Determination of n-alkane partitioning within phosphatidylethanolamine Lα/HII phases (original) (raw)
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Biochimica et Biophysica Acta (BBA) - Biomembranes, 1984
increasing T m at high concentrations. Our X-ray diffraction data show that this reversal of T m is a consequence of the induction of an unusual gel phase, where the lipid hydrocarbon chains from apposing monolayers fully interpenetrate or interdigitate. The properties of this interdigitated phase also explain the lipid chain length dependence of the reversal in the T m versus ethanol concentration curves and the narrow width of the transition at high ethanol concentrations, as well as spectroscopic and calorimetric data from lipid suspensions containing other drugs such as methanol, benzyl alcohol, phenyl ethanol, and cldorpromazine.
Curvature, order, and dynamics of lipid hexagonal phases studied by deuterium NMR spectroscopy
Biochemistry, 1993
Solid-state deuterium (2H) N M R spectroscopy enables one to study both equilibrium and dynamical properties of membrane constituents at the molecular level and can yield significant insights regarding the organization of non-bilayer lipid aggregates. We have investigated a representative unsaturated phosphatidylethanolamine, viz., 1-perdeuteriopalmitoy1-2-linoleoyl-sn-glycero-3-phosphoethanolamine, in the lamellar, or La, phase and the reversed hexagonal, or HII, phase. Phosphorus-31 ( 3 1 P ) N M R studies of PLPE-d31 in the HII phase revealed that the chemical shift anisotropy of the phosphoethanolamine head groups, ACT, was scaled by the expected geometrical factor of -1 /2 relative to the lamellar state. However, we found the occurrence of a further reduction in the 2H N M R quadrupolar splittings, AVQ, of the 2H-labeled palmitoyl acyl chain segments. These observations point toward the role of interfacial curvature with regard to properties of reverse hexagonal phase lipids, and indicate that the pivotal position or neutral surface of approximately constant area may lie near the glycerol or polar head group region. Variations in the acyl chain packing due to curvature of the aqueous interface yield significant differences in the segmental order profiles as determined by 2H N M R spectroscopy. The latter reflect the local orientational order of the acyl chains and can be used together with simple statistical theories to extract positional or structural information. Average projected acyl chain lengths and mean interfacial or crosssectional areas for PLPE-d31 in the different phases have been calculated. In addition, we describe a new means of estimating the radius of curvature of HII phase lipid aggregates utilizing 2H N M R spectroscopy, which is based on the difference between the lamellar and hexagonal phase order profiles. Here the radius of curvature, R,, is defined as the distance from the center of the water core to the lipid/water interface, near the carbonyl segments of the acyl chains, giving R, = 25.4-28.1 A for PLPE-831 in the HII phase at 60 OC. This value is in good agreement with previous X-ray diffraction studies of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Alternatively, the data yield for the radius of the central water core that R, = 17.8-20.5 A at 60 "C. The differences in geometry also lead to higher quadrupolar echo relaxation rates (R2J for the lipid acyl segments closest to the aqueous interface in the HII versus the La phase. We propose that this enhancement is due to an additional relaxation mechanism found in the hexagonal phases, namely, translational diffusion of lipids about the cylinder axes. For comparison, the normal hexagonal (HI) and lamellar (La) phases of a lyotropic system comprising perdeuterated potassium laurate were also studied. This research indicates clearly that the packing and dynamical properties of the acyl chains of phospholipids depend on the curvature of the aqueous interface and, thus, the aggregate geometry. The latter is related to the average shape of lipids in their respective phases and to the curvature free energy, which in the planar state may influence protein-mediated functions of membranes.
Biochemistry, 1990
The quadrupolar splitting profiles of methylene groups along the acyl chains of perdeuteriated dimyristoylphosphatidylcholine (DMPC-dS4) in mixtures with dioleoylphosphatidylethanolamine (DOPE) were studied by 2H N M R. The quadrupolar splittings, obtained for lipid mixtures in the bilayer state, were measured as functions of temperature and PE:PC ratio and were used to obtain the approximate gauche probabilities a t a given chain position, pB. Ratios (R) ofpB for C13, C12, and C1 1 relative to that of the plateau region were used to characterize the effect of increasing P E on the gauche content of PC chains. At all temperatures studied (including the bilayer to hexagonal phase transition region), for each ratio R (e.g., RC13,P), the relative gauche content of the D M P C chains was similar over the range of 2 5 4 5 % PE. DOPE is viewed in simple terms as having a "conical" shape; if this geometry applies to the acyl chain region of the molecule, a greater lateral pressure would be expected toward the center of the bilayer as the P E content is increased, resulting in a decreased gauche content, relative to the plateau, of those methylene groups of PC. The failure to observe the predicted increase in lateral pressure has ramifications for the cone-shape molecular model. The overall "cone shape" of P E is seen to arise from the smaller size of the head-group relative to the acyl chains; however, the acyl chain region itself is not rigidly cone-shaped and is better represented by a flexible ''balloon''. These results were supported by small-angle X-ray diffraction, which showed a decreasing trend in the area per molecule with increasing P E content.
The Journal of Physical Chemistry, 1991
only when the membrane processes are concentration independent. Appendix 11. Parameters for a Uniform, Consuming Membrane (Eqs 20 and 27) We consider the generalized diffusion/reaction eq 22 to hold in the membrane, with initial and boundary conditions given by eqs 20. Taking Laplace transforms, these equations become a2z s Z = D-i Z O < x < h ax2 Z(x=O,s) = K C / s f(x=h,s) = 0 The solution of these equations is KC* sinh 4(xh) s sinh 4h Z(x,s) =-where 4 = ((s + 7) / D) ' / 2 The elements of the admittance matrix corresponding to forward transport, Yll(s) and Y12(s), are defined by Y , , (S) = (-D ar/ax),,, = D K~ coth qh (11.1.1) Y,,(s) = (D ar/ax),,, =-D K~ csch qh (11.1.2) where the absence of the minus sign in the second member of the latter equation is due to the convention that flux into the membrane is positive. Since we are considering a uniform membrane, we must also have Y22(s) = Yll(s) and Ytl(s) = Y I 2 (s) .
Chemistry and Physics of Lipids, 1989
The orientational order and rotational dynamics of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-l,3,5-hexatrienyl)phenyl]ethyl] carbon yl]-3-sn-phosphatidylchofine (DPH-PC) in dilinoleoylphosphatidylethanolamine (DLPE) and 1-palmitoyl-2-oleoylphosphatidylcholine 0aOPC) binary lipid mixtures were investigated. A previous study (Biochim. Biophys. Acta 731 (1983) 177) indicated that the empirical phase diagram of POPC/DLPE can roughly be divided into three zones. They are the lamellar (15% PC and higher), intermediate (5-15% PC) and inverted hexagonal (0-50/0 PC) phases. As the lipids changed from the lamellar to intermediate phase, the order parameter increased at all temperatures (1-50°C). On the contrary, the rotational diffusion decreased at high temperatures (20-50°C) but increased at low temperatures (I-10°C). These results indicate that the intermediate phase is in a stressed state at high temperatures but in a highly mobile amorphous state at low temperatures. As the lipid progressed from the intermediate toward hexagonal phase, the order parameter decreased abruptly at all temperatures. The ratio of order parameter in the intermediate phase to that in the hexagonal phase was calculated. This ratio was found to increase linearly with temperature, indicating that a distinct change in the packing symmetry of lipids occurred as temperature increased. From the intermediate to hexagonal phase, the rotational diffusion increased slightly at high temperatures but declined abruptly at low temperatures. These results further agreed with the stressed and amorphous natures of the intermediate phases as described above.
Chemistry and Physics of Lipids, 1996
The effect of 1-palmitoyl lysophosphatidylcholine (PLPC) on the phase behaviour of 1,2-dipalmitoyl phosphatidylethanolamine (DPPE) in excess water (34 wt%) has been examined by differential scanning calorimetry, scanning dilatometry and isothermal compressibility measurements. Mole percentages of PLPC in DPPE between 14 and 62% have been studied over the temperature range 30-75°C. The temperature dependence of orientational ordering at selected sites in 2H-labelled PLPC and 2H20 has been determined from measurement of time-averaged chemical shift anisotropies and quadrupole splittings in the 3~p_ and 2H-NMR spectra. These data have been used to further characterize phase behaviour. At less than equimolar contents of PLPC, when a single phase transition with a reduced transition temperature is observed, spectral and calorimetric data indicate complete miscibility of the two lipid components. An equimolar mixture of PLPC and DPPE shows a sharp first order transition at 47.3°C and a second order transition at 62.5°C. NMR data are consistent with the existence of a defective bilayer at intermediary temperatures. In this range it is proposed that PLPC molecules prefer regions with high curvature in the vicinity of the defects, while DPPE molecules are mostly confined to flatter regions of the bilayer. A possible molecular model is described. At temperatures above 62.5°C, PLPC and DPPE are completely miscible and exist as lamellae. At higher PLPC content ( > 50 mol"/,,), thermodynamic and spectral data are indicative of phase separation of the two components over the temperature range examined.
Theoretical models of phospholipid systems have indicated that both intramolecular and intermolecular forces are important in governing their acyl chain order. Knowledge of the nature and magnitude of these interactions is central to understanding the balance of forces present in lipid lamellar phases, which in turn is related to their microscopic and macroscopic behavior. It is possible to explore the contribution of intermolecular interactions using lipid systems with the same headgroup and acyl chain identity by variation of the ratio of the headgroups to acyl chains. In this paper, deuterium (2H) NMR spectroscopy has been used to gain information on the orientational order of an acyl chain perdeuterated lipid, l-perdeuteriopalmitoyl-sn-Bly~3-phosphocholine (PaLpc-d31), in various molecular environments. The orientational order of PaLpGd31 was studied in four different lamellar phases, including pure PaLPC-d31 (containing 10 wt % H20), diplmitoyl-phosphatidyl~holine/PaLPC-d3~ (3:1), palmitic a~id/PaLPC-d~~ (l:l), and ch~lesterol/PaLPC-d~~ (1:l) (each containing 50 wt % HzO). ' H NMR spectra were obtained for the low-temperature and liquid-crystalline (L,) states of each of these mixtures. In the low-temperature state, the f i t three systems yielded ZH NMR spectra characteristic of all-trans chains undergoing axial diffusion, with the methyl groups rotating about their C3 axes. The molecular order, as judged by the presenoe of spectral discontinuities and moment analysis, was found to be almost identical in the low-tempratwe phases. A different behavior was observed for the ch~lesterol/PaLPC-d~~ (1:l) sample in that the maximum splitting was close to the all-trans rotating value, with a profile of quadrupolar splittings due to inmased disorder near the chain ends. The f i t three systems underwent orderdisorder phase transitions near the same midpoint temperature (range of T, values 40-48 "C), whereas the ch~lesterol/PaLPC-d~~ (1:l) sample did not display a transition over the temperature range studied. In the L, phase, where order profdm were determined as a function of acyl chain segmglt position, the segmental ordering d i f f e r e d significantly among the samples. The differences were interpreted using a simple diamond lattice model for the acyl chain configurational statistics, as a means of comparing the effective lengths, (L), projected along the bilayer normal and estimated chain cross-sectional areas, (A), of PaLPC-d31 in the various mixtures. The derived values of (L) and (A) can be understood qualitatively in terms of average packing parameters related to the balance of forces in the headgroup and acyl chain regions, or alternatively the curvature free energy of the membrane lipid-water interface. In lamellar phases of pure P~L P C-C~~~ the curvature stnap is potentially large, and interdigitation of the acyl chains of the appoaed monolayers may occur. However, in mixtures of PaLPC-d31 with 1 , 2-d i p a l m i t o y l-s n-B l y~~3-p h o s p~~~e (DPPC), the curvature elaatic stress is apparently relieved by an increase in the cross-sectional acyl chain area, (A), Le. corresponding to an increase in configurational f d o m. The data were also compared to the results of statistical theories to yield additional knowledge of the intermolecular forces. These studies indicate how the segmental ordering reflects intermolecular interactions within a given lamellar phase. Avmge properties of the entire system such as average cross-sectional arm accessible to each acyl chain relative to the headgroup area can be modulated by these interactions. Such intermolecular interactions may be related to the prcaence of lipid diversity in biological membranes.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1988
The effect el mono., di. and triaeyiglyeerols on the bilayer to hexagonal phase (1-1 n) transition was studied by. differential sc~min 8 calerimetry and StP.NMR spectresce~. The acylglycemls were mixed with either dlelaidoyl~ylethanoline or with l.palmitoyl-2.oleeyll~letbmohmine. Acylglycerols of laurie, olek and shmrte acids were mgized. All el the aeylglyeerols lowered the bilayer to H u phase transition ~.
Chemistry and Physics of Lipids, 1994
The bilayer stability of liposomes containing unsaturated phosphatidylethanolamine (PE) has been investigated by measuring the C~O and CH 2 stretching frequencies of the lipids at different lipid compositions and temperatures. Binary mixtures of 1-palmitoyl-2-oleoylphosphatidyleholine (POPC) and dilinoleoyl-PE (DLPE) are known to exhibit lamellar liquid crystalline (L,), inverted hexagonal (Hn) and metastable intermediate (I) phases. Abrupt increases in the C~-O and CH2 stretching frequencies at 65-75 and 90-95% PE, respectively, were found as the PE content of the DLPE/POPC mixtures was increased from 0 to 100%. These transitions were associated with the La-I and I-HII phase transitions of the DLPE/POPC mixtures, accordingly. The effects of three lipid perturbants, butylated hydroxytoluene (BHT), diacylglyeerol (DG) and cholesterol (CL), on the above L~-I and I-H n transitions were also examined. All perturbants were found to be effective in shifting the L~-I transition of the DLPC/POPC mixtures to a lower PE% as detected by the C=O stretching frequency measurements. On the other hand, the perturbants appeared to eliminate the I-Hn transition of the DLP~POPC mixtures as detected by the CH2 stretching frequency measurements. The effectiveness of the perturbants in promoting the La-I phase transition of the DLPE/POPC mixtures followed the order of DG > BHT > CL. The phase boundaries of this L~-I transition at different lipid and perturbant compositions were further quantitated by the use of three-dimensional contour plots (Cheng, Chem. and Phys. Lipids, 60, 119-125 (1991)) of the C~O frequency as a function of DLPE/POPC and perturbant/lipid compositions. It was concluded that the lipid C=O vibrational frequency is a useful and non-invasive parameter for examining the bilayer stability of liposomes.
Low amounts of PEG-lipid induce cubic phase in phosphatidylethanolamine dispersions
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1997
Ž . By using time-resolved X-ray diffraction we demonstrate that low amounts 5-10 mol% of a phospholipid with two saturated hydrocarbon acyl chains 14 carbon atoms long and PEG550 chain covalently attached to its phosphoethanolamine Ž . Ž polar head group, DMPE PEG550 , induce spontaneous formation of a cubic phase with lattice constant 20.5 nm cubic . Ž. aspect a8, space group Im3m in aqueous dispersions of dielaidoylphosphatidylethanolamine DEPE . This phase displays a highly resolved X-ray diffraction pattern with 17 low-angle reflections. The cubic phase was found to intrude in the Ž . Ž . temperature range between the lamellar liquid crystalline L phase and the inverted hexagonal phase H known to form a II Ž . in pure DEPErwater dispersions. A higher DMPE PEG550 amount of 20 mol% was found to eliminate the non-lamellar phases in the temperature scale up to 1008C. DMPE grafted with PEG5000 only shifts the L -H transition of DEPE to a II higher temperatures but does not promote formation of cubic phase. These findings indicate that, consistent with their bulky head groups, the PEG-lipids decrease the tendency for negative interfacial mean curvature of the DEPE bilayers.