An analysis of the X-ray interchain peak profile in dipalmitoylglycerophosphocholine

Phase behavior of a binary lipid system containing long- and short-chain phosphatidylcholines

A new phase state, named the U phase, was observed in DPPC–diC8PC mixtures at low DPPC contents.

Regional cooperativity in the phase transitions of dipalmitoylphosphatidylcholine bilayers: the lipid tail triggers the isothermal crystallization process

We have a long-standing interest to explore the answer of the question: Which part of the amphiphilic molecule triggers the phase transition of the self-assembled aggregates consisting of these amphiphiles? This is an important issue regarding the phase transition kinetics of amphiphiles. To this end, we studied the phase transition behaviors of dipalmitoylphosphatidylcholine (DPPC) by differential scanning calorimetry, synchrotron X-ray scattering, Fourier transform infrared spectroscopy, and image analysis. We found that different parts (head, interface, and tail) of DPPC molecules all exhibit nonsynchronous changes during the sub-, pre-, and main transitions. Particular efforts have been devoted to studying the isothermal subgel (L(c')) formation process. It was found that only the lipid interface and tail regions change, and only when the rearrangement of the lipid hydrocarbon chain packing reaches a certain extent can the interfacial C═O groups be induced to undergo vibrational environment changes. The result means that the hydrocarbon tail is the part that triggers the gel (L(β')) to L(c') phase transition. The present work deepens our understanding on the phase transition mechanisms of DPPC and may shed light on those of other phospholipids and other types of amphiphiles.

An alternative interpretation of nuclear magnetic resonance observations in the gel state of lipid bilayers

In the established interpretation of nuclear magnetic resonance (NMR) spectra of phospholipid bilayers in the gel state, the molecules are assumed to perform rotational diffusion about their long axis. Here we present an alternative model of the molecular mobility in this phase, which considers the positions of the lipid molecules in the two-dimensional bilayer lattice as fixed within the NMR timescale. Instead we assume an intramolecular two-site hopping of the hydrocarbon chains about their long axis. It is shown that deuterium NMR spectra of chain-labeled compounds are very sensitive to the precise angle of this flip-flop motion near 90 degrees , so that the diversity of these gel-phase spectra is easily explained by slight variations of this angle. In addition, it is argued that the axial symmetry of (13)C spectra of carbonyl-labeled phospholipids might also result from this intramolecular mobility.

Pretransitional effects in dimyristoylphosphatidylcholine vesicle membranes: optical dynamometry study

We used micron-sized latex spheres to probe the phase state and the viscoelastic properties of dimyristoylphosphatidylcholine (DMPC) bilayers as a function of temperature. One or two particles were manipulated and stuck to a DMPC giant vesicle by means of an optical trap. Above the fluid-gel main transition temperature, T(m) congruent with 23.4 degrees C, the particles could move on the surface of the vesicle, spontaneously (Brownian motion) or driven by an external force, either gravity or the laser beam's radiation pressure. From the analysis of the particle motions, we deduced the values of the membrane hydrodynamic shear viscosity, eta(s), and found that it would increase considerably near T(m). Below T(m), the long-distance motion of the particles was blocked. We performed experiments with two particles stuck on the membrane. By optical dynamometry, we measured the elastic resistance of the membrane to a variation in the interparticle distance and found that it would decrease considerably (down to zero) when the temperature was increased to T(m). We propose an interpretation relating the elastic response to the membrane curvature modulus, k(C). In this scheme, the two-bead dynamometry experiments provide a direct measurement of k(C) in the P'(beta) phase of lipid bilayers.

Phase stability of phosphatidylcholines in dimethylsulfoxide solutions

The temperature dependence of the phase stability of dispersions of dimyristoyl, dipalmitoyl, and distearoyl derivatives of phosphatidylcholines in excess aqueous dimethylsulfoxide has been examined by differential scanning calorimetry and synchrotron x-ray diffraction methods. There was a close correlation between the enthalpic transitions and the structural changes associated with the pre- and main transitions of the phospholipids in the range of concentrations up to mole fractions of dimethylsulfoxide in water of 0.1333. The temperature of the pre- and main transitions of the three phospholipids were found to increase linearly with increasing mole fraction of dimethylsulfoxide. The difference in phase stability between the lamellar gel and ripple phases induced by increasing dimethylsulfoxide concentration resulted in disappearance of the ripple phase and direct transition between lamellar gel and lamellar liquid-crystal phases. The effect of changing the properties of the solvent by the addition of dimethylsulfoxide on the dimensions of dipalmitoylphosphatidylcholine and solvent layers of the bilayer repeat structure has been determined from electron density distribution calculations. The lamellar repeat spacing recorded at 25 degrees C decreased from 6.36 nm in aqueous dispersion to 6.04 nm in a dispersion containing a mole fraction of 0.1105 dimethylsulfoxide. The results indicate that dipole interactions between solvent and phospholipid and dielectric properties of the solvent are important factors in the determination of the structure of saturated phosphatidylcholines.

Phospholipid phase transitions: kinetics and structural mechanisms

A brief review is given on the principles and methods used to investigate structural phase transitions in phospholipid supramolecular structures. The conceptual differences of approaches close to and far from equilibrium are addressed, and the consequences in terms of the limits of interpretation for different types of methods, in particular referring to jump-relaxation and steady-state techniques, are surveyed. With the emphasis on connecting dynamic and structural information, the results obtained so far from different techniques are reviewed, and the open questions addressed. The more recent advances by millisecond time-resolved X-ray diffraction with synchrotron radiation and their main results obtained for transitions triggered by IR-laser temperature jumps are summarized. As a major novel aspect in the field, the necessity of considering martensitic, diffusionless transformation mechanisms and the occurrence of intermediate structures is highlighted.

Detection of structural defects in phosphatidylcholine membranes by small-angle neutron scattering. The cluster model of a lipid bilayer

The oriented DPPC multilayers hydrated by D2O have been studied by a small-angle neutron scattering method in the Guinier range, and the gyration radius of the structural inhomogeneities has been estimated at about 29 A. They are interpreted as the annular defects between adjacent clusters uniting the all-trans chain 'segments' adjacent to the polar head group regions. The angle of the 'segment' tilt is determined by the hydrated polar group area (59.2 A2 for DPPC bilayers) and has been estimated to be about 44 degrees under the given experimental conditions. The hydrocarbon interior of a bilayer can be suggested as a 'sandwich' that is formed by two clustered layers (approx. 7 A of the thickness) and the central disordered (liquid) layer. The average cluster size along the bilayer surface is estimated to be approx. 24 A which correlates with the estimations of the short order region dimensions from the halfwidth of the X-ray 'packing' reflex (4.6 A)-1. The average interchain separation of approx. 5 A and the average cross-section area of a chain in a cluster (21.4 A2) were estimated from the reflex position and the chain cross-section geometry. The total volume of defects and the fraction of a bilayer surface occupied by them were estimated too.

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C

We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

A crystalline lipid phase in a dry biological system: evidence from X-ray diffraction analysis of Typha latifolia pollen

The temperature limits for germination in Typha latifolia pollen lie within the range 4-40 degrees C. These limits correlate at the low-temperature end with the 'crystallization' of endogenous triacylglycerols and on the high-temperature end with the 'melting' of a gel-like lipid component in intact pollen. X-ray diffraction analysis was used to structurally characterize and to trace the latter gel-like lipid from the intact pollen through a range of pollen lipid fractions. We tentatively identify this component as a fatty acyl sterol ester and present evidence that it resides in the exine of the pollen grain. Its thermotropic behavior is insensitive to pollen hydration. The possibility of interpreting a crystalline lipid phase as being membrane-derived when in fact it originates from contaminating non-membranous neutral lipid is discussed. The total lipid content of T. latifolia pollen is 123 mg/g dry weight, of which 37% is polar lipid. The neutral lipid consists primarily of triacylglycerols and of the aforementioned sterol ester, which represents 0.34% (w/w) of pollen dry weight. The polar lipid fraction has phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid as major components with lesser amounts of phosphatidylglycerol and phosphatidylinositol. Palmitic (16:0) and linoleic (18:2) acids, in a 1:2 molar ratio, constitute the major fatty acids of both polar and neutral lipid fractions with lesser amounts of linolenic (18:3), oleic (18:1) and stearic (18:0) acid in evidence.

Synthesis, calorimetry, and X-ray diffraction of lecithins containing branched fatty acid chains

Lecithins with branched fatty acid chains were synthesized and characterized by differential scanning calorimetry and X-ray diffraction. The influence of three chemical alterations on the phase transition parameters were investigated: length of the branches in 2-position of the acyl chains, position of the branches in the acyl chains, and position of the branched fatty acid chains in the glycerol backbone. The results show that the branched phosphatidylcholines (PCs) have a reduced gel-to-liquid-crystalline phase transition temperature (Tm) compared to the corresponding straight-chain PCs. Depending on both the length of the branches in 2-position of the acyl chains and the position of the branches in the acyl chains, the Tm-values pass through a minimum. The systematic change of the main-transition temperatures Tm is connected with a modified structural polymorphism. If the length of the branches increases three types of polymorphism were observed.

Phase properties of the aqueous dispersions of n-octadecylphosphocholine

Properties of the aqueous dispersions of n-octadecylphosphocholine are examined by differential scanning calorimetry, fluorescence depolarization, light scattering, 31P-NMR, pig pancreatic phospholipase A2 binding, and X-ray diffraction. On heating, these dispersions exhibit a sharp lamellar to micelle transition at 20.5 degrees C. The lamellar phase consists of frozen (gel-state) alkyl chains which do not bind phospholipase A2. The kinetics of the transition are asymmetric: the micelle to lamellar transition is very slow and the lamellar to micelle transition is fast. It is suggested that the lamellar phase is a frozen chain bilayer in which the chains interdigitate.

Effect of lanthanum ions on the phase transitions of lecithin bilayers

Interaction of lanthanum ions (La3+) with 1,2 dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) causes an increase in Tc, the temperature of maximal excess heat capacity, and the width of the gel-to-liquid crystalline transition. At a mole ratio of La3+ to DPPC sufficient to remove the hydrocarbon chain tilt angle of DPPC, the changes in the thermodynamic parameters of the pretransition are minor, Tc and the width were unaltered and the enthalpy was reduced by only 10%. This suggests that the change in tilt angle is not a necessary concomitant of the pretransition.

Kinetics of the main phase transition of hydrated lecithin monitored by real-time X-ray diffraction

A method is described for observing and recording in real-time x-ray diffraction from an unoriented hydrated membrane lipid, dipalmitoylphosphatidylcholine (DPPC), through its thermotropic gel/liquid crystal phase transition. Synchrotron radiation from the Cornell High Energy Synchrotron Source (Ithaca, New York) was used as an x-ray source of extremely high brilliance and the dynamic display of the diffraction image was effected using a three-stage image intensifier tube coupled to an external fluorescent screen. The image on the output phosphor was sufficiently intense to be recorded cinematographically and to be displayed on a television monitor using a vidicon camera at 30 frames X s1. These measurements set an upper limit of 2 s on the DPPC gel----liquid crystal phase transition and indicate that the transition is a two-state process. The real-time method couples the power of x-ray diffraction as a structural probe with the ability to follow kinetics of structural changes. The method does not require an exogenous probe, is relatively nonperturbing, and can be used with membranes in a variety of physical states and with unstable samples. The method has the additional advantage over its static measurement counterpart in that it is more likely to detect transiently stable intermediates if present.

Theoretical conformational analysis of phospholipids. I. Study of the interactions between phospholipid molecules by use of semi-empirical methods with the explicit introduction of polar headgroup interactions

We present a theoretical conformational analysis of a system composed of seven dipalmitoylphosphatidylethanolamine molecules in interaction. The combined use of classical semi-empirical methods for the polar headgroup region with mechanical statistical calculations for the aliphatic chains permits the evaluation of the free energy for a phospholipids molecule. The free energy variation in function of the mean intermolecular interchain distance gives information about the main lipid bilayer phase transition. It appears, however, necessary to take into account the hydration of the polar headgroups.

1H-NMR study of the three low temperature phases of DPPC-water systems

The three phases of dipalmitoylphosphatidylcholine-water dispersions, occurring below the main transition are studied by a moment analysis of 1H-nuclear magnetic resonance (NMR) spectra. The subtransition, recently detected by Chen, S. C., J. M. Sturtevant, and B. J. Gaffney, 1980, Pro. Natl. Acad. Sci. USA, 77:5060-5063, is characterized by a sharp drop in the second moment at 12 degrees C as a result of increasing the temperature. Interesting features of this phase transition are a hysteresis of 11 K and extremely slow kinetics. It is interpreted as the onset of a flip-flop of the hydrocarbon chains about their long axis. At the pretransition, this type of motion is assumed to change into a fast rotation. The proposed models for the three phases are confirmed by computer calculations of theoretical values for the second and fourth moments of the corresponding NMR signals.

Phase transitions in phosphatidylcholine multibilayers

The (2)H NMR spectrum of a multilamellar dispersion of 1-myristoyl-2-[14,14,14-(2)H(3)]myristoyl-sn-glycero-3-phosphocholine with 1 mol% cholesterol in excess water has been recorded at temperatures between -15 degrees C and 36 degrees C. Motionally averaged quadrupole coupling constants nu(Q) and motionally induced asymmetry parameters eta are obtained by spectral analysis. Values of these quantities indicate that, at temperatures below -4 degrees C, any rotational motion of the molecules about their molecular long axis is slow on the NMR time scale. At temperatures immediately above the pretransition these same parameters show that a fast-rotational motion is occurring about the molecular long axis. This rotational motion is hindered in that the molecules flip about a twofold symmetry axis. Between -4 degrees C and the pretransition, spectra appear as the superposition of two powder patterns, one corresponding to the pattern observed below -4 degrees C and the other to the pattern above the pretransition. The relative contribution of the latter increases with temperature until the pretransition is reached. These data have been interpreted in two ways: either the sample between -4 degrees C and the pretransition contains two populations of rapidly and slowly rotating molecules, or there is only a single population of molecules undergoing a 180 degrees flipping motion on the time scale of the NMR measurement. The latter interpretation is more consistent with other experimental findings. At the temperature of the main transition the hydrocarbon chains melt. In the absence of cholesterol, spectra are more complex in that the line shape is reproduced by the superposition of three spectral powder patterns between -4 degrees C and the pretransition and by the superposition of two spectral patterns above the pretransition. It is postulated that these two patterns observed above the pretransition are in direct correspondence to the two ripple structures observed by freeze-fracture electron microscopy in the absence of cholesterol.

Structure and dynamics of phospholipid membranes: an electron spin resonance study employing biradical probes

The large zero-field splitting of rigid biradicals makes them important candidates for spin probes of phospholipid membranes. Here we develop an electron spin resonance line-shape model for such probes on the basis of the stochastic Liouville equation. Particular emphasis is given to the slow-diffusional regime, characteristic of bilayers in the gel phase. The theory is employed to study the line shapes of bis(verdazyl) biradicals, incorporated into oriented multibilayers of dimyristoylphosphatidylcholine. Computer simulations of the angular-dependent spectra provide the orientational distribution functions and rotational correlation times of the spin probes. They occupy two different sites in bilayer membrane. The orientational distribution of the spin probes is related to the structure of the phospholipid phases. In the L beta' phase the hydrocarbon chains are uniformly tilted by delta = 23 degrees with respect to the bilayer normal. For the P beta' phase we observe a random distribution of tilt angles from delta = 0 degree to delta = 19 degrees, indicating that the chains orient perpendicular to the local (rippled) bilayer surfaces. This structure has not been established previously. In agreement with other studies we find no tilt for the L alpha phase. The order parameters of the hydrocarbon chains increase with decreasing temperature, jumping from S less than or equal to 0.6 to S greater than or equal to 0.8 at the main transition. From the rotational correlation times of the spin probes, intrinsic bilayer viscosities of 0.08 P less than or equal to eta less than or equal to 20 P (50 degrees C greater than or equal to T greater than or equal to 1 degree C) are determined. An Arrhenius plot provides activation energies of the viscous flow. The values increase from Evisc approximately 10 kcal/mol in the L alpha phase to Evisc approximately 18 kcal/mol in the L beta' phase.

NMR study of synthetic lecithin bilayers in the vicinity of the gel-liquid--crystal transition

1H, 2H, and 31P NMR methods have been employed in the study of dimyristoyl lecithin bilayers hydrated with D2O in the gel (L beta'), intermediate (P beta') and liquid-crystalline (L alpha) phases. For D2O/lipid molar ratios, n, in the range 7 less than or equal to n less than or equal to 11 discontinuities are observed in the deuterium NMR splittings at both main and pretransitions. A partial phase diagram based on NMR and differential scanning calorimetry data is presented. 1H NMR dipolar splittings are observed for macroscopically oriented samples in all three phases. Changes in the 1H splittings are correlated with 2H and 31P data and interpreted to show that the chain tilt in the gel phase undergoes a discontinuous change on transition to the intermediate phase, which brings the chain axes closer to the bilayer normal. An estimate of chain tilt in the gel phase is made on the basis of NMR data and found to be approximately 23 degrees for a sample with n = 11 at 18 degrees C.

A proton NMR moment study of the gel and liquid-crystalline phases of dipalmitoyl phosphatidylcholine

Measurements of the proton second and fourth moments have been undertaken for multilamellar dispersions, in excess water, of protiated and chain-perdeuterated dipalmitoyl phosphatidylcholine (DPPC) in the temperature range -20 to 50 degrees C. The comparison of the measured moment with the rigid lattice M2 and the calculated second moment values in the presence of certain motions gives insight into the dynamic structure of the methylene chains of DPPC. This study demonstrates that at -15 degrees C there is still a significant amount of methylene chain motion or disorder in DPPC. At 35 degrees C the moment values indicate that the methylene chains are not in the fully extended all-trans conformation and they may also be rotationally disordered. At the pretransition there is a decrease in magnitude of the proton second moments, which can be accounted for by an increase in the lateral diffusion rate to a value greater than 10(-11) cm2/s. This work suggests that at temperatures just below the main transition the chain conformation is considerably different from the common model structure (P beta') in which the chains are fully extended. In the liquid-crystalline phase the proton moment data are in agreement with data from other techniques on the liquid-like nature of the methylene chains. It is demonstrated how the values of the moment ratio M4r/(M2r)2, which are relatively constant within each phase, can be used to calculate the molar fractions of coexisting liquid-crystalline and gel-phase phospholipid at temperatures near the main transition.

Fluorescence lifetime and time-resolved polarization anisotropy studies of acyl chain order and dynamics in lipid bilayers

The time-resolved fluorescence intensity and anisotropy decays of cis- and trans-parinaric acids and phosphatidylcholines labeled with trans-parinaric acid have been characterized in bilayers formed by several phosphatidylcholines and by dipalmitoylphosphatidylcholine-cholesterol mixtures, at several temperatures. Both a conventional free-running nitrogen flashlamp and the novel synchrotron source at the Stanford Linear Accelerator Center (SLAC) were used as excitation sources for a modified single photon counting fluorescence lifetime apparatus. The measured emission decay kinetics of both isomers of parinaric acid were biexponential in all but one of the lipid systems examined. The fluorescence anisotropy of parinaric acid was large and constant in gel phase lipids, but showed a very rapid (approximately 2 GHz) decay of large amplitude in fluid lipids. In all lipid systems studied, the fluorescence anisotropy decayed to a nonzero asymptote, in striking contrast to the behavior observed in viscous solvent solutions. The asymptotic anisotropy was used to calculate an "order parameter" of the emission transition dipole. The value of the order parameter is quite close to that obtained by deuterium NMR. Cholesterol increased the order parameter measured in fluid dipalmitoylphosphatidylcholine but did not substantially affect the rate of angular relaxation. Experiments conducted with trans-parinaroylphosphatidylcholines yielded results virtually identical with those obtained with trans-parinaric acid.

Evidence for acyl chain trans/gauche isomerization during the thermal pretransition of dipalmitoyl phosphatidylcholine bilayer dispersions

In order to clarify, in dipalmitoyl phosphatidylcholine multilayers, the effect of the 34 degrees C thermal pretransition on the acyl chain intramolecular disordering process, Raman spectra of dipalmitoyl phosphatidylcholine gels at 20 and 34 degrees C were compared in the 1000--1200 cm-1 skeletal C-C stretching region. In addition to an overall intensity decrease associated with a change in chain packing characteristics, the growth of intensity in the 1080--1090 and 1122 cm-1 regions in the 34--20 degrees C) difference spectrum clearly indicates that the thermal pretransition is accompanied by an increase in the population of hydrocarbon chain gauche rotamers toward the center of the bilayer.

Mode of binding of cytochrome b5 to phospholipid bilayers in lamellar structure

X-ray diffraction studies were made on the multilamellar systems produced by incubation of phospholipid bilayers and the membrane protein, cytochrome b5, or non-membrane proteins (albumin, ovalbumin and beta-lactoglobulin A) at pH 8.1 in aqueous 5 mM CaCl2 solutions. Detergent-extracted cytochrome b5 (soluble aggregate) forms two types of lamellar phase with dipalmitoyl phosphatidylcholine bilayers, depending upon the incubation temperature. One type, which has a repeat distance of 114 A, is formed above 34 degrees C, where the binding of cytochrome b5 to the bilayers is hydrophobic. The other type, with a repeat distance of 153 A, is formed below 34 degrees C, where the binding is electrostatic. It is also suggested that cytochrome b5 is monomeric in the former phase but remains aggregated in the latter phase. When dimyristoyl phosphatidylcholine is used, the boundary temperature for the two types shifts to 12 degrees C. These boundary temperatures coincide with the thermal pretransition points of hydrated dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, respectively. Trypsin-treated cytochrome b5 (monomeric) and the three non-membrane proteins exhibit only binding of the electrostatic type to the bilayers, indipendently of the incubation temperature. The observed repeat distances suggest that in these cases two layers of protein molecules are incorporated between the bilayers.

The organization of n-alkanes in lipid bilayers

The interaction of n-alkanes (C6--C16) with phosphatidylcholine has been studied by the combined use of differential scanning calorimetry, X-ray diffraction and monolayer techniques. It has been found that the thermal properties and ultrastructure of lipid-alkane vesicles are strongly dependent on the length of the n-alkanes. Long alkanes, such as tetradecane and hexadecane, increase the transition temperature of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine, while the X-ray data indicate that these long alkanes align parallel to the lipid acyl chains. In contrast, shorter alkanes, such as hexane and octane, decrease and broaden the thermal transition and electron density profiles show that these alkanes increase bilayer width by partitioning between the apposing monolayers of the bilayer. For lipids in the gel and liquid crystalline states, the short alkanes form an alkane region in the geometric center of the bilayer.

Differences in hydrocarbon chain tilt between hydrated phosphatidylethanolamine and phosphatidylcholine bilayers. A molecular packing model

Both wide-angle and lamellar x-ray diffraction data are interpreted in terms of a difference in hydrocarbon chain tilt between fully hydrated dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE). Although the hydrocarbon chains of multilayers of DPPC tilt ty approximately 30 degrees relative to the normal to the plane of the bilayer, as previously reported by others, the hydrocarbon chains of DPPE appear to be oriented approximately normal to the plane of the bilayer. It is found that the chain tilt in DPPC bilayers can be reduced by either: (a) adding an n-alkane to the bilayer interiors or (b) adding lanthanum ions to the fluid layers between bilayers. A molecular packing model is presented which accounts for these data. According to this model, DPPC chains tilt because of the size and conformation of the PC polar head group.

Thermotropic phase transitions of phosphatidylcholines with odd-numbered n-acyl chains

Diacyl phosphatidylcholines with n-C13, -C15, -C17 and -C19 saturated acyl chains have been synthesized and their phase transitions in the presence of excess water monitored by differential thermal analysis. The C15-, C17- and C19-diacyl species show gel to liquid-crystalline transitions and pretransitions like those of the even-chain phosphatidylcholines. A plot of the main phase transition temperature, Tc, vs. acyl chain length is a smooth curve on which the Tc values of both odd- and even-chain species fall, while a similar plot for the pretransition temperature, Tp, shows significant alternation of Tp between odd- and even-chain species. Consideration of these results in terms of the physical basis of the odd-even alternation of phase transition temperatures in homologous series of paraffinic compounds suggests that the acyl chains of disaturated phosphatidylcholines are tilted with respect to the bilayer normal below Tp but become perpendicular to the bilayer surface above the pretransition temperature.

Exchange and aggregation in dispersions of dimyristoyl phosphatidylcholine vesicles containing myristic acid

Processes occurring in dispersions of dimyristoyl phosphatidylcholine containing myristic acid have been studied by light scattering of dilute dispersions (concn. less than or equal to 1 mg/ml) at temperatures above and below the phase transition temperatures of these dispersions. The transition temperatures increase with increasing mol fraction of myristic acid. Above these temperatures, vesicles with different mol fractions of myristic acid exchange lipid molecules. The exchange process leads to vesicles having phase transition temperatures and radii, which ar both intermediate between the initial transitions and radii, respectively. In contrast with the observations above the phase transitions, it was found that when dimyristoyl phosphatidylcholine/myristic acid vesicles were cooled to a few degrees below the phase transition, larger particles were formed. These observations are consistent with a mechanism consisting of vesicle aggregation followed by fustion of the aggregated vesicles. The aggregation process is of second order in the vesicle concentration, and its rate increases with increasing mol fraction of myristic acid.

The membrane volume occupied by anesthetic molecules: a reinterpretation of the erythrocyte expansion data

Seeman and coworkers (Seeman, P. (1972) Pharmacol. Rev. 24, 583--655) calculated that anesthetic agents expand membrane volume ten times more than the van der Waals volume of the agent alone. Their calculation was based on the assumption that the thickness of the erythrocyte membrane expands at the same rate as the surface area. However, recent data on bilayer membranes demonstrate that an expansion of membrane surface area is accompanied by a decrease in membrane thickness. A reinterpretation of their erythrocyte area expansion data using an appropriate contraction of membrane thickness suggests the volume in a membrane occupied by anesthetic molecules is approximately equal to their van der Waals volume.