Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 2. Infrared and 31P NMR spectroscopic studies

The polymorphic phase behavior of aqueous dispersions of a number of representative phosphatidylcholines with methyl iso-branched fatty acyl chains was investigated by Fourier transform infrared (FT-IR) and phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. For the longer chain phosphatidylcholines, where two transitions are resolved on the temperature scale, the higher temperature event can unequivocally be assigned to the melting of the acyl chains (i.e., a gel/liquid-crystalline phase transition), whereas the lower temperature event is shown to involve a change in the packing mode of the methylene and carbonyl groups of the hydrocarbon chains in the gel state (i.e., a gel/gel transition). The infrared spectroscopic data suggest that the methyl iso-branched phosphatidylcholines assume a partially dehydrated, highly ordered state at low temperatures, resembling the Lc phase recently described for the long-chain n-saturated phosphatidylcholines. At higher temperatures, some branched-chain phosphatidylcholines appear to assume a fully hydrated, loosely packed gel phase similar to but not identical with the P beta, phase of their linear saturated analogues. Thus, the iso-branched phosphatidylcholine gel/gel transition corresponds, at least approximately, to a summation of the structural changes accompanying both the subtransition and the pretransition characteristic of the longer chain n-saturated phosphatidylcholines. The infrared spectroscopic data also show that, in the low-temperature gel state, there are significant differences between the odd- and even-numbered isoacylphosphatidylcholines with respect to their hydrocarbon chain packing modes as well as to their head group and interfacial hydration states.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fatty acyl chain length and structure on the lamellar gel to liquid-crystalline and lamellar to reversed hexagonal phase transitions of aqueous phosphatidylethanolamine dispersions

The lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transitions of aqueous dispersions of a number of synthetic phosphatidylethanolamines containing linear saturated, branched chain, and alicyclic fatty acyl chains of varying length were studied by differential scanning calorimetry, 31P nuclear magnetic resonance spectroscopy, and X-ray diffraction. For any given homologous series of phosphatidylethanolamines containing a single chemical class of fatty acids, the lamellar gel/liquid-crystalline phase transition temperature increases and the lamellar liquid-crystalline/reversed hexagonal phase transition temperature decreases with increases in hydrocarbon chain length. For a series of phosphatidylethanolamines of the same hydrocarbon chain length but with different chemical structures, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary markedly and in the same direction. In particular, at comparable effective hydrocarbon chain lengths, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary in parallel, such that the temperature difference between these two phase transitions is nearly constant. Moreover, at comparable effective acyl chain lengths, the d spacings of the lamellar liquid-crystalline phases and of the inverted hexagonal phases are all similar, implying that the thickness of the phosphatidylethanolamine bilayers at the onset of the lamellar liquid-crystalline/reversed hexagonal phase transition and the diameter of the water-filled cylinders formed at the completion of this phase transition are comparable and independent of the chemical structure of the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Gel-state disorder in the presence of methyl iso- and anteiso-branched-chain substituents

The hydrocarbon chain orientational order parameters of membranes of Acholeplasma laidlawii B enriched with large quantities of a linear saturated, a methyl iso-branched, or a methyl anteiso-branched fatty acid plus small quantities of various isomeric monofluoropalmitic acid probes were determined via fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR) over a range of temperatures spanning the gel to liquid-crystalline phase transitions (determined by differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar regardless of the particular fatty acyl structure, showing a region of relatively constant order preceding a region of progressive decline in order toward the methyl terminus of the acyl chain. In the gel state, the order profile of the linear saturated fatty acid enriched membranes was characteristically flat, with little head to tail gradation of order. In contrast, the methyl iso-branched and the methyl anteiso-branched enriched membranes exhibited a local disordering in the gel phase reflected in a very pronounced head to tail gradient of order, which remained at temperatures below the lipid phase transition. In addition, the methyl iso- and anteiso-branched fatty acid enriched membranes were overall more disordered than the membrane containing only linear saturated fatty acyl groups. Thus, at a constant value of reduced temperature below the lipid phase transition, overall order decreased in the progression 15:0 greater than 16:0i greater than 16:0ai, suggesting that these methyl-branched substituents lower the lipid phase transition by disrupting the gel phase lipid chain packing.(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)     

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.     

Phosphatidylcholine-cholesterol interactions: bilayers of heteroacid lipids containing linoleate lose calorimetric transitions at low cholesterol concentration

Model membranes composed of cholesterol plus one of two phosphatidylcholines (PC), each containing a saturated and a dienoic acyl chain, have been studied by differential scanning calorimetry. The gel to liquid-crystalline phase transition temperature of 1-palmitoyl-2-linoleoyl PC was -19.5 degrees C and that of 1-stearoyl-2-linoleoyl PC was -13.7 degrees C. The addition of cholesterol to the phosphatidylcholines in aqueous dispersion resulted in the progressive removal of the phase transition as observed by differential scanning calorimetry. Per mole of sterol in the membrane, cholesterol was more effective at reducing the enthalpy change of the phase transitions of these bilayers containing dienoic phosphatidylcholines than it is in eliminating the transition of membranes made with other phospholipids that contain more saturated chains. No transitions in membranes made with palmitoyl-linoleoyl PC or stearoyl-linoleoyl PC could be detected calorimetrically when 17 mol% cholesterol was present.     

Modifications of lipid structure and their influence on mesomorphism in model membranes: the influence of hydrocarbon chains

The influence of hydrocarbon chains on the temperature (TG-LC) of the gel to liquid-crystalline phase transition of model membranes has been investigated over an extensive variety of phosphatidylcholines (PC). The TG-LC is dependent upon the length of the hydrocarbon chains, on whether or not the chains are saturated or have been modified in some way, and on the position of any modification along the chain. For PC having two different acyl chains (heteroacid PC) in the sn-1 and sn-2 positions, the TG-LC is dependent on the chain position and on the inequivalence of chain penetration into the bilayer. Positional isomers of PC have different TG-LC. The first two double bonds introduced in each chain of a PC cause a much greater reduction in TG-LC and in the enthalpy change of the transition than does the subsequent introduction of additional double bonds. Dipolyunsaturated PC have uncooperative (broad) transitions that occur at low temperatures and have small enthalpy changes. While each PC has unique transitional characteristics, there are a number of patterns in the TG-LC which emerge on consideration of all the available data. One such pattern may be useful in predicting TG-LC from analytical data on the composition and positions of acyl chains of various lipids.     

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.     

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)     

Monolayer and calorimetric studies of phosphatidylcholines containing branched-chain fatty acids and of their interactions with cholesterol and with a bacterial hopanoid in model membranes

Although methyl iso- and anteiso-branched fatty acids occur widely in the membrane lipids of prokaryotic microorganisms, relatively little is known about the physical properties of phospholipids containing these fatty acids. We report here a monolayer and differential scanning calorimetric characterization of several synthetic phosphatidylcholines containing branched-chain fatty acids, and describe the interactions of these phospholipids with cholesterol and with a bacterial hopanoid. We find that monolayers as well as bilayers of methyl isobranched- and especially of methyl anteisobranched-fatty-acid-containing phosphatidylcholines exhibit a reduced solid-to-fluid phase transition temperature in comparison with linear saturated fatty acid-containing phosphatidylcholines of comparable chain length. We also find that the liquid-condensed or gel states of branched-chain fatty acid-containing phosphatidylcholines are partially disordered relative to those of phospholipids containing linear saturated fatty acids, although the presence of a methyl branch has only a small effect on hydrocarbon chain packing in the liquid-expanded or liquid-crystalline states. The presence of cholesterol was found to produce a marked condensation of liquid-expanded films and a small condensation of liquid-condensed films, whether the phosphatidylcholine contained linear or branched-chain fatty acyl constituents. The presence of a bacterial hopanoid produced similar, although slightly smaller, monolayer-condensing effects, indicating that these compounds may perform a cholesterol-like function in bacterial membranes.     

Structure and cohesive properties of sphingomyelin/cholesterol bilayers.

Thermal, structural, and cohesive measurements have been obtained for both bovine brain sphingomyelin (BSM) and N-tetracosanoylsphingomyelin (C24-SM) in the presence and absence of cholesterol. A goal of these experiments has been to clarify the mechanisms responsible for the strong interaction between sphingomyelin and cholesterol. Differential scanning calorimetry shows that fully hydrated bilayers of BSM and C24-SM have main endothermic phase transitions at 39 and 46 degrees C, respectively, that reflect the melting of the acyl chains from a gel to a liquid-crystalline phase. For each lipid, the addition of cholesterol monotonically reduces the enthalpy of this transition, so that at equimolar cholesterol the transition enthalpy is zero. The addition of equimolar cholesterol to either BSM or C24-SM coverts the wide-angle X-ray diffraction reflection at 4.15 A to a broad band centered at 4.5 A. Electron density profiles of gel-phase C24-SM bilayers contain two terminal methyl dips in the center of the bilayer, indicating that the lipid hydrocarbon chains partially interdigitate so that the long saturated 24-carbon acyl chains in one monolayer cross the bilayer center and appose the shorter sphingosine chains from the other monolayer. The incorporation of cholesterol adds electron density to the hydrocarbon chain region near the head group and removes the double terminal methyl dip. These wide- and low-angle X-ray data indicate that cholesterol packs into the hydrocarbon chain region near the sphingomyelin head group, fluidizes the methylene chains near the center of the bilayer compared to the gel phase, and reduces the extent of methylene chain interdigitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

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.     

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.     

Chain Arrangements in the Gel State and the Transition Temperatures of Phosphatidylcholines

Measures of chain length, inequivalence of chain length, and chain position have been incorporated into a parameter, D, which we call the “relative chain inequivalence.” D has been calculated for a number of saturated phosphatidylcholines (PC) containing one type of chain (homoacid PC), saturated PC containing two different acyl chains (heteroacid PC), and heteroacid PC containing one saturated and one unsaturated chain. The gel to liquid-crystalline transition temperatures and D are related in a regular pattern, which suggests similarity of chain packing in the gel. This pattern may have useful predictive value.     

Effects of halothane on dipalmitoylphosphatidylcholine liposomes: a Raman spectroscopic study

Raman spectroscopy has been used to monitor the concentration of halothane (1-bromo-1-chloro-2,2,2-trifluoroethane) in 20% aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) as well as to follow changes in the acyl chain order within the hydrocarbon interior of the liposomes. Temperature profiles for the gel to liquid-crystalline phase transitions for the liposomes were constructed from changes in peak height intensity ratios in the C-H stretching mode and C-C stretching mode regions. Halothane present at the clinical level produces a change of -0.5 degrees C in the phase transition temperature. A limiting transition temperature of about 21 degrees C and saturation of the gel phase occur when the molar ratio of halothane to DPPC reaches about 1.25. At molar ratios above 2.1, the liquid-crystalline phase is also saturated with halothane. Calculations of the distribution of halothane between the various phases in the system are presented and used to interpret literature data as well as the present experiments. Ideal solution theory accounts rather well for the depression in the transition temperature over most of the mole ratio range, an outcome which implies that halothane is excluded from the hydrocarbon interior but not the head-group region in the gel phase. The role of halothane in the head-group region is discussed.     

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.     

An analysis of the relationship between fatty acid composition and the lamellar gel to liquid-crystalline and the lamellar to inverted nonlamellar phase transition temperatures of phosphatidylethanolamines and diacyl-α-D-glucosyl glycerols

The lamellar gel to lamellar liquid-crystalline (Lbeta/Lalpha) and lamellar liquid-crystalline to inverted hexagonal (Lalpha/H(II)) phase transitions of a number of phosphatidylethanolamines (PEs) and diacyl-alpha-D-glucosyl-sn-glycerols (alpha-D-GlcDAGs) containing linear saturated, linear unsaturated, branched or alicyclic hydrocarbon chains of various lengths were examined by differential scanning calorimetry and low-angle X-ray diffraction. As reported previously, for each homologous series of PEs or alpha-D-GlcDAGs, the Lbeta/Lalpha phase transition temperatures (Tm) increase and the Lalpha/H(II) phase transition temperatures (Th) decrease with increases in hydrocarbon chain length. The Tm and the especially the Th values for the PEs are higher than those of the corresponding alpha-D-GlcDAGs. For PEs having the same effective hydrocarbon chain length but different chain configurations, the Tm and Th values vary markedly but with an almost constant temperature interval (deltaT(L/NL)) between the two phase transitions. Moreover, although the Tm and Th values of the PEs and alpha-D-GlcDAGs are equally sensitive on the temperature scale to variations in the length and chemical configuration of the hydrocarbon chains, the deltaT(L/NL) values are generally larger in the PEs and vary less with the hydrocarbon chain structure. This suggests that the PE headgroup has a greater ability to counteract variations in the packing properties of different hydrocarbon chain structures than does the alpha-D-GlcDAG headgroup. With decreasing chain length, this ability of the PE headgroup to counteract the hydrocarbon chain packing properties increases, significantly expanding the temperature interval over which the Lalpha phase is stable relative to the corresponding regions in the alpha-D-GlcDAGs. Overall, these findings indicate that the PEs have a smaller propensity to form the H(II) phase than do the alpha-D-GlcDAGs with an identical fatty acid composition. In contrast to our previous report, there is some variation in the d-spacings of these various PEs (and alpha-D-GlcDAGs) in both the Lalpha and H(II) phases when the hydrocarbon chain structure is changed while the effective chain length is kept constant. These hydrocarbon chain structural modifications produce different d-spacings in the Lalpha and H(II) phases, but those changes are consistent between the PEs and alpha-D-GlcDAGs, probably reflecting differences in the hydrocarbon chain packing constraints in these two phases. Overall, our experimental observations can be rationalized to a first approximation by a simple lateral stress model in which the primary bilayer strain results from a mismatch between the actual and optimal headgroup areas and the primary strain in the H(II) phase arises from a simple hydrocarbon chain packing term.     

The effect of the phase transition on the hydration and electrical conductivity of phospholipids.

Adsorption isotherms for various saturated phosphatidylcholines have been obtained. Lipids above and below their phase transition temperature differ only in the amount of water adsorbed and not in the nature of their adsorption isotherms. Cholesterol has an effect similar to that of increasing unsaturation in the hydrocarbon chains. Decreasing the length of the hydrocarbon chains for lipids below their phase transition temperature has no effect on the isotherms. If the chain length is short enough so that the lipids are above their transition temperature, however, a large increase in water adsorption occurs. All of the phospholipids exhibit a rapid increase of electrical conductivity for a few water molecules adsorbed per lipid molecule. All of the phospholipids show a saturation in conductivity at greater amounts of adsorbed water; the shape of the saturation region depends on whether the lipids are above or below their phase transition temperature. The activation energy for the electrical conductivity process depends on whether the hydrated lipids are in the "liquid-like" of the crystalline state, being lower for phospholipids in the liquid-like state. If the lipids are hydrated above their phase transition temperatures, their activation energies are lower than if they are hydrated below the transition temperature. Cholesterol lowers the activation energy. The phosphatidylcholines can be characterized by different activation energies, depending both upon their physical state and the presence of unsaturation in their hydrocarbon chains.     

Phase behavior of synthetic phosphatidylglycerols and binary mixtures with phosphatidylcholines in the presence and absence of calcium ions

Using differential thermal analysis, scanning calorimetry and light scattering, transition temperatures and enthalpy data for the gel to liquid crystalline phase transitions of five synthetic phosphatidylglycerol sodium salts (PG-Na+) were measured. The values obtained were almost identical with literature values for the corresponding phosphatidylcholines (PC). However, transition temperatures for the fully protonated forms of the saturated phosphatidylglycerols (PG-H+) were approximately 20 degrees C higher. For binary mixtures of PG-Na+ and PC in which the acyl chains of the two species were identical, the width of the thermal transition for the phase change was not appreciably greater than that observed with either of the two components alone. In contrast, mixing of PG-Na+ and PC with different chain lengths increases the transition width. In the presence of Ca2+, narrow transitions were also observed with mixtures of PG and PC when the chain length of the PG-Ca2+ was equal to or two carbons shorter than the PC but the transition width was clearly increased when the chain length of the PG-Ca2+ was two carbons longer than the PC. Mixing lipids with greater differences in chain length or mixing saturated lipids with unsaturated lipids in the presence of Ca2+ produced two minima in the thermograms, clearly indicative of phase separation. In sum, these results provide evidence for a high degree of miscibility of the phosphoglycerol and phosphocholine head groups, either in the presence or absence of Ca2+, such that the characteristic phase behavior of each mixture is determined primarily by differences in the hydrocarbon chain structure.