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 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.     

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)     

Enigmatic thermotropic phase behavior of highly asymmetric mixed-chain phosphatidylcholines that form mixed-interdigitated gel phases.

Twelve saturated mixed-chain phosphatidylcholines have been identified for which the thermotropic phase behavior observed upon cooling from the L alpha phase is dependent upon the thermal history of the sample in the gel phase. If fully hydrated samples of these lipids are cooled and soon thereafter examined by differential scanning calorimetry, one observes a single highly cooperative endotherm (the chain-melting phase transition) upon heating, and on subsequent cooling, a single exotherm that may occur at temperatures as much as 4-6 degrees C below that of the single endotherm observed upon heating. In contrast, if the samples are incubated in the gel state at low temperatures for prolonged periods of time, one observes a single heating endotherm as before, but two sharp exotherms upon cooling. The latter transitions occur at temperatures close to that of the single endotherm observed upon heating and the single cooling exotherm observed prior to incubation in the gel state. The combined enthalpy of the two cooling exotherms is the same as that of the single heating endotherm or the single cooling exotherm initially observed. Infrared spectroscopic and X-ray diffraction studies indicate that the structural conversions characteristic of liquid-crystalline/gel phase transitions occur at both of those cooling exotherms. Of the 12 lipids that exhibit this unusual behavior, nine fulfill the previously defined structural requirements for the formation of the so-called mixed-interdigitated gel phase, and there is evidence in the literature that one of the three remaining lipids also forms such a structure. Infrared spectroscopic studies of the other two lipids indicate that their gel phases exhibit spectroscopic features that closely resemble those of lipids that meet the previously defined structural criteria for the formation of mixed-interdigitated gel phases and that differ markedly from those of both saturated symmetric-chain and saturated mixed-chain phosphatidylcholines that do not normally form mixed-interdigitated gel phases. Also, electron density reconstructions based on small-angle X-ray diffraction studies of the gel phases of those two lipids indicate that the thickness of their gel phase bilayers is consistent with their forming mixed-interdigitated gel phases. Thus the unusual thermotropic phase behavior described here may be a general characteristic of phosphatidylcholines that form mixed-interdigitated gel phases. This unusual behavior is not associated with any major change in any of several physical properties of these lipid bilayers but may arise from an alteration of the size and/or structure of microdomains present in the liquid-crystalline phase.     

The effect of hydrostatic pressure on the bilayer structure of phosphatidylcholines containing omega-cyclohexyl fatty acyl chains

The barotropic behavior of aqueous dispersions of two representative omega-cyclohexyl phosphatidylcholines was investigated by pressure-tuning Fourier transform infrared spectroscopy. In the even-numbered homologue, 1,2-di-14-cyclohexyltetradecanoyl-sn-glycero-3-phosphocholine (14cyPC), the lipid molecules are orientationally disordered until the applied pressure reaches 2.1 kbar. This pressure marks the onset of correlation field splitting of the scissoring and rocking modes of the linear chain methylenes, as well as that of the cyclohexyl ring methylenes. It indicates immobilization of the entire acyl chains, whereby the zig-zag planes of the neighboring straight chain all-trans methylenes are oriented mainly perpendicular to each other. As judged from the magnitude of the correlation field splittings, the interchain interaction is weaker in 14cyPC than that in linear lipids (e.g., DMPC or DPPC). Upon an increase in pressure, up to 20 kbar, the zig-zag methylene planes in 14cyPC undergo a gradual transformation to a parallel orientation. In the odd-numbered homologue, 1,2-di-13-cyclohexyltridecanoyl-sn-glycero-3-phosphocholine (13cyPC), there is no correlation field splitting originating from the straight chain methylenes (up to 21 kbar). The linear, nonbranched segments of the omega-cyclohexyl chains in 13cyPC are closely packed with the all-trans methylene zig-zag planes oriented parallel to each other. There is, however, correlation field splitting of the ring methylenes, indicating interring interactions between the bulky cyclohexyl rings in opposing bilayer leaflets. There are major structural differences between the even- and odd-numbered homologues in the interfacial region, which remain even at high pressures. The ester carbonyl C = O stretching band in 14cyPC is a composite of two discrete bands which do not change considerably in intensity or frequency in the pressure range 2-20 kbar. In contrast, 13cyPC possesses an additional, low-frequency C = O stretching component at low pressures. As the pressure increases, the three component bands coalesce into a single C = O stretching band. Our results suggest equally oriented, fully hydrogen-bonded carbonyl groups in 13cyPC at pressures above approx. 10 kbar.     

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.     

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)     

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)     

Role of head group structure in the phase behavior of amino phospholipids. 2. Lamellar and nonlamellar phases of unsaturated phosphatidylethanolamine analogues

Three types of analogues of unsaturated phosphatidylethanolamines (PE) have been prepared: phosphatidyl-omega-amino-1-alkanols, N-alkyl-PE's, and C2-alkyl-PE's, with alkyl substitution of carbon-2 of the ethanolamine head group. The physical properties of dioleoyl, dielaidoyl, and 1-palmitoyl-2-oleoyl phospholipids with these head groups have been examined by calorimetry, 31P NMR, freeze-fracture electron microscopy, and X-ray diffraction. N-Alkylation of PE, or substitution of the ethanolamine moiety by 3-amino-1-propanol or 4-amino-1-butanol, decreases the transition temperature of the hydrated gel phase (Tc) and considerably increases the temperature of the lamellar to hexagonal II transition (TH). The pattern of these effects for various PE analogues suggests that head group size and hydrophobicity as well as hydrogen bonding are important determinants of the phase behavior of these lipids. C2-Alkylated PE analogues exhibit several rather surprising properties, notably the ready formation of a quasi-crystalline "high-melting" solid phase even for di-cis-unsaturated species and substantially lower TH values than are observed for the parent PE species. The behavior of these compounds suggests that "hydration forces" can be more important than considerations of lipid "dynamic shape" in predicting the relative stabilities of lamellar vs. nonlamellar phases for at least some zwitterionic phospholipids.     

Platelet-aggregating effects of platelet-activating factor-like phosphollpids formed by oxidation of phosphatidylcholines containing an sn-2-polyunsaturated fatty acyl group

Previously, we reported the formation of four kinds of pfaosphatidylcholines (PC) with a short-chain monocarboxylate, dicarboxylate, dicarboxylate semialdehyde or w-hydroxymonocarboxylate group by oxidation of PCs containing polyunsaturated fatty acid (PUFA) in an FeSO₄ /ascorbate /EDTA system. In this study, we identified these novel phospholipids by GC-MS as oxidation products of two alkyl ether-linked PCs. 1-O-hexadecyl-2-docosahexaenoyl and 1-O-hexadecyl-2-arachidonoyl-sn-glycero-3-phosphocholine (GPC). The sn-2-acyl moieties of oxidatively fragmented PCs derived from PCs containing docosahexaenoate were one methylene unit shorter than those detected as major oxidation products of PCs containing arachidonate. The platelet-aggregations induced by the oxidized PCs were all inhibited by FR-900452, an antagonist of platelet activating factor (PAF). The PAF-like activity of oxidized 1-O-hexadecyl-2-docosahexaenoyl-GPC, which was equivalent of 1372 ± 262 pmol 16: 0-PAF/μmol starting PC, was 5 times that of oxidized 1-O-hexadecyl-2-arachidonoyl-GPC and 150 times that of oxidized 1-palmitoyl-2-docosahexaenoyl-GPC, suggesting that both an sn-1-alkyl ether linkage and an sn-2-acyl group with a short chain length are important structural requirements for induction of platelet aggregation. These possibilities were confirmed by experiments on the platelet-aggregating activities of synthetic PAF-like compounds. Quantitative measurements by GC-MS of PAF-like phospholipids formed by lipid peroxidation and the activities of synthetic PAF-like phospholipids, suggested that the activities of most oxidized PCs containing PUFA were ascribable to those of PCs with an sn-2-short-chain monocarboxylate group.     

The influence of unsaturation on the phase transition temperatures of a series of heteroacid phosphatidylcholines containing twenty-carbon chains

A series of heteroacid sn-1,2 phosphatidylcholines (PC) with twenty-carbon fatty acyl chains has been synthesized. Each PC contained eicosanoate (20:0) in the sn-1 position and one of a group of eicosaenoic acids with increasing numbers of cis double bonds in the sn-2 position. The double bonds were at positions Δ11 (20:1), Δ11,14 (20:2), Δ11,14,17 (20:3), or Δ5,8,11,14 (20:4). The disaturated PC containing two eicosanoate chains was also studied. Aqueous dispersions of these PC were analyzed by differential scanning calorimetry, and data for the gel to liquid-crystalline transitions (given as PC: T_c (° C), T_max (° C), ΔH(kcal/mol)) were as follows - 20:0-20:0 PC: 66.8, 68.4, 15; 20:0-20:1 PC: 19.8, 22.2, 8; 20:0-20:2 PC: −4.3, 1.8, 5; 20:0-20:3 PC: 1.2, 4.4, 7; 20:0-20:4 PC: −10.7, −6.8, 3. Double bonds in excess of two per chain did not substantially change the transition temperatures of these heteroacid PC. There was a small effect of the location of the multiple double bonds on the transition temperature. The data is consistent with the model that the transition temperatures are determined by a balance between a decrease in the packing density in the gel and a decrease in the rotational freedom of the chains in the liquid crystal, both caused by the double bonds ((1983) Biochemistry 22, 1466–1473).     

Studies of mixed-chain diacyl phosphatidylcholines with highly asymmetric acyl chains: a Fourier transform infrared spectroscopic study of interfacial hydration and hydrocarbon chain packing in the mixed interdigitated gel phase.

The mixed interdigitated gel phases of unlabeled, specifically 13C = O-labeled, and specifically chain-perdeuterated samples of 1-O-eicosanoyl, 2-O-lauroyl phosphatidylcholine and 1-O-decanoyl, 2-O-docosanoyl phosphatidylcholine were studied by infrared spectroscopy. Our results suggest that at the liquid-crystalline/gel phase transition temperatures of these lipids, there is a greater redistribution in the populations of free and hydrogen-bonded ester carbonyl groups than is commonly observed with symmetric chain n-saturated diacyl phosphatidylcholines. The formation of the mixed interdigitated gel phase coincides with the appearance of a marked asymmetry in the contours of the C = O stretching band, a process which becomes more pronounced as the temperature is reduced. This asymmetry is ascribed to the emergence of a predominant lipid population consisting of free sn1- and hydrogen-bonded (hydrated) sn2-ester carbonyl groups. This suggests that the region of the mixed interdigitated bilayer polar/apolar interface near to the sn1-ester carbonyl group is less hydrated than is the case with the noninterdigitated gel-phase bilayers formed by normal symmetric chain phosphatidylcholines. In the methylene deformation region of the spectrum, the unlabeled lipids exhibit a pronounced splitting of the CH2 scissoring bands. This splitting is significantly attenuated when the short chains are perdeuterated and collapses completely upon perdeuteration of the long chains, irrespective of whether the long (or short) chains are esterified to the sn1 or sn2 positions of the glycerol backbone. These results are consistent with a global hydrocarbon chain packing motif in which the zigzag planes of the hydrocarbon chains are perpendicular to each other and the sites occupied by long chains are twice as numerous as those occupied by short chains. The experimental support for this chain-packing motif enabled more detailed considerations of the possible ways in which these lipid molecules are assembled in the mixed interdigitated gel phase. Generally, our results are compatible with a previously proposed model in which the mixed interdigitated gel phase is an assembly of repeat units which consists of two phosphatidylcholine molecules forming a triple-chain structure with the long chains traversing the bilayer and with the methyl termini of the shorter chains opposed at the bilayer center. Our data also suggest that the packing format which is most consistent with our results and previously published work is one in which the hydrocarbon chains of each repeat unit are parallel to each other with the repeat units themselves being perpendicularly packed.     

Phase equilibria of membrane lipids from Acholeplasma laidlawii: importance of a single lipid forming nonlamellar phases

A basis for the reorganization of the bilayer structure in biological membranes is the different aggregate structures formed by lipids in water. The phase equilibria of all individual lipids and several in vivo polar lipid mixtures from acyl chain modified membranes of Acholeplasma laidlawii were investigated with different NMR techniques. All dioleoyl (DO) polar lipids, except monoglucosyldiglyceride (MGDG), form lamellar liquid crystalline (L alpha) phases only. The phase diagram of DOMGDG reveals reversed cubic (III), reversed hexagonal (HII), and L alpha phases. In mixtures of DOMGDG and dioleoyldiglycosyldiglyceride (DODGDG), the formation of an III (or HII) phase is enhanced by DOMGDG and low hydration or high temperatures. For in vivo mixtures of all polar DO lipids, a transition from an L alpha to an III phase is promoted by low hydration or high temperatures (50 degrees C). The phospholipids are incorporated in this III phase. Likewise, III and HII phases are formed at similar temperatures in a series of in vivo mixtures with different extents of acyl chain unsaturation. However, their melting temperatures (Tm) vary in an expected manner. All cubic and hexagonal phases, except the III phase with DOMGDG, exist in equilibrium with excess water. The maximum hydration of MGDG and DGDG is similar and increases with acyl chain unsaturation but is substantially lower than that for, e.g., phosphatidylcholine. The translational diffusion of the lipids in the cubic phases is rapid, implying bicontinuous structures. However, their appearances in freeze-fracture electron microscope pictures are different. The III phase of DOMGDG belongs to the Ia3d space group.(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)     

2H nuclear magnetic resonance order parameter profiles suggest a change of molecular shape for phosphatidylcholines containing a polyunsaturated acyl chain.

Solid-state 2H nuclear magnetic resonance spectroscopy was used to determine the orientational order parameter profiles for a series of phosphatidylcholines with perdeuterated stearic acid, 18:0d35, in position sn-1 and 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 in position sn-2. The main phase transition temperatures were derived from a first moment analysis, and order parameter profiles of sn-1 chains were calculated from dePaked nuclear magnetic resonance powder patterns. Comparison of the profiles at 37 degrees C showed that unsaturation causes an inhomogenous disordering along the sn-1 chain. Increasing sn-2 chain unsaturation from one to six double bonds resulted in a 1.6-kHz decrease in quadrupolar splittings of the sn-1 chain in the upper half of the chain (or plateau region) and maximum splitting difference of 4.4 kHz at methylene carbon 14. The change in chain order corresponds to a decrease in the 18:0 chain length of 0.4 +/- 0.2 A with 18:2 omega 6 versus 18:1 omega 9 in position sn-2. Fatty acids containing three or more double bonds in sn-2 showed a decrease in sn-1 chain length of 0.7 +/- 0.2 A compared with 18:1 omega 9. The chain length of all lipids decreased with increasing temperature. Highly unsaturated phosphatidylcholines (three or more double bonds in sn-2) had shorter sn-1 chains, but the chain length was somewhat less sensitive to temperature.(ABSTRACT TRUNCATED AT 250 WORDS)     

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 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)     

Bilayers of dipalmitoyl-3-sn-phosphatidylcholine: Conformational differences between the fatty acyl chains

$\text{Dipalmitoyl}\text{-3-sn-}\text{phosphatidylcholine}$ is specifically deuterated at the C-2 position of the fatty acyl chains. Using deuterium magnetic resonance it is then possible to probe the chain conformation in the vicinity the polar head groups. Three separate quadrupole splittings are observed for bilayers of 1,2[2′-²H₂] $\text{palmitoyl}\text{-3-sn-}\text{phosphatidylcholine}$, indicating that the two chains behave differently. The synthesis of phosphatidylcholines each deuterated in only one chain allows the assignment of the three resonances. It is concluded that the beginnings of the two chains have orientations parallel and perpendicular to the bilayer normal. The data further suggest the possibility of two long-lived conformations of the glycerol constituent.     

A new class of lipoxygenase inhibitor. Polyunsaturated fatty acids containing sulfur

A series of unsaturated and polyunsaturated fatty acids with a sulfur atom substituting for a methylene unit of the chain has been prepared and characterized. The syntheses were accomplished by the Wittig coupling of the ylid derived from the triphenylphosphonium salt of 9-bromononanoic acid with aldehydes containing sulfur. The newly formed double bond had predominately the natural Z geometry even when the starting aldehyde was conjugated with the sulfur atom. The sulfides 13-thia-9(Z)-octadecenoic acid ($\text{2}$), 13-thia-9(Z), 11(E)-octadecadienoic acid ($\text{5}$) and 13-thia-9(E), 11(E)-octadecadienoic acid ($\text{6}$) were readily converted into their sulfoxide derivatives by treatment with an equivalent amount of m-chloroperoxybenzoic acid. The structures of the novel compounds were confirmed by the application of ir, uv, ¹H-nmr, ¹³C-nmr, and (as methyl esters) chemical ionization mass spectrometry. Two members of this new family of fatty acids ($\text{5}$ and $\text{6}$) were found to inhibit the catalysis of the oxygenation of linoleic acid by soybean type-1 lipoxygenase. The analysis of the kinetic data for compound $\text{5}$ indicated that the type of inhibition was reversible competitive with an inhibition constant of 30 μM.     

Liposomes composed of partially hydrogenated egg phosphatidylcholines: fatty acid composition, thermal phase behavior and oxidative stability

Partially hydrogenated egg phosphatidylcholines (PHEPC) represent a new class of raw materials for liposome-based drug products. PHEPC were manufactured from native egg phosphatidylcholine (EPC) to iodine values (IV) 40, 30, 20, 10, and 1. Hydrogenation resulted in a complete loss of arachidonic acid (20:4) and docosahexaenoic acid (22:6) in IV 40 EPC and a progressive conversion of linoleic (18:2) and oleic acid (18:1) to stearic acid (18:0) at higher degrees of hydrogenation (IV 30, 20, 10, 1). Hydrogenation lead to formation of trans-fatty acid isomers maximally 18.5 mol% in IV 20 EPC. Liposomes made from IV 20, IV 10 and IV 1 EPC had marked phase transitions between 20 and 60 degrees C. PHEPC showed increased resistance to oxidation as measured by oxygen uptake during 2,2'-azobis-(2-amidinopropane) hydrochloride (AAPH) initiated accelerated oxidation. Various applications of these new materials in the manufacture of liposomes and liposome based drug products are discussed.