Bilayer interactions of ether- and ester-linked phospholipids: dihexadecyl- and dipalmitoylphosphatidylcholines

Mixed phospholipid systems of ether-linked 1,2-dihexadecylphosphatidylcholine (DHPC) and ester-linked 1,2-dipalmitoylphosphatidylcholine (DPPC) have been studied by differential scanning calorimetry and X-ray diffraction. At maximum hydration (60 wt % water), DHPC shows three reversible transitions: a main (chain melting) transition, TM = 44.2 degrees C; a pretransition, TP = 36.2 degrees C; and a subtransition, TS = 5.5 degrees C. DPPC shows two reversible transitions: TM = 41.3 degrees C and TP = 36.5 degrees C. TM decreases linearly from 44.2 to 41.3 degrees C as DPPC is incorporated into DHPC bilayers; TP exhibits eutectic behavior, decreasing sharply to reach 23.3 degrees C at 40.4 mol % DPPC and then increasing over the range 40-100 mol % DPPC; TS remains constant at 4-5 degrees C and is not observed at greater than 20 mol % DPPC. At 50 degrees C, X-ray diffraction shows a liquid-crystalline bilayer L alpha phase at all DHPC:DPPC mole ratios. At 22 degrees C, DHPC shows an interdigitated bilayer gel L beta phase (bilayer periodicity d = 47.0 A) into which approximately 30 mol % DPPC can be incorporated. Above 30 mol % DPPC, a noninterdigitated gel L beta' phase (d = 64-66 A) is observed. Thus, at T greater than TM, DHPC and DPPC are miscible in all proportions in an L alpha bilayer phase. In contrast, a composition-dependent gel----gel transition between interdigitated and noninterdigitated bilayers is observed at T less than TP, and this leads to eutectic behavior of the DHPC/DPPC system.     

Thermal phase behaviour and structure of hydrated mixtures between dipalmitoyl- and dihexadecylphosphatidylcholine

Mixtures of 1,2-dipalmitoyl- and 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine (DPPC and DHPC) in dispersion with excess water were studied by differential scanning calorimetry (DSC) and X-ray diffraction techniques. The transition parameters of the main gel-to-liquid crystalline transition show a monotonous dependence on the composition, indicating ideal miscibility of the two lipids, in keeping with the closely similar structures of the pure, hydrated lipids in the P beta' and L alpha states. The pre-transition shows a depression to a minimum temperature of 23 degrees C occurring around equimolar mixtures. Below the pre-transition temperatures, the L beta' gel phase of DPPC maintains bimolecular structure up to DHPC admixtures of 50 mol%, with adaptations in hydrocarbon chain packing and multilayer periodicity. On the side of DHPC, the interdigitated gel structure shows full solubility for DPPC up to equimolarity without major structural changes. The crystalline Lc-phase of DPPC exhibits immiscibility with DHPC, demonstrated by the fact that the subtransition is abolished already at less than 15 mol% DHPC. DHPC, below its subtransition, can accommodate up to 50 mol% DPPC within an interdigitated layer structure with unperturbed, crystalline hydrocarbon chain packing.     

High-pressure infrared spectroscopy of ether- and ester-linked phosphatidylcholine aqueous dispersions.

Infrared spectra of aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and its ether-linked analogue, 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC), were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 20 kbar. Although these two lipids differ only in the linkages to the saturated hydrocarbon chains, significant differences were observed in their barotropic behavior. Most notable were the magnitudes of the pressure-induced correlation field splittings of the methylene scissoring and rocking modes, and the relative intensities of the corresponding component bands. In the case of the scissoring mode, not only can the correlation field component band be resolved at a lower pressure in DHPC (1.2 kbar, as compared with 2.2 kbar in DPPC), but the initial magnitude of the correlation field splitting in DHPC, particularly less than 9 kbar, is significantly greater than that observed in DPPC. These differences are attributed to the presence of an interdigitated lamellar gel phase in DHPC. At all pressures where the correlation field component band delta'CH2 can be resolved, the relative peak height/intensity ratio R = I delta'/I delta is greater in DPPC than in DHPC, suggesting that this parameter may be useful as a test of interdigitation.     

High-pressure 31P NMR study of dipalmitoylphosphatidylcholine bilayers.

High-pressure 31P NMR was used for the first time to investigate the effects of pressure on the structure and dynamics of the phosphocholine headgroup in pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar aqueous dispersions and in DPPC bilayers containing the positively charged form of the local anesthetic tetracaine (TTC). The 31P chemical shift anisotropies, delta sigma, and the 31P spin-lattice relaxation times, T1, were measured as a function of pressure from 1 bar to 5 kbar at 50 degrees C for both pure DPPC and DPPC/TTC bilayers. This pressure range permitted us to explore the rich phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as gel I (P beta'), gel II (L beta'), gel III, gel IV, gel X, and the interdigitated, Gi, gel phase. For pure DPPC bilayers, pressure had an ordering effect on the phospholipid headgroup within the same phase and induced an interdigitated Gi gel phase which was formed between the gel I (P beta') and gel II (L beta') phases. The 31P spin-lattice relaxation time measurements showed that the main phase transition (LC to gel I) was accompanied by the transition between the fast and slow correlation time regimes. Axially symmetric 31P NMR lineshapes were observed at pressures up to approximately 3 kbar but changed to characteristic axially asymmetric rigid lattice lineshapes at higher pressures (3.1-5.1 kbar).(ABSTRACT TRUNCATED AT 250 WORDS)     

Packing characteristics of two-component bilayers composed of ester- and ether-linked phospholipids.

The miscibility properties of ether- and ester-linked phospholipids in two-component, fully hydrated bilayers have been studied by differential scanning calorimetry (DSC) and Raman spectroscopy. Mixtures of 1,2-di-O-hexadecyl-rac-glycero-3-phosphocholine (DHPC) with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DHPE) and of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with 1,2-di-O-hexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) have been investigated. The phase diagram for the DPPC/DHPE mixtures indicates that these two phospholipids are miscible in all proportions in the nonrippled bilayer gel phase. In contrast, the DHPC/DPPE mixtures display two regions of gel phase immiscibility between 10 and 30 mol% DPPE. Raman spectroscopic measurements of DHPC/DPPE mixtures in the C-H stretching mode region suggest that this immiscibility arises from the formation of DHPC-rich interdigitated gel phase domains with strong lateral chain packing interactions at temperatures below 27 degrees C. However, in the absence of interdigitation, our findings, and those of others, lead to the conclusion that the miscibility properties of mixtures of ether- and ester-linked phospholipids are determined by the nature of the phospholipid headgroups and are independent of the character of the hydrocarbon chain linkages. Thus it seems unlikely that the ether linkage has any significant effect on the miscibility properties of phospholipids in biological membranes.     

The phase behavior of mixed aqueous dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol.

The phases and transition sequences for aqueous dispersions of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron microscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperature over the range 20 degrees-90 degrees C. It is concluded that DPPC and 1,2-DPG form two complexes in the gel phase, the first one with a DPPC/1,2-DPG molar ratio of 55:45 and the second one at a molar ratio of approximately 1:2, defining three different regions in the phase diagram. Two eutectic points are postulated to occur: one at a very low 1,2-DPG concentration and the other at a 1,2-DPG concentration slightly higher than 66 mol%. At temperatures higher than the transition temperature, lamellar phases were predominant at low 1,2-DPG concentrations, but nonlamellar phases were found to be predominant at high proportions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixtures was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high lamellar repeat spacings seen by x-ray diffraction. Ripple phase characteristics were also found in the fluid lamellar phases occurring at concentrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fourier transform infrared spectroscopy of the dehydration of the lipid-water interface induced by the presence of 1,2-DPG. The biological significance of the presence of diacylglycerol in membrane lipid domains is discussed.     

Effects of the local anesthetic tetracaine on the structural and dynamic properties of lipids in model membranes: a high-pressure Fourier transform infrared study

High-pressure Fourier transform infrared (FT-IR) spectroscopy was used to study the effects of a local anesthetic, tetracaine, on the structural and dynamic properties of lipids in model membranes. The model membrane systems studied were multilamellar aqueous dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) in the absence and presence of a physiological concentration of cholesterol (30 mol %). The infrared spectra were measured at 28 degrees C in a diamond anvil cell as a function of pressure up to 25 kbar. The results indicate that the effects of tetracaine on the structure of pure DMPC bilayers in the gel state are dependent on the state of charge of the anesthetic. The uncharged tetracaine disorders the lipid acyl chains while the charged form induces the formation of an interdigitated gel phase. The presence of cholesterol in the latter system prevents the formation of the interdigitated phase, whereas in the former system it disorders the lipid acyl chains in the gel state. Moreover, it is shown that the addition of uncharged tetracaine to interdigitated DHPC bilayers does not alter the interdigitated state of the hydrocarbon chains.     

Effects of hydrostatic pressure on the molecular structure and endothermic phase transitions of phosphatidylcholine bilayers: a Raman scattering study

The temperature dependences of the Raman spectra of aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were monitored at different but constant pressures between 1 and 1210 bar. The changes observed in these Raman spectra are discussed in terms of the effects of high pressure on the phase state and molecular structure of lipid bilayers. It is demonstrated that the temperature of the endothermic gel to liquid-crystal phase transition, as well as the temperature of the pretransition, increases linearly with increasing hydrostatic pressure. The dTm/dP values obtained from a wide range of pressures are 20.8 degrees C X kbar-1 for DPPC and 20.1 degrees C X kbar-1 for DMPC. The dTp/dP value for DPPC is 16.2 degrees C X kbar-1. It is also shown that the volume change that occurs at the gel to liquid-crystal transition is not constant; i.e., d delta Vm/dP decreases by 6.2% (DPPC) or 6.3% (DMPC) per kilobar pressure. The volume change at the pretransition is also pressure dependent; the d delta Vp/dP value of DPPC decreases by 4.7% per kilobar pressure.     

Rapid reversible formation of a metastable subgel phase in saturated diacylphosphatidylcholines.

Formation of well ordered lamellar subgel (SGII) phase in aqueous dispersions of L-dipalmitoylphosphatidylcholine upon cooling from the lamellar gel phase, without low-temperature equilibration, is observed in real time using synchrotron x-ray diffraction. It has the same lamellar repeat period as the gel phase from which it was formed but differs in its wide-angle diffraction pattern. The SGII phase forms at about 7 degrees C upon cooling at 2 degrees C/min. In temperature jump experiments at 1 degree C/s from 50 to -5 degrees C, the relaxation time of the lamellar gel-SGII transition is found to be approximately 15 s. The conversion between the lamellar gel and SGII phase is cooperative and rapidly reversible. Upon heating, it coincides in temperature with an endothermic event with a calorimetric enthalpy of 0.35 kcal/mol, the so-called sub-subtransition. Similar sub-subtransitions are also observed calorimetrically at temperatures approximately 10 degrees C below the subtransition, without low-temperature storage, in aqueous dispersions of L-dimyristoylphosphatidylcholine and L-distearoylphosphatidylcholine, but not in racemic DL-dipalmitoylphosphatidylcholine. The formation of the equilibrium lamellar crystalline Lc phase appears to take place only from within the SGII phase.     

Deuterium NMR investigation of ether- and ester-linked phosphatidylcholine bilayers

Deuterium nuclear magnetic resonance (2H NMR) spectra of specifically head-group- and chain-deuterated ester- and ether-linked phosphatidylcholine bilayers were studied as a function of temperature over the range -33 to 50 degrees C. Head-group-deuterated dihexadecylphosphatidylcholine ([alpha-2H2]DHPC) bilayers yield line shapes and spin-lattice relaxation times similar to those observed for its ester-linked counterpart, dipalmitoylphosphatidylcholine ([alpha-2H2]DPPC), in the high-temperature ripple and L alpha bilayer phases. These results indicate the ether linkage has no effect on the dynamics or the orientational order at the alpha-C2H2 segment of the phosphocholine head group. At all temperatures, the 2H NMR spectra of chain-deuterated 1,2[1',1'-2H2]DHPC bilayers exhibit a reduced spectral width compared to 1,2[2',2'-2H2]DPPC bilayers. The most significant feature of the deuterated alkyl chain spectrum of DHPC at 45 degrees C is the observation of four separate quadrupolar splittings from the alpha-methylene segments of the alkyl chains, in comparison to the three quadrupolar splittings reported previously from the alpha-methylene segments of the acyl chains of DPPC. Spin-lattice relaxation experiments performed on DHPC suggest an assignment of the two smaller and the two larger quadrupolar splittings to separate alkyl chains, respectively. Low-temperature (T less than or equal to -20 degrees C) gel-phase spectra of deuterated head-group [alpha-2H2]DHPC remain an order of magnitude narrower than those observed for [alpha-2H2]DPPC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure and thermodynamics of the dihexadecylphosphatidylcholine-water system

X-ray small- and wide-angle diffraction, differential scanning calorimetry (DSC), temperature scanning densitometry (TSD) and electron microscopy were used to study the lyotropic and thermotropic properties of the system 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine-water over a wide range of compositions from the dry lipids to a large excess of water, and in the temperature range between 0 degrees C and 150 degrees C. The results were used to construct a temperature-composition phase diagram. The phases have been characterized with respect to their molecular arrangements and hydrocarbon chain packing subcells. In the fully hydrated state (greater than 45 wt% H2O) four thermotropic phases were found, with readily reversible transitions at 5 degrees C, 32.5 degrees C and 43.6 degrees C, respectively. The two lower temperature phases deviate from all others in consisting of bilayers with fully interdigitated hydrocarbon chains, while above 32.5 degrees C the structures resemble closely those of the analog diester lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). At hydrations between 30 and 45 wt% H2O, and below 32 degrees C, interdigitated and non-interdigitated multilayers coexist in one coherent phase. A bilayer tilting mechanism is proposed for the formation of this coexistence of two regular structures. Below 30 wt% H2O, hydrated 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) exists in lamellar, non-interdigitated bilayers, showing very weak interbilayer swelling. There, the water molecules appear to occupy voids between the polar headgroups.     

Phospholipids chiral at phosphorus. Characterization of the sub-gel phase of thiophosphatidylcholines by use of X-ray diffraction, phosphorus-31 nuclear magnetic resonance, and Fourier transform infrared spectroscopy

A recent study using differential scanning calorimetry (DSC) showed that the thermotropic phase behavior of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DPPsC) is sensitive to the configuration at phosphorus and that the Rp isomer displayed only a broad transition at 45.6 degrees C [Wisner, D. A., Rosario-Jansen, T., & Tsai, M.-D. (1986) J. Am. Chem. Soc. 108, 8064-8068]. We have employed X-ray diffraction, 31P NMR, and Fourier transform infrared (FT-IR) spectroscopy to characterize various phases of the isomers of DPPsC, to compare the structural differences between 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and isomers of DPPsC, and to identify structural factors responsible for the unique behavior of the RP isomer. The results from all three techniques support the previous proposal based on DSC studies that (SP)- and (RP + SP)-DPPsC undergo a subtransition, a pretransition, and a main transition analogous to those of DPPC, while (RP)-DPPsC is quite stable at the subgel phase and undergoes a direct subgel----liquid-crystalline transition at 46 degrees C. Quantitative differences between DPPC and DPPsC (i.e., the effect of sulfur substitution rather than the configurational effect) in the subgel phase have also been observed in the chain spacing, the motional averaging, and the factor group splitting (revealed by X-ray diffraction, 31P NMR, and FT-IR, respectively). In particular, DPPsC isomers are motionally rigid and show enhanced factor group splitting in the subgel phase. These results suggest that DPPsC is packed in different subcells relative to DPPC in the subgel phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polymorphic phase behavior of platelet-activating factor.

Vibrational Raman and 31P NMR spectroscopic experiments have been performed as a function of temperature on aqueous dispersions of 1-0-octadecyl-2-acetoyl-sn-glycero-3-phosphocholine, a chemically synthesized platelet-activating factor. In the temperature range of -7 to 30 degrees C, the C(18)/PAF-H2O system is shown, upon heating, to undergo two thermal phase transitions centered at 9.2 degrees and 18.4 degrees C. The low temperature transition, attributed to the interdigitated lamellar gel (II)----gel (I) phase transition, is characterized by the breakdown of large lamellar organizations into small, but aggregated, bilayer vesicles. The high-temperature transition corresponds to the interdigitated lamellar gel (I)----micellar transition. The molecular ordering and packing structure of C(18)/PAF in the two lamellar phases and phase transition regions are described. It appears that the interdigitated lamellar gel (I) phase is unique for C(18)/PAF dispersions when compared with the behavior of other chemically closely related phospholipids in excess water.     

Comparisons of lipid dynamics and packing in fully interdigitated monoarachidoylphosphatidylcholine and non-interdigitated dipalmitoylphosphatidylcholine bilayers: cross polarization/magic angle spinning 13C-NMR studies

13C-NMR spectra have been obtained at 50.3 MHz for monoarachidoylphosphatidylcholine (MAPC) and dipalmitoylphosphatidylcholine (DPPC) dispersions from 25 degrees C to 55 degrees C and for DPPC polycrystals at 25 degrees C using the cross polarization/magic angle spinning technique. Differential scanning calorimetric studies on DPPC and MAPC dispersions show comparable lipid phase transitions with transition temperatures at 41 degrees C and 45 degrees C, respectively, and thus enable the comparison of thermal, structural and dynamic differences between these two systems at corresponding temperatures. Conformational-dependent 13C chemical shift studies on DPPC dispersions demonstrate not only the coexistence of the tilted gel (L beta') and liquid-crystalline (L alpha) phases in the rippled gel (P beta') phase, but also the presence of an intermediate third microscopic phase as evidenced by three resolvable peaks for omega - 1 methylene carbon signals at the temperature interval between Tp and Tm. By comparing chemical shifts of MAPC in the hydrocarbon chain region with those of DPPC at similar reduced temperatures, it can be concluded that the packings are perturbed markedly in the middle segment of the fatty acyl chain during the lamellar to micellar transition. However, terminal methylene and methyl groups of interdigitated MAPC lamellae were found to be more ordered than those of non-interdigitated DPPC bilayers in the gel state. Interestingly, the terminal methyl groups of MAPC in the micelles remain to be relatively ordered; in fact, they are more ordered than the corresponding acyl chain end of DPPC in the liquid-crystalline state. Analysis of data obtained from rotating frame proton spin-lattice relaxation measurements shows a highly mobile phosphocholine headgroup, a rigid carbonyl group and an ordered hydrocarbon chain for lamellar MAPC in the interdigitated state. Furthermore, results suggest that free rotations of the glycerol C2-C3 bond within MAPC molecules may occur in the interdigitated bilayer, whereas intramolecular exchange between two conformations of the glycerol backbone in DPPC become possible at temperatures close to the pretransition temperature. Without isotope enrichment, we conclude that high-resolution solid-state 13C-NMR is indeed a useful technique which can be employed to study the packing and dynamics of phospholipids.     

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

While hydrated dipalmitoyl phosphatidylcholine (DPPC) forms tilted chain L beta' bilayers in the gel phase, the ether-linked analogue dihexadecyl phosphatidylcholine (DHPC) exhibits gel phase polymorphism. At low hydration DHPC forms L beta' phases but at greater than 30% H2O a chain-interdigitated gel phase is observed (Ruocco, M. J., D. S. Siminovitch, and R. G. Griffin. 1985. Biochemistry. 24:2406-2411; Kim, J.T., J. Mattai, and G.G. Shipley. 1987. Biochemistry. 26:6599-6603). In this study we report the behavior of a phosphatidylcholine (PC) with both types of chain linkage, 1-hexadecyl-2-palmitoyl-sn-glycero-3-phosphocholine (HPPC). HPPC has been investigated as a function of hydration using differential scanning calorimetry (DSC) and x-ray diffraction. By DSC, over the hydration range 5. 1-70.3 wt% H2O, HPPC exhibits two reversible transitions. The reversible main chain-melting transition decreases from 69 degrees C, reaching a limiting value of 40 degrees C at full hydration. X-ray diffraction patterns of hydrated HPPC have been recorded as a function of hydration at 20 degrees and 50 degrees C. At 50 degrees C, melted-chain L alpha bilayer phases are observed at all hydrations. At 20 degrees C, at low hydrations (less than 34 wt% H2O) HPPC exhibits diffraction patterns characteristic of bilayer gel phases similar to those of the gel phase of DPPC. In contrast, at greater than or equal to 34 wt% H2O, HPPC shows a much reduced bilayer periodicity, d = 47 A, and a single sharp reflection at 4.0 A in the wide angle region. This diffraction pattern is identical to that exhibited by the interdigitated phase of DHPC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pressure Locking of the Subgel Phase of Hydrated Dipalmitoyl Phosphatidylcholine Bilayers: A Raman Spectroscopic Study

Raman spectra of aqueous dispersions of 1,2-dipalmitoyl-phosphatidylcholine (DPPC) have been measured as a function of pressure (up to 46 kbar) for samples incubated at 2°C and for nonincubated DPPC samples subjected to equally high pressure. The nature of the transition from the GII gel phase of the hydrated lipid into the subgel phase on incubation is entirely different from that of the transition from the GII gel phase into the GIII gel phase of the nonincubated lipid. The GIII gel phase has a monoclinic interchain packing, while the subgel phase exhibits a triclinic interchain structure. It is shown that pressure cannot induce the transition from the GII gel phase to the subgel phase; however, it does stabilize the subgel phase above the subtransition temperature. The mechanism for the formation of the subgel phase and the complex phase behavior of the gel phase of DPPC are rationalized in terms of the dynamic properties of the acyl chains of the lipid molecule.     

On the subtransitions of deuterated derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine

By means of Fourier transform infrared spectroscopy the subtransition temperature of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the derivatives in which either or both of the acyl chains were deuterated were determined. It was found that, while the temperatures of the gel to liquid-crystal and the pre-transitions decrease with increasing deuteration, the temperature of the subtransition is independent of the degree of deuteration.     

The structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine codispersed with a branched-chain phosphatidylcholine

The structure and thermotropic phase behaviour of a fully hydrated binary mixture of dipalmitoylphosphatidylcholine and a branched-chain phosphatidylcholine, 1, 2-di(4-dodecyl-palmitoyl)-sn-glycero-3-phosphocholine, were examined using differential scanning calorimetry, synchrotron X-ray diffraction and freeze-fracture electron microscopy. The branched-chain lipid forms a nonlamellar phase when dispersed alone in aqueous medium. Mixed aqueous dispersions of the two phospholipids containing less than 33 mol% of the branched-chain lipid form lamellar phases over the whole temperature range were studied (4 degrees C to 60 degrees C). When present in proportions greater than 33 mol% it induces a hexagonal phase in mixed aqueous dispersions with dipalmitoylphosphatidylcholine at temperatures above the fluid phase transition. At temperatures below 35 degrees C a hexagonal phase coexists with a gel bilayer phase. The lamellar<-->nonlamellar transition can be explained satisfactorily on the basis of the shape of the molecule expressed in terms of headgroup and chain cross-sectional areas. At temperatures below 35 degrees C macroscopic phase separation of two gel phases takes place. Freeze-fracture electron microscopy revealed that one gel phase consists of bilayers with a highly regular, periodic superstructure (macro-ripples) whereas the other phase forms flat, planar bilayers. The macro-ripple phase appears to represent a relaxation structure required to adapt to the packing constraints imposed by the incorporation of the branched-chain lipid into the dipalmitoylphosphatidylcholine host bilayer. The data suggest that structural changes that take place on cooling the mixed dispersion below the lamellar<-->nonlamellar phase transition temperature cannot be adequately described using the molecular form concept. Instead it is necessary to take into account the detailed molecular form of the guest lipid as well as its physical properties.     

31P NMR spectra of rod outer segment and sarcoplasmic reticulum membranes show no evidence of immobilized components due to lipid-protein interactions

31P NMR studies of rod outer segment (ROS) and sarcoplasmic reticulum (SR) membranes have been performed under conditions where broad and narrow spectral components can be clearly resolved. Control studies of an anhydrous, solid powder of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), as well as aqueous binary mixtures of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), demonstrate clearly that broad spectral components can be detected. For the codispersions of DSPC and DOPC in the mixed-phase region at 22 degrees C, the 31P NMR spectra consist of a superposition of a broad component and a narrow, axially symmetric component, due to coexisting solid and liquid-crystalline domains, which are in slow exchange on the 31P NMR time scale. The 31P NMR spectra of the native ROS and SR membranes, however, consist of only a narrow component, to within experimental error, indicating that most or all of the phospholipids are in the liquid-crystalline (L alpha) phase at 22 degrees C. The above conclusions are in agreement with many, but not all, previous studies [see, e.g., Yeagle, P.L. (1982) Biophys. J. 37, 227-239]. It is estimated that at most 10% of the phospholipids in the ROS and SR membranes could give rise to broad 31P NMR spectral components, similar to those seen for anhydrous or solid-phase lipids, corresponding to approximately 7 phospholipids/rhodopsin molecule and approximately 11 phospholipids/Ca2+-ATPase molecule, respectively.     

Interaction of alamethicin with ether-linked phospholipid bilayers: oriented circular dichroism, 31P solid-state NMR, and differential scanning calorimetry studies

The arrangement of the antimicrobial peptide alamethicin was studied by oriented circular dichroism, 31P solid-state NMR, and differential scanning calorimetry in ether-linked phospholipid bilayers composed of 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine (DHPC). The measurements were performed as a function of alamethicin concentration relative to the lipid concentration, and results were compared to those reported in the literature for ester-linked phospholipid bilayers. At ambient temperature, alamethicin incorporates into the hydrophobic core of DHPC bilayers but results in more lipid disorder than observed for ester-linked 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid bilayers. This orientational disorder appears to depend on lipid properties such as bilayer thickness. Moreover, the results suggest that alamethicin inserts into the hydrophobic core of the bilayers (at high peptide concentration) for both ether- and ester-linked lipids but using a different mechanism, namely toroidal for DHPC and barrel-stave for POPC.