Structural analysis of hydrated egg lecithin and cholesterol bilayers. I. X-ray diffraction

Cholesterol condensation of alpha-linolenic and gamma-linolenic acid-containing phosphatidylcholine monolayers and bilayers

Cholesterol is demonstrated to condense phosphatidylcholine (PC) monolayers and bilayers containing stearic acid in the sn-1 position and alpha-linolenic acid in the sn-2 position (18:0, alpha-18:3 PC) but has no effect when gamma-linolenic acid occupies the sn-2 position (18:0,gamma-18:3 PC). Cholesterol-induced condensation is measured by area/molecule determinations made on monolayers using a Langmuir trough, while condensation in bilayers is followed by the fluorescent dyes merocyanine (MC540) and dansyllysine. Permeability to erythritol is also demonstrated to be diminished by cholesterol for the condensable 18:0,alpha-18:3 PC bilayer membranes but not the 18:0,gamma-18:3 PC membranes. alpha- and gamma-linolenic acid are isomers containing 18 carbons and three unsaturations. Both fatty acids have unsaturations at positions 9 and 12 and differ only in the location of the third unsaturation, at either position 6 for gamma-linolenic acid (an omega-6 fatty acid) and at position 15 for alpha-linolenic acid (an omega-3 fatty acid). Here lipid-cholesterol interaction is used to distinguish the effect of position of unsaturation on membrane structure.

Direct determination of layer packing for a phospholipid solid solution at 0.32-nm resolution

Electron diffraction intensity data were collected from a 2:3 binary solid solution of two homologous phosphatidylethanolamines (1,2-dimyristoyl-sn-glycerophosphoethanolamine and 1,2-dipalmitoyl-sn-glycerophosphoethanolamine) epitaxially oriented by cocrystallization with naphthalene. The layer packing was determined directly by predicting the value of 12 of the 17 phases from sigma 1- and sigma 2-triplet invariants in space group P1. A reverse Fourier transform of the resulting potential maps provides estimates for three other phases and the two remaining ones were found by generating maps for the 2(2) = 4 possible phase combinations and then testing the smoothness of the potential profile of the hydrocarbon chain packing. The same phase solution can be found by translating a molecular model (based on the known x-ray crystal structure of a shorter homologue) past the unit cell origin. The solid solution is found to retain a stable polar group packing while the statistical occupancy of two terminal-chain carbons is expressed by a reduced potential profile at the nonpolar interface at the bilayer center.

Membrane interaction of calcium channel antagonists modulated by cholesterol. Implications for drug activity

The interactions of lipophilic calcium channel antagonists with the membrane lipid bilayer are complex and highly dependent on membrane composition and structure. Variability in membrane lipid composition (e.g. cholesterol content, acyl chain saturation) can dramatically affect the membrane partitioning of calcium channel antagonists. The membrane binding properties of these drugs did not correlate with traditional measurements of drug lipophilicity, such as a simple hydrocarbon system (Table 2). These data indicate the need for directly measuring the binding of drug molecules to membranes in order to understand better certain pharmacological parameters, including bioavailability, under both normal and pathological conditions in which membrane composition is altered. The interaction of certain lipophilic amphipathic drugs with the membrane lipid bilayer may be an important component of their overall receptor binding mechanism. Specifically, the membrane bilayer may serve to concentrate and orient these drug molecules with respect to a hydrophobic receptor site at the protein receptor/membrane bilayer interface. Thus, the design of drugs which target membrane bound receptors should take into consideration the interaction of the drug molecule with the membrane lipid compartment. This understanding of drug/membrane interactions may lead to the development of drugs with more desirable pharmacokinetics, greater efficacy, and reduced side effects.

Structure of polymerizable lipid bilayers. V. Synthesis, bilayer structure and properties of diacetylenic ether and ester lipids

Four diacetylenic phosphatidylcholines (PC's) have been synthesized and the structures of bilayers of these lipids have been determined at low resolution by low-angle X-ray diffraction. The PC's all have 18-carbon chains but differ with respect to the ether/ester linkage at the sn-1 and sn-2 positions and the relative position of the diacetylene moiety: diester-PC (1): 1,2-bis(octadeca-4',6'-diynoyl)-sn-glycero-3-phosphocholine diester-PC (2): 1-(octadeca-4',6'-diynoyl)-2-(octadeca-5',7'-diynoy l)-sn- glycero-3-phosphocholine diester-PC (3): 1,2-bis(octadeca-8',10'-diynoyl)-sn-glycerol-3-phosphocholin e diether-PC (4): 1-O-(octadeca-4',6'-diynyl)-2-O-(octadeca-5",7"-din yl)-sn- glycero-3-phosphocholine Only (1) exhibits the typical bilayer profile, whereas (2), (3) and (4) show evidence of interdigitation and/or significant disorder. Only (1) polymerized effectively upon illumination with 254 nm light, turning deep blue in seconds, indicating the formation of long, well-ordered polydiacetylenic structures. Liposomes of these derivatives were tested for permeability by osmotic swelling. Polymerized liposomes of (1) underwent osmotic swelling with urea, glycerol, and acetamide more rapidly than did liposomes of stearoyl-oleoyl-PC, but the initial rates of osmotic swelling of polymerized liposomes of (1) were 3-10-times lower than those of unpolymerized liposomes of (1). Blue polymerized multilayer samples of (1) exhibited an irreversible thermochromic transition to red at approx. 40 degrees C. Differential scanning calorimetry with liposome suspensions of (1) revealed an endotherm at 28.3 degrees C with a transition enthalpy of 40 J/g. PC (1) is a potentially useful diacetylenic lipid which exhibits facile, complete polymerization and a bilayer thickness comparable to that of biomembrane lipids.

Lipid-drug interaction: a structural analysis of pindolol effects on model membranes

The ternary system constituted by distearoylphosphatidylcholine, pindolol (a vasodilator drug) and water has been investigated by using X-ray diffraction and calorimetric techniques. The structural modifications induced by the drug have been determined and a possible interaction model has been derived. In particular, the pindolol content-temperature dependent phase diagram shows the occurrence of two new phases: the first is an interdigitated gel, and the second is a lamellar structure presenting an unusual mixed disordered-ordered conformation of the hydrocarbon chains (L alpha beta). The comparative analysis of electron density profiles relative to the L alpha beta phase, reveals significant modifications in the paraffinic region of the lipid layer. In agreement with thermodynamic results, the structural data suggest that the drug induces a stiffening and a tightening of the hydrocarbon chains. Moreover, the hydrophilic properties of the membrane (particularly in P beta, and L alpha beta phases) present an evident dependence with the drug concentration.

Liposome interactions with hydroxyapatite crystals: a possible mechanism in the calcification of atherosclerotic plaques

Some stages in the calcification of atherosclerotic plaques may involve associations between lipids and hydroxyapatite (HA) by surface interactions. Liposomes, artificial membranous lipid vesicles, have been used in this study as model structures for biological calcification processes. Liposome (containing cholesterol and phosphatidylcholine in most cases) suspensions were prepared by sonication, after which HA seed crystals were added to the suspensions and stirred at 37 degrees C. Aliquots of the liposome suspensions were analyzed for particle size distribution and by transmission electron microscopy and electron diffraction. The results showed that HA induced aggregation of liposomes and modifications of the microscopic shapes of the liposomes in the aggregates. These data can be explained by the electron diffraction pattern where superimposition of liposome reflection and crystal reflection exists and may suggest organic-inorganic interaction. The potential of HA crystals to induce formation of liposome aggregates may be seen as a step in atherosclerotic plaques calcification.

Modulation of the interbilayer hydration pressure by the addition of dipoles at the hydrocarbon/water interface

The effects of the cholesterol analog 5 alpha-cholestan-3 beta-ol-6-one (6-ketocholestanol) on bilayer structure, bilayer cohesive properties, and interbilayer repulsive pressures have been studied by a combination of x-ray diffraction, pipette aspiration, and dipole potential experiments. It is found that 6-ketocholestanol, which has a similar structure to cholesterol except with a keto moiety at the 6 position of the B ring, has quite different effects than cholesterol on bilayer organization and cohesive properties. Unlike cholesterol, 6-ketocholestanol does not appreciably modify the thickness of liquid-crystalline egg phosphatidylcholine (EPC) bilayers, and causes a much smaller increase in bilayer compressibility modulus than does cholesterol. These data imply that 6-ketocholestanol has both its hydroxyl and keto moieties situated near the water-hydrocarbon interface, thus making its orientation in the bilayer different from cholesterol's. The addition of equimolar 6-ketocholestanol into EPC bilayers increases the magnitude, but not the decay length, of the exponentially decaying repulsive hydration pressure between adjacent bilayers. Incorporation of equimolar 6-ketocholestanol into EPC monolayers increases the dipole potential by approximately 300 mV. These data are consistent with our previous observation that the magnitude of the hydration pressure is proportional to the square of the dipole potential. These results mean that 6-ketocholestanol, despite its location in the bilayer hydrocarbon region, approximately 10 A from the physical edge of the bilayer, modifies the organization of interlamellar water. We argue that the incorporation of 6-ketocholestanol into EPC bilayers increases the hydration pressure, at least in part, by increasing the electric field strength in the polar head group region.

Structure and cohesive properties of sphingomyelin/cholesterol bilayers

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

X-ray diffraction reconstruction of the inverted hexagonal (HII) phase in lipid-water systems

The structure of the inverted hexagonal (HII) phase in biological lipid-water systems is studied to examine the physical interactions which drive the polymorphic phase behavior and which are also thought to play a relevant role in biological membrane function. A method is derived which yields the complex phase factors of the HII phase diffraction amplitudes from examination of a single sample. This method is applied to a low-resolution Fourier reconstruction of the HII phase in dioleoylphosphatidylethanolamine (DOPE) + water, specifically to examine deviations from the presumed circular model of the HII phase. It is found that the average radius of the water core, Rw, as determined from a Fourier reconstruction, is in good agreement with previously measured values of Rw obtained from more time-consuming traditional methods [Tate, M. W., & Gruner, S. M. (1989) Biochemistry 28, 4245]. In addition to the average value of Rw, the Fourier reconstruction also can be used to determine the true shape of the water core. It is found that the water core is circular to within 5% of Rw when the unit cell size is less than approximately 75 A. Above 75 A, however, a definite shape deformation becomes apparent, with radial noncircularities of 5-10%, probably in response to the increased entropic cost of packing the hydrocarbon chains into the anisotropic environment of the HII unit cell [Kirk, G. L., Gruner, S. M., & Stein D. E. (1984) Biochemistry 23, 1093]. As a more direct probe of the packing anisotropy, Fourier reconstructions of DOPE + dodecane and DOPE + squalene systems were compared with the reconstruction of DOPE. These oils are known to promote the low temperature occurrence of the HII phase, presumably by a reduction in the hydrocarbon packing stress. In support of this hypothesis, the alkanes were observed to relax the water core to a circular shape for even large lattices. In addition, anisotropy of the electron density near the end of the lipid chains is reduced when alkane is added, implying a more uniform hydrocarbon packing environment, consistent with the results of neutron diffraction upon the addition of deuterated decane [Turner, D. C., Gruner, S. M., & Huang, J. (1992) Biochemistry (following paper in this issue)].

Incorporation of phosphonic acid diesters into lipid model membranes. Part II. X-ray and neutron diffraction studies

Mixtures of egg phosphatidylcholine and phosphonic acid diethyl or dibutyl esters of the general type RP(O)(OR')2 with R = hexane or dodecane were studied at room temperature in the fluid lamellar state by X-ray and by neutron diffraction. Generally a molar ratio of lipid and ester of 1:0.5 was used. Additionally an equimolar lipid/ester mixture of hexane phosphonic acid diethyl ester was studied. Depending on the ester used and its concentration a single L alpha-phase was observed above a certain water content which changes to an L alpha + water two phase system at high water concentration. Despite the large amounts of the amphiphilic ester molecules incorporated in the membrane and their high molecular asymmetry, the mixtures qualitatively show the typical hydration and swelling behaviour of non-charged lipid membranes. However, the incorporation of the esters induces a higher hydration capacity, a lateral extension and a decrease in membrane thickness. The position of the ester molecules and their orientation in the membrane were determined by neutron diffraction using partially deuterated esters. The esters were found to be located with their phosphonic moiety near or in the lipid/water interface. The lamellar structure contradicts this location of the cone-shaped ester molecules which should increase the tendency to form hexagonal structures. However, the experimental findings can be understood if one considers a partial interdigitation of the last hydrocarbon groups of the lipid chains accompanied by a larger disorder in the hydrophobic centre of the membrane. In the case of hexane phosphonic acid dibutyl ester, a vertical translocation of the ester takes place below a certain water content where it is distributed between two locations at the lipid water interface and the centre of the membrane.

Transbilayer distribution of bromine in fluid bilayers containing a specifically brominated analogue of dioleoylphosphatidylcholine

We describe in this paper the transbilayer distribution of the bromines of the specifically halogenated phospholipid 1-oleoyl-2-(9,10-dibromostearoyl)-sn-glycero-3- phosphocholine (OBPC). The distribution was determined by X-ray diffraction of oriented multilayers of mixtures of OBPC and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 66% relative humidity by the general approach of Franks et al. (1978) [Nature 276, 530-532]. The bromine distribution of OBPC in the fluid L alpha phase is described accurately by a pair of Gaussian functions located 7.97 +/- 0.27 A from the center of the bilayer with l/e half-widths of 4.96 +/- 0.62 A. We find that OBPC bilayers are accurately described as DOPC bilayers with an additional bromine distribution centered at the position of the double bond of DOPC and conclude that OBPC is an excellent structural isomorph for DOPC under the conditions of these experiments. The distribution obtained is the complete and fully resolved transbilayer image of the halogen label because the broad distribution of the bromines is due entirely to thermal disorder and not to experimental limitations [Wiener, M. C., & White, S. H. (1991a) Biophys. J. 59, 162-173]. The observed width of the bromine distribution indicates that virtually all of the hydrocarbon interior is accessible to the bromines. The distance between the bromine/double-bond position and the headgroup phosphate position was determined from one-dimensional Patterson maps and found to be approximately 12 A. The application of accurately determined bromine distributions to the quantitative interpretation of fluorescence quenching experiments is discussed. A method for the self-consistent global analysis of diffraction data from mixtures that permits the use of data sets with different instrumental scale factors is developed in an Appendix.

Interaction of electric dipoles with phospholipid head groups. A 2H and 31P NMR study of phloretin and phloretin analogues in phosphatidylcholine membranes

Phloretin, 4-hydroxyvalerophenone, and 2-hydroxy-omega-phenylpropiophenone are lipophilic dipolar substances that modify ionic conductances of bilayer membranes. The structural changes at the level of the head groups and the hydrocarbon chains as induced by the incorporation of phloretin and its analogues were investigated with deuterium and phosphorus nuclear magnetic resonance. Membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were selectively deuterated at the choline head group and at the hydrocarbon chains, and 2H and 31P NMR spectra were recorded with varying concentrations of dipolar agents. Incorporation of phloretin leaves the bilayer structure intact, induces only a small disordering of the hydrocarbon chains, and has no significant effect on the head-group dynamics. On the other hand, quite distinct structural changes are observed for the phosphocholine head group. While the -P-N+ dipole is oriented approximately parallel to the membrane surface for pure POPC bilayers, addition of phloretin, and to a lesser extent 4-hydroxyvalerophenone and 2-hydroxy-omega-phenylpropiophenone, rotates the N+ end of the -P-N+ dipole closer to the hydrocarbon layer. The resulting normal component of the -P-N+ dipole partly compensates the electric field of the dipolar agents. In addition to this structural change, phloretin also modifies the hydration layer at the lipid-water interface. Much less 2H2O is adsorbed to the membrane surface when the bilayer contains phloretin, 4-hydroxyvalerophenone, or 2-hydroxy-omega-phenylpropiophenone. Moreover, a rather large change in the residual phosphorus chemical shielding anisotropy argues in favor of hydrogen-bond formation between the phosphate segment and the phloretin hydroxyl groups.

Location of ion-binding sites in the gramicidin channel by X-ray diffraction

We report the first X-ray diffraction on gramicidin in its membrane-active form by using uniformly aligned multilayer samples of membranes containing gramicidin and ions (T1+, K+, Ba2+, Mg2+ or without ions). From the difference electron density profiles, we found a pair of symmetrically located ion-binding sites for T1- at 9.6 (+/- 0.3) A and for Ba2+ at 13.0 (+/- 0.2) A from the midpoint of the gramicidin channel. The location of Ba(2+)-binding sites is near the ends of the channel, consistent with the experimental observation that divalent cations do not permeate but block the channel. The location of T1(+)-binding sites is somewhat of a surprise. It was generally thought that monovalent cations bind to the first turn of the helix from the mouth of the channel. (It is now generally accepted that the gramicidin channel is a cylindrical pore formed by two monomers, each a single-stranded beta 6.3 helix and hydrogen-bonded head-to-head at their N termini.) But our experiment shows that the T1(+)-binding site is either near the bottom of or below the first helix turn.

Nonbilayer phases of membrane lipids

Numerous liquid crystalline biomembrane lipids are known to exhibit non-lamellar phases characterized by curvature of their component lipid monolayers. An understanding of the phase stability of these systems begins with analysis of the energy of bending the monolayers, the interactions which lead to the bending energy, and the geometrical constraints which lead to competing energy terms which arise when the monolayers are bent and packed onto lattices with different structures. Diffraction and other techniques suitable for probing lipid phase structure are described. A phenomenological model is reviewed which successfully explains many of the qualitative features of lipid mesomorphic phase behavior. A key result of this model is that lipid bilayer compositions which are close to the non-lamellar phase boundaries of their phase diagrams are characterized by a frustrated elastic stress which may modulate the activity of imbedded membrane proteins and which may provide a rationale for the prevalence of non-lamellar-tending lipid species in biomembrane bilayers. Areas in need of future research are discussed.

Location of two antioxidants in oriented model membranes. Small-angle x-ray diffraction study

Small-angle x-ray diffraction has been applied in locating either butylated hydroxytoluene (BHT) or delta-tocopherol and their brominated analogues at a concentration of 40 mol% in oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) or DPPC + 15 mol% cholesterol at 20 degrees C. Phases were determined using swelling experiments with structure factors plotted in reciprocal space, creating a relatively smooth curve as the amount of water between the bilayers was changed. Continuous Fourier transforms were also calculated using sampling theory (Shannon, C. E. 1949. Proc. Inst. Radio Engrs. NY. 37:10-21) to further test the consistency of the phase assignments. Fourier synthesis of structure factors resulted in absolute electron density profiles for different bilayers to a resolution of 5-6 A. In addition, difference Patterson maps were constructed to confirm the positions of the bromine atoms in the unit cell. Analysis of the data indicates the following: (a) The BHT molecules are dispersed throughout the alkyl-chain region in DPPC samples with and without cholesterol. (b) The chromanol ring of delta-tocopherol is in the vicinity of the glycerol backbone-headgroup region in samples of DPPC or DPPC + 15 mol% cholesterol. (c) Difference Patterson maps confirm the localization of bromine atoms in the various delta-tocopherol samples and lack of bromine localization in the various BHT samples.

Diacetylenic lipid microstructures: structural characterization by X-ray diffraction and comparison with the saturated phosphatidylcholine analogue

Thermotropic and lyotropic mesomorphism in the polymerizable lecithin 1,2-ditricosa-10,12-diynoyl-sn-glycero-3-phosphocholine and its saturated analogue, 1,2-ditricosanoyl-sn-glycero-3-phosphocholine, has been investigated by wide- and low-angle X-ray diffraction of both powder and oriented samples and by differential scanning calorimetry. Previous studies have shown that the hydrated diacetylenic lipid forms novel microstructures (tubules and stacked bilayer sheets) in its low-temperature phase. The diffraction results indicate that at low temperatures fully hydrated tubules and sheets have an identical lamellar repeat size (d001 = 66.4 A) and crystalline-like packing of the acyl chains. Chain packing in the lamellar crystalline phase is hydration independent. A model for the polymerizable lecithin with (1) fully extended all-trans methylene segments, (2) a long-axis tilt of 32 degrees, and (3) minimal chain interdigitation seems most reasonable on energetic grounds, is consistent with the diffraction data (to 3.93-A resolution), and is likely to support facile polymerization. Above the chain "melting" transition the lamellar repeat of the polymerizable lipid increases to 74 A. The conformational similarity between tubules, sheets, and the dry powder is corroborated by calorimetry, which reveals a cooling exotherm at the same temperature where tubules form upon cooling hydrated sheets. The data suggest that although a high degree of conformational order is a pertinent feature of tubules, this character alone is not sufficient to account for tubule formation. The conformation of the corresponding saturated phosphatidylcholine appears to be similar to that of other saturated phosphatidylcholines in the lamellar gel phase. Furthermore, above the main transition temperature, the dry, saturated lipid shows evidence of a P delta phase (112 degrees C), whereas the diacetylenic lipid appears to exhibit a centered rectangular phase, R alpha (55 degrees C).

On the use of deuterium nuclear magnetic resonance as a probe of chain packing in lipid bilayers

The packing of hydrocarbon chains in the bilayers of lamellar (L alpha) phases of soap/water and phospholipid/water mixtures has been studied by deuterium NMR spectroscopy and X-ray diffraction. A universal correlation is shown to exist between the average C-D bond order parameter SCD of hydrocarbon chains and the average area per chain ach, irrespective of the chemical structure of the surfactant (hydrophilic group, number of chains per molecule, and chain length), composition, and temperature. The practical utility of the correlation is illustrated by its application to the characterization of the distribution of various hydrophobic and amphiphilic solutes in bilayers. The distribution of hydrocarbons within a bilayer is shown to depend upon their molecular structure in a manner which highlights the nature of the molecular interactions involved. For example, benzene is shown to be fairly uniformly distributed across the bilayer with an increasing tendency to distribute into the center at high concentrations. In contrast, the more complex hydrocarbon tetradecane preferentially distributes into the center of the bilayer at low concentrations, while at higher concentrations it intercalates between the surfactant chains. Alcohols such as benzyl alcohol, octanol, and decanol all interact similarly with the bilayer in so far as they are pinned to the polar/apolar interface, presumably by involvement of the hydroxyl group in a hydrogen bond. But the response of the surfactant chains to the void volume created in the center of the bilayer is dependent upon the distance of penetration of the alcohol into the bilayer. For benzyl alcohol, the shortest molecule, this void volume is taken up by the disordering of the chains, while for decanol, the longest molecule, it is absorbed by interdigitation of the chains of apposing monolayers. For octanol, the chain interdigitation mechanism is dominant at low concentrations, but there is a transition to chain disordering at high concentrations. Finally, it is shown that the correlation provides a useful test for statistical mechanical models of chain ordering in lipid bilayers.

Fluid bilayer structure determination by the combined use of x-ray and neutron diffraction. I. Fluid bilayer models and the limits of resolution

This is the first in a series of papers concerned with methods for the determination of the structures of fluid phospholipid bilayers in the liquid-crystalline (L alpha) phase. The basic approach is the joint refinement of quasimolecular models (King and White, 1986. Biophys. J. 49:1047-1054) using x-ray and neutron diffraction data. We present here (a) the rationale for quasimolecular models, (b) the nature of the resolution problem for thermally disordered bilayers, and (c) an analysis of the resolution of experiments in which Gaussian functions are used to describe the distribution of submolecular components. We show that multilamellar liquid-crystalline bilayers are best described by the convolution of a perfect lattice function with a thermally disordered bilayer unit cell. Lamellar diffraction measurements on such a system generally yield only 5-10 orders of diffraction data from which transbilayer profiles of the unit cell can be constructed. The canonical resolution of these transbilayer profiles, defined as the Bragg spacing divided by the index of the highest recorded diffraction order, is typically 5-10 A. Using simple model calculations, we show that the canonical resolution is a measure of the widths of the distributions of constituents of the unit cell rather than a measure of the spatial separation of the distributions. The widths provide a measure of the thermal motion of the bilayer constituents which can be described by Gaussian functions. The equilibrium positions of the centers of the distributions can be determined with a precision of 0.1-0.5 A based upon typical experimental errors.

Fluid bilayer structure determination by the combined use of x-ray and neutron diffraction. II. "Composition-space" refinement method

This is the second of two papers describing a method for the joint refinement of the structure of fluid bilayers using x-ray and neutron diffraction data. We showed in the first paper (Wiener, M. C., and S. H. White. 1990. Biophys. J. 59:162-173) that fluid bilayers generally consist of a nearly perfect lattice of thermally disordered unit cells and that the canonical resolution d/hmax is a measure of the widths of quasimolecular components represented by simple Gaussian functions. The thermal disorder makes possible a "composition space" representation in which the quasimolecular Gaussian distributions describe the number or probability of occupancy per unit length across the width of the bilayer of each component. This representation permits the joint refinement of neutron and x-ray lamellar diffraction data by means of a single quasimolecular structure that is fit simultaneously to both diffraction data sets. Scaling of each component by the appropriate neutron or x-ray scattering length maps the composition space profile to the appropriate scattering length space for comparison to experimental data. Other extensive properties, such as mass, can also be obtained by an appropriate scaling of the refined composition space structure. Based upon simple bilayer models involving crystal and liquid crystal structural information, we estimate that a fluid bilayer with hmax observed diffraction orders will be accurately represented by a structure with approximately hmax quasimolecular components. Strategies for assignment of quasimolecular components are demonstrated through detailed parsing of a phospholipid molecule based upon the one-dimensional projection of the crystal structure of dimyristoylphosphatidylcholine. Finally, we discuss in detail the number of experimental variables required for the composition space joint refinement. We find fluid bilayer structures to be marginally determined by the experimental data. The analysis of errors, which takes on particular importance under these circumstances, is also discussed.

Relaxometry of brain: why white matter appears bright in MRI

The remarkable success of magnetic resonance imaging of adult brain relates to the unusually large ratio of the longitudinal relaxation rates 1/T1 of white and gray matter, approximately 2:1 at physiological temperature and traditional imaging fields. Several investigators have conjectured that myelin is the source of the greater 1/T1 of white matter without, however, suggesting details of the molecular mechanisms responsible. From measurements of the magnetic field dependence of 1/T1 (NMRD profiles) of adult and neonatal gray and white matter at 5 and 35 degrees C, we find a thermally activated contribution to the NMRD profile of adult white matter that is not present in the profiles of either adult gray or neonatal gray and white matter. We attribute this contribution to myelin and develop a quantitative model that accounts for the unique relaxation behavior of myelinated white matter. We find that myelin water, 15% of the total, has a relatively short T1 that arises from an unexpectedly large interaction with myelin lipid; when cast in terms of an interaction over the entire myelin bilipid-water interface, it is sevenfold greater than the analogous protein-water interfacial interaction. Its magnitude remains to be accounted for, but cholesterol, known to alter the relaxation rates of lipid protons, may play an important role. The contribution of myelin to 1/T1 at physiological temperatures is attributed to thermally activated transmembrane diffusion of water and, hence, more rapid mixing of axonal and the rapidly relaxing myelin water molecules.

Amiodarone-liposome interaction: a multinuclear NMR and X-ray diffraction study

Amiodarone, a potent antiarrhythmic drug, is widely used in cardiology. Its electrophysiological effects, as well as many of its side effects, seem to involve lipids. We report here a multinuclear NMR and X-ray diffraction study of amiodarone in egg phosphatidylcholine liposomes and lipid multilayers. In proton NMR experiments, amiodarone alters the signal from the lipid trimethyl ammonium group for pH values ranging from 3.2 to 8.4; cholesterol does not cause this alteration. The addition of SCN- changes both the proton and phosphorus NMR spectra of liposomes containing amiodarone. For both proton and carbon NMR, amiodarone modifies the signal from the lipid methylene groups, but to a far lesser extent than does cholesterol. Incorporation of amiodarone in EPC bilayers also modifies the low-angle X-ray diffraction patterns, decreasing the lamellar repeat period at low water contents, but swelling the fluid spaces between bilayers at high water contents. Electron density profiles and modeling studies using the X-ray data indicate that amiodarone decreases the bilayer thickness and adds electron density at the interfacial region of the bilayer. Our analysis of the NMR and X-ray data indicates that the iodine atoms of amiodarone are located near the hydrocarbon/water interface and that the tertiary amine of amiodarone is in the headgroup region of the bilayer.

Neutron diffraction from cannabinoids in phospholipid membranes

Neutron diffraction measurements have been utilized to study the effects of delta 9-tetrahydrocannabinol (delta 9-THC) and delta 8-tetrahydrocannabinol (delta 8-THC) incorporated in phospholipid membranes of dipalmitoylphosphatidylcholine (DPPC). Low-angle diffraction indicated that these cannabinoids induce increases in interlamellar spacing similar to those produced by cholesterol. Wide-angle diffraction indicated significant differences in how the intralamellar structure is affected by the inclusion of either cannabinoids or cholesterol. Similar weight percentages of cholesterol and cannabinoids in membranes yielded different thermal analysis profiles but the profiles for membranes with either delta 8 or delta 9-THC were similar. Since the neutron diffraction results for inclusions of delta 8 and delta 9-THC were also similar, this suggests that the difference in psychoactivity of delta 8 and delta 9-THC is probably due to interactions with membrane proteins rather than with phospholipids.

Structure of polymerizable lipid bilayers: water profile of a diacetylenic lipid bilayer using elastic neutron scattering

Elastic neutron scattering experiments have been used to study the hydration of multibilayers of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC). Previously published FTIR spectroscopic data had suggested, based on shifts in the carbonyl (C = O) stretch frequencies, that the phosphocholine headgroup in these polymerizable lipid bilayers was much less hydrated than that of saturated phosphatidylcholines. Our results demonstrate that the DC8,9PC headgroup is at least as well hydrated as that of dipalmitoylphosphatidylcholine (DPPC), a saturated lipid, under the same conditions.

X-ray diffraction analysis of myelin lipid/proteolipid protein multilayers

To examine the proposal that myelin proteolipid protein underlies the adhesion of neighboring membranes in central nervous system (CNS) myelin, we carried out X-ray diffraction studies on the structure and interactions of model bilayers composed of total myelin lipids plus proteolipid apoprotein (PLP). Total myelin lipids were used because their heterogeneity was expected to provide an appropriate environment for the integral membrane protein to achieve its native conformation and establish appropriate contacts with the apposed bilayer. We found that incorporation of PLP into myelin lipid bilayers, whether organized into multilamellar vesicles or oriented multilayers, did not appreciably affect the lamellar period, which ranged from 65-71 A. In oriented multilayers, the wide-angle spacing at approximately 4.8 A, which arises from the lateral packing of lipid chains and is perpendicular to the lamellar diffraction, was less oriented and weaker in bilayers containing the protein. These results indicate that PLP was incorporated into the bilayers and had a disordering effect on the hydrocarbon chains but did not extend into the spaces between bilayers. Bilayer profiles calculated from the lamellar diffraction to about 15 A spacing did not show any major changes in the distribution of electron density, suggesting that to moderate resolution, the protein was distributed uniformly across the width of the lipid bilayer. Periodicities measured from osmotically stressed multilamellar vesicles did not depend on the presence of PLP, indicating that the protein did not form stabilizing contacts between bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholesteryl esters on the accessibility of LH/hCG receptors and membrane lipid fluidity in rat testes

Incubation of rat testicular membranes with cholesteryl hemisuccinate resulted in an increase in both membrane lipid microviscosity and 125I-labelled hCG specific binding. The purpose of this investigation was to establish which functional groups of cholesteryl hemisuccinate are important for the stimulatory effects. The data obtained showed that only esters of cholesterol with dicarboxylic acids, not those of monocarboxylic acids, increase the accessibility of LH/hCG receptors and membrane rigidity. Experiments with cholesteryl sulfates showed that there are polar groups on C3 carbon of cholesterol having no stimulatory effect on receptors, although an increase in membrane rigidity occurred. The side-chain of cholesterol is important for the stimulatory action. Androstenolone hemisuccinate was ineffective in this respect. On the other hand, partially modified side-chains (hemisuccinates of beta-sitosterol and stigmasterol) did not result in a marked reduction of the stimulatory action. The carboxyl group of cholesteryl hemisuccinate must be 'free': its esterification abolishes the stimulatory effect of cholesteryl hemisuccinate on both the LH/hCG receptor and membrane microviscosity. These results suggest that an intact carboxyl group of ester and the side-chain of cholesterol are indispensable for the stimulatory effect of cholesteryl hemisuccinate on the accessibility of LH/hCG receptors.

Structure of polymerizable lipid bilayers. I--1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine, a tubule-forming phosphatidylcholine

This report presents the first X-ray diffraction data on diacetylenic phospholipids. The tubule-forming polymerizable lipid, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC), was studied by low angle X-ray diffraction from partially dehydrated oriented multibilayers in both polymerized and unpolymerized form. Bilayers of this material were found to be highly ordered, yielding as many as 16 orders of lamellar diffraction, in both the polymerized and unpolymerized states. The unit cell dimension was very small for a lipid of this size. In addition to the features usually observed in the electron density profile structure of phospholipid bilayers, the electron-dense diacetylenic portions of the fatty acyl chain produced electron density maxima at two well-defined levels on each side of the bilayer approximately 15 A and 9 A from the bilayer midplane. A model molecular conformation deduced from the one-dimensional electron density map features all-trans acyl chains tilted at approximately 28 degrees from the bilayer normal that are interdigitated with chains of the opposing monolayer by approximately two carbons at the bilayer center. The linear diacetylene moieties on beta- and gamma-chains appear at different positions along the bilayer normal axis and are roughly parallel to the bilayer surface. This model is discussed in terms of a polymerization mechanism.

Thermomechanical and transition properties of dimyristoylphosphatidylcholine/cholesterol bilayers

Mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) have been used to examine the effects of cholesterol on the chain crystallization transitions and thermomechanical properties in phospholipid bilayer membranes. The mechanical properties--elastic moduli and level of tension at membrane rupture--were derived from micropipet pressurization of giant single-walled vesicles. Also, the micropipet method allowed temperature-dependent area transitions to be measured at constant membrane tension. X-ray diffraction measurements were made on selected lipid/cholesterol mixtures. Wide-angle patterns and electron density profiles were used to measure bilayer thickness as an indication of chain tilt and fluidity. Vesicle area versus temperature plots showed that the main acyl chain crystallization transition of DMPC broadened and shifted to higher temperatures. Both above and below the broad transition, the elastic area compressibility modulus, K, was greatly increased with cholesterol addition. The value for the 1:1 DMPC/Chol complex was found to be approximately 700 dyn/cm, comparable to that for DMPC in the L beta' phase. However, for all concentrations above 12.5 mol % (which was weakly solid), vesicle bilayers behaved as surface liquids with no surface shear rigidity even at temperatures well below the DMPC phase transition. Area changes over the broadened transitions were reduced by cholesterol and disappeared with the addition of 50 mol % to leave the thermal area expansivity at 1.3 X 10(-3)/degrees C. These area changes are consistent with separate formation of a 1:1 DMPC/Chol complex that does not condense plus residual free lipid and lipid loosely associated with the 1:1 complex that freezes normally.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature, pressure and cholesterol effects on bilayer fluidity; a comparison of pyrene excimer/monomer ratios with the steady-state fluorescence polarization of diphenylhexatriene in liposomes and microsomes

Pyrene excimer/monomer (E/M) ratios have been compared with the steady-state fluorescence polarization (P) of diphenylhexatriene (DPH) in multilamellar liposomes of dilaurylphosphatidylcholine and rat liver microsomes. The purpose was to use the well-understood properties of DPH to reveal the nature of bilayer fluidity which pyrene manifests as an E/M ratio. Reducing the temperature (from 37 degrees C to 8 degrees C), increasing the hydrostatic pressure (from 0.1 to 70 MPa), and, in liposomes, cholesterol enrichment (up to 0.30 mole fraction) separately decreased the E/M ratios and increased P. The pyrene membrane/buffer partition coefficient was affected by temperature but not by pressure, and in the case of cholesterol enrichment, it was assumed to be unaffected. Plots of P as a function of the E/M ratio showed the two to be closely correlated (r = 0.99 in liposomes and 0.96 in microsomes), independent of the treatment used to reduce fluidity. The apparent activation volume and enthalpy for excimer formation was calculated and compared with published data. Pyrene E/M ratios probably reflect the intermolecular volume (fluidity) of the outer region of the bilayer, which is reduced by a decrease in temperature and an increase in pressure and cholesterol. DPH reports the bilayer interior, which is similarly ordered by the experimental treatments. The regional distinction between the two probes, however, accounts for the divergence of E/M ratios and P, which has been reported in membranes enriched with fluidizing fatty acids.

The partial molar volume of water in biological membranes

A new algorithm is presented for interpreting the hydration dependence of x-ray diffraction measurements. The method assumes that the volume of the hydrocarbon phase of the lipid bilayer is not affected by hydration and that the volume expansion between bilayers at maximum hydration is caused by incorporation of water molecules whose partial molar volume is that of pure bulk water. These simple assumptions lead to a determination of the area expansion (and hence change in hydrocarbon-phase thickness) as a function of hydration. An analysis is made of x-ray data of the L alpha and L beta' phases of dimyristoyl phosphatidylcholine and the L alpha phase of egg phosphatidylcholine. The partial molar volume of water depends critically on the degree of lipid hydration and the presence of voids between the head groups of adjacent lipids. The calculated head-group spacings at minimum hydration are consistent with those obtained from neutron diffraction and indicate that the methyl groups of the choline are almost in contact with corresponding groups in the opposing bilayer. This calls into question the origin of the repulsive forces observed in dehydration experiments.

Electron diffraction structure analysis of phospholipids

The use of lamellar electron diffraction data from epitaxially oriented phospholipid crystals for quantitative structure analysis is described in this review of the technique. It is seen that an appropriate correction for crystal texture, which is justified by the analysis of low-dose lattice images, enables these intensity data to be used much as they would be used in X-ray crystallography. Analyses of two classes of phospholipids are reviewed, revealing that the recently determined lamellar structures of ether-linked phosphatidylethanolamine and phosphatidylcholine and quite similar to the acyl-chain structures. Preliminary analysis of 1,2-dihexadecyl-sn-glycerophospho-N-methyl ethanolamine indicates that the headgroup conformation may be similar to that of the ether-linked lecithin.