, and C(18)C(14)PE exhibited little or no tendency toward regeneration of the crystalline phase. In contrast, bilayers of C(18C(12)PE and C(18)C(10)PE exhibited a metastability of the liquid-crystalline phase in the temperature interval between Tm and Tm+h, which can allow for the regeneration of the crystalline phase under certain conditions.Bilayers of C(18)C(12)PE exhibited an additional metastability of the noninterdigitated gel phase.     

Studies of the thermotropic phase behavior of phosphatidylcholines containing 2-alkyl substituted fatty acyl chains: a new class of phosphatidylcholines forming inverted nonlamellar phases.

We have synthesized a number of 1,2-diacyl phosphatidylcholines with hydrophobic substituents adjacent to the carbonyl group of the fatty acyl chain and studied their thermotropic phase behavior by differential scanning calorimetry, 31P-nuclear magnetic resonance spectroscopy, and x-ray diffraction. Our results indicate that the hydrocarbon chain-melting phase transition temperatures of these lipids are lower than those of the n-saturated diacylphosphatidylcholines of similar chain length. In the gel phase, the 2-alkyl substituents on the fatty acyl chains seem to inhibit the formation of tightly packed, partially dehydrated, quasi-crystalline bilayers (Lc phases), although possibly promoting the formation of chain-interdigitated bilayers. In the liquid-crystalline state, however, these 2-alkyl substituents destabilize the lamellar phase with respect to one or more inverted nonlamellar structures. In general, increases in the length, bulk, or rigidity of the alkyl substituent result in an increased destabilization of the lamellar gel and liquid-crystalline phases and a greater tendency to form inverted nonlamellar phases, the nature of which depends upon the size of the 2-alkyl substituent. Unlike normal non-lamella-forming lipids such as the phosphatidylethanolamines, increases in the length of the main acyl chain stabilize the lamellar phases and reduce the tendency to form nonlamellar structures. Our results establish that with a judicious choice of a 2-alkyl substituent and hydrocarbon chain length, phosphatidylcholines (and probably most other so-called "bilayer-preferring" lipids) can be induced to form a range of inverted nonlamellar structures at relatively low temperatures. The ability to vary the lamellar/nonlamellar phase preference of such lipids should be useful in studies of bilayer/nonbilayer phase transitions and of the molecular organization of various nonlamellar phases. Moreover, because the nonlamellar phases can easily be induced at physiologically relevant temperatures and hydration levels while avoiding changes in polar headgroup composition, this new class of 2-alkyl-substituted phosphatidylcholines should prove valuable in studies of the physiological role of non-lamella-forming lipids in reconstituted lipid-protein model membranes.     

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.     

Calorimetric and spectroscopic studies of the phase behavior and organization of lipid bilayer model membranes composed of binary mixtures of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

The thermotropic phase behavior of hydrated bilayers derived from binary mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and 31P-nuclear magnetic resonance spectroscopy. Binary mixtures of DMPC and DMPG that have not been annealed at low temperatures exhibit broad, weakly energetic pretransitions (approximately 11-15 degrees C) and highly cooperative, strongly energetic gel/liquid-crystalline phase transitions (approximately 23-25 degrees C). After low temperature incubation, these mixtures also exhibit a thermotropic transition form a lamellar-crystalline to a lamellar gel phase at temperatures below the onset of the gel/liquid-crystalline phase transition. The midpoint temperatures of the pretransitions and gel/liquid-crystalline phase transitions of these lipid mixtures are both maximal in mixtures containing approximately 30 mol% DMPG but the widths and enthalpies of the same thermotropic events exhibit no discernable composition dependence. In contrast, thermotropic transitions involving the Lc phase exhibit a very strong composition dependence, and the midpoint temperatures and transition enthalpies are both maximal with mixtures containing equimolar amounts of the two lipids. Our spectroscopic studies indicate that the Lc phases formed are structurally similar as regards their modes of hydrocarbon chain packing, interfacial hydration and hydrogen-bonding interactions, as well as the range and amplitudes of the reorientational motions of their phosphate headgroups. Our results indicate that although DMPC and DMPG are highly miscible, their mixtures do not exhibit ideal mixing. We attribute the non-ideality in their mixing behavior to the formation of preferential PC/PG contacts in the Lc phase due to the combined effects of steric crowding of the DMPC headgroups and charge repulsion between the negatively charged DMPG molecules.     

Calorimetric and spectroscopic studies of the polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines.

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines was investigated by differential scanning calorimetry, 31P-nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Upon heating, aqueous dispersions of dried samples of the short- and medium-chain homologues (n < or = 17) exhibit single, highly energetic transitions from a dry, crystalline form to the fully hydrated, liquid-crystalline bilayer at temperatures higher than the lamellar gel-liquid-crystalline phase transition exhibited by fully hydrated samples. In contrast, the longer chain homologues (n > or = 18) first exhibit a transition from a dehydrated solid form to the hydrated L beta gel phase followed by the gel-liquid-crystalline phase transition normally observed with fully hydrated samples. The fully hydrated, aqueous dispersions of these lipids all exhibit reversible, fairly energetic gel-liquid-crystalline transitions at temperatures that are significantly higher than those of the corresponding phosphatidylcholines. In addition, at still higher temperatures, the longer chain members of this series (n > or = 16) exhibit weakly energetic transitions from the lamellar phase to an inverted nonlamellar phase. Upon appropriate incubation at low temperatures, aqueous dispersions of the shorter chain members of this homologous series (n < or = 16) form a highly ordered crystal-like phase that, upon heating, converts directly to the liquid-crystalline phase at the same temperature as do the aqueous dispersions of the dried lipid. The spectroscopic data indicate that unlike the n-saturated diacyl phosphatidylcholines, the stable crystal-like phases of this series of phosphatidylethanolamines describe an isostructural series in which the hydrocarbon chains are packed in an orthorhombic subcell and the headgroup and polar/apolar interfacial regions of the bilayer are effectively immobilized and substantially dehydrated. Our results suggest that many of the differences between the properties of these phosphatidylethanolamine bilayers and their phosphatidylcholine counterparts can be rationalized on the basis of stronger intermolecular interactions in the headgroup and interfacial regions of the phosphatidylethanolamine bilayers. These are probably the result of differences in the hydration and hydrogen bonding interactions involving the phosphorylethanolamine headgroup and moieties in the polar/apolar interfacial regions of phosphatidylethanolamine bilayers.     

Low-temperature 2H NMR spectroscopy of phospholipid bilayers containing docosahexaenoyl (22:6 omega 3) chains.

Polyunsaturated fatty acids are widely distributed components of biological membranes and are believed to be involved in many biological functions. However, the mechanisms by which they act on a molecular level are not understood. To further investigate the unique properties of omega 3 polyunsaturated phospholipid bilayers, deuterium nuclear magnetic resonance (2H NMR) studies have been made of the liquid-crystalline (L alpha) and gel phases of a homologous series of mixed-chain phosphatidylcholines containing docosahexaenoic acid: (per-2H-n:0)(22:6)PC, where n = 12, 14, 16, and 18. The moments of the 2H NMR lineshapes have been evaluated, and from these the warming and cooling main phase transition temperatures were determined. The transition temperatures of the mixed-chain series were found to be significantly lower than those of the corresponding lipids in the disaturated series, di(per-2H-n:0)PC, with hystereses ranging from 2 to 14 degrees C. Distinct effects of the docosahexaenoyl chain on bilayer order were found, though these effects varied across the mixed-chain series. In evaluating the moment data, an empirical method for normalizing the moments with respect to differences in temperature was applied, in addition to using the reduced temperature method. For the systems studied here, the method of normalization had no significant effect on the interpretation of the moment data.     

Asymmetric glycerophospholipids impart distinctive biophysical properties to lipid bilayers

Phospholipids are a diverse group of biomolecules consisting of a hydrophilic headgroup and two hydrophobic acyl tails. The nature of the head and length and saturation of the acyl tails are important for defining the biophysical properties of lipid bilayers. It has recently been shown that the membranes of certain yeast species contain high levels of unusual asymmetric phospholipids consisting of one long and one medium-chain acyl moiety, a configuration not common in mammalian cells or other well-studied model yeast species. This raises the possibility that structurally asymmetric glycerophospholipids impart distinctive biophysical properties to the yeast membranes. Previously, it has been shown that lipids with asymmetric length tails form a mixed interdigitated gel phase and exhibit unusual endotherm behavior upon heating and cooling. Here, however, we address physiologically relevant temperature conditions and, using atomistic molecular dynamics simulations and environmentally sensitive fluorescent membrane probes, characterize key biophysical parameters (such as lipid packing, diffusion coefficient, membrane thickness, and area per lipid) in membranes composed of both length-asymmetric glycerophospholipids and ergosterol. Interestingly, we show that saturated but asymmetric glycerophospholipids maintain membrane lipid order across a wide range of temperatures. We also show that these asymmetric lipids can substiture of unsaturated symmetric lipids in the phase behaviour of ternary lipid bilayers. This may allow cells to maintain membrane fluidity, even in environments that lack oxygen, which is required for the synthesis of unsaturated lipids and sterols.     

New structural model for mixed-chain phosphatidylcholine bilayers

Multilamellar suspensions of a mixed-chain saturated phosphatidylcholine with 18 carbon atoms in the sn-1 chain and 10 carbon atoms in the sn-2 chain have been analyzed by X-ray diffraction techniques. The structural parameters for this lipid in the gel state are quite different than usual phosphatidylcholine bilayer phases. A symmetric and sharp wide-angle reflection at 4.11 A indicates that the hydrocarbon chains in hydrated C(18):C(10)PC bilayers are more tightly packed than in usual gel-state phosphatidylcholine bilayers and that there is no hydrocarbon chain tilt. The lipid thickness is about 12 A smaller than would be expected in a normal bilayer phase, and the area per molecule is 3 times the area per hydrocarbon chain. In addition, the bilayer thickness increases upon melting to the liquid-crystalline state, whereas normal bilayer phases decrease in thickness upon melting. On the basis of these data, we propose a new lipid packing model for gel-state C(18):C(10)PC bilayers in which the long C(18) chain spans the entire width of the hydrocarbon region of the bilayer and the short C(10) chain aligns or abuts with the C(10) chain from the apposing molecule. This model is novel in that there are three hydrocarbon chains per head group at the lipid-water interface. Calculations show that this phase is energetically favorable for mixed-chain lipids provided the long acyl chain is nearly twice the length of the shorter chain. In the liquid-crystalline state C(18):C(10)PC forms a normal fluid bilayer, with two chains per head group.(ABSTRACT TRUNCATED AT 250 WORDS)