Fourier-transform infrared spectroscopy study of dioleoylphosphatidylcholine and monooleoylglycerol in lamellar and cubic liquid crystals

The liquid-crystalline phases of the systems monooleoylglycerol (MO)/water, dioleoylphosphatidylcholine (DOPC)/water, and MO/DOPC/water have been studied by Fourier-transform infrared (FTIR) spectroscopy. In the latter ternary system, the sn-3 OH group of MO competes with water to interact with the polar head group of DOPC, and an intramolecular hydrogen bonding of MO is broken up. The hydration of the ester carbonyl groups in the lamellar phases of both the MO/water and DOPC/water systems increases with increasing water content. Similarly, the addition of small amounts either of MO to a DOPC/water system or of DOPC to an MO/water system also results in an increase in the hydration of the ester carbonyl groups. This leads to an unfavorable hydrocarbon-water contact which is counteracted by the formation of a cubic phase, except for the DOPC/water system, where the lamellar phase is stable also at the highest water concentrations. The phase behavior of the different systems can be described in terms of lipid monolayer curvature and molecular packing in the lipid aggregates. Finally, it is shown by the water association band in the FTIR spectrum that the water hydrogen bonding is considerably different in the liquid-crystalline phases than in bulk water.

Influences of membrane curvature in lipid hexagonal phases studied by deuterium NMR spectroscopy

The presence of reversed hexagonal phase, HII, favoring lipids in membranes has been proposed to be significant in various biological processes. Therefore an understanding of the HII phase and the transition from the lamellar to hexagonal phase is of importance. We have applied deuterium NMR spectroscopy to study the bilayer and reversed hexagonal phases of 1-perdeuteriopalmitoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamin e. The difference in packing between the HII and L alpha phases leads to smaller segmental order parameters in the former case. Since the order profiles are sensitive to the geometry of the aggregates, they can be used to extract structural information about the phases. We present a new means of calculating the radius of curvature, R1, for the HII phase from 2H NMR data. This method gives a value of R1 = 18.1 A, which is in agreement with current understanding of the structure of the HII phase and with x-ray diffraction data.

Reversed hexagonal phase formation in lecithin-alkane-water systems with different acyl chain unsaturation and alkane length

Investigations of lipid-alkane systems are important for an understanding of the interactions between lipids and hydrophobic/amphiphilic peptides or other hydrophobic biological molecules. A study of the formation of nonlamellar phases in several phosphatidylcholine (PC)-alkane-2H2O systems has been performed. The PC molecules chosen in this work are dipalmitoyl-PC (DPPC), 1-palmitoyl-2-oleoyl-PC (POPC), dioleoyl-PC (DOPC), and dilinoleoyl-PC (DLiPC), lipids that in excess water form just a lamellar liquid-crystalline phase up to at least 90 degrees C. The addition of n-alkanes (C8-C20) to these PC-2H2O systems induces the formation of reversed hexagonal (HII) and isotropic phases. The water and dodecane concentrations required to form these phases depend on the degree of acyl chain unsaturation of the PC molecules and increase in the order DLiPC approximately DOPC less than POPC less than DPPC. The most likely explanation to this result is that the diameter of the lipid-water cylinders in the HII phase grows gradually larger with increased acyl chain saturation and more water and dodecane are consequently needed to fill the water cylinders and the void volumes between the cylinders, respectively. The ability of the alkanes to promote the formation of an HII phase is strongly chain length dependent. Although the number of alkane carbon atoms added per DOPC molecule in the DOPC-n-alkane-2H2O mixtures was kept constant, this ability decreased on going from octane to eicosane. The thermal history of a DPPC-n-dodecane-2H2O sample was important for its phase behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase equilibria of the ternary system 1-palmitoyl-sn-glycero-3-phosphocholine/oleic acid/water studied by NMR

Part of a phase diagram for the system 1-palmitoyl-sn-glycero-3-phosphocholine (PamGroPCho)/oleic acid/water has been constructed from mainly 31P-NMR data and a previous determination of the phase equilibria of the binary PamGroPCHo/water system. It was found that the appearance of the phase diagram is very similar to those found for several simple soap/fatty acid/water or soap/long-chain alcohol/water systems. The most striking features observed are: (1) the lamellar phase can swell towards very high water contents (2) vesicles are formed after sonication and (3) the cubic liquid crystalline phase disappears upon addition of very small amounts of oleic acid. The self-association of the amphiphiles and the shape of the aggregates are discussed in terms of existing first-order approximative theories.

Hydrophobic molecules in lecithin-water systems. I. Formation of reversed hexagonal phases at high and low water contents

The system dioleoylphosphatidylcholine (DOPC)-n-dodecane-2H2O was investigated with different nuclear magnetic resonance (NMR) techniques: (a) a tentative phase diagram was determined by 2H- and 31P-NMR, (b) translational diffusion coefficients were determined for the three components with the pulsed magnetic field gradient NMR technique, and (c) order parameters for perdeuterated n-dodecane were obtained by 2H-NMR. n-Dodecane induces the formation of reversed hexagonal (HII) phases at low and high water concentrations, and cubic phases at low water contents. The translational diffusion coefficients of n-dodecane in a cubic phase with 6 mol water per mol DOPC, and in an HII phase with 48 mol water per mol DOPC, were just approximately 2.5 times lower than in pure dodecane. Perdeuterated dodecane gave large quadrupole splittings in a lamellar phase, much smaller in an HII phase at low water contents, and a narrow single peak in an HII phase at high water contents. This latter observation indicates that a large fraction of the dodecane molecules is located in separate regions between the water cylinders. Our results support the model given by Gruner concerning the aggregation of membrane lipids in the presence of hydrophobic molecules.

Relationship between three-dimensional arrays of "lipidic particles" and bicontinuous cubic lipid phases

Several lipid-water mixtures form phases that give rise to freeze-fracture replicas exhibiting three-dimensional regular arrays of closely packed globular elements, often called "lipidic particles". These phases have often been poorly classified with respect to long-range organization and symmetry and have in most cases been asserted to be built up by closed lipid aggregates, such as reversed micelles. However, studies of phases giving rise to the above-mentioned freeze-fracture replicas, with X-ray diffraction and the nuclear magnetic resonance pulsed field gradient diffusion technique, have revealed that they are cubic liquid-crystalline phases and with one exception bicontinuous phases, i.e., cubic phases in which both the hydrocarbon and the water regions are continuous. Up to now the only known exception is a cubic phase composed of closed rod-shaped micelles of the normal type. Thus it is not possible to decide from a freeze-fracture image of a cubic phase, showing three-dimensional arrays of "lipidic particles", if the phase is bicontinuous or composed of closed lipid aggregates. Hitherto, it has not been shown that a biological membrane lipid-water system is able to form a cubic liquid-crystalline phase consisting of reversed micelles. The existence of such a phase is also improbable considering the location in the phase diagrams of cubic phases formed by biological membrane lipid-water systems.

Metabolic changes of membrane lipid composition in Acholeplasma laidlawii by hydrocarbons, alcohols, and detergents: arguments for effects on lipid packing

The packing of lipids into different aggregates, such as spheres, rods, or bilayers, is dependent on the hydrophobic volume, the hydrocarbon-water interfacial area, and the hydrocarbon chain length of the participating molecules, according to the self-assembly theory [Israelachvili, J. N., Marcelja, S., & Horn, R. G. (1980) Q. Rev. Biophys. 13, 121-200]. The origin of the participating molecules should be of no importance with respect to their abilities to affect the above-mentioned parameters. In this investigation, Acholeplasma laidlawii, with a defined acyl chain composition of the membrane lipids, has been grown in the presence of three different classes of foreign molecules, known to partition into model and biological membranes. This results in an extensive metabolic alteration in the lipid polar head group composition, which is expressed as changes in the molar ratio between the lipids monoglucosyldiglyceride (MGDG) and diglucosyldiglyceride (DGDG), forming reversed hexagonal and lamellar phases in excess water, respectively. The formation of nonlamellar phases by A. laidlawii lipids depends critically upon the MGDG concentration [Lindblom, G., Brentel, I., Sjölund, M., Wikander, G., & Wieslander, A. (1986) Biochemistry (preceding paper in this issue)]. The foreign molecules tested belong to the following groups: nonpolar organic solvents, alcohols, and detergents. Their effects on the gel to liquid crystalline phase transition temperature (Tm), on the order parameter of the acyl chains, and on the phase equilibria between lamellar and nonlamellar liquid crystalline phases in lipid-water model systems are known in several instances.(ABSTRACT TRUNCATED AT 250 WORDS)

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

A basis for the reorganization of the bilayer structure in biological membranes is the different aggregate structures formed by lipids in water. The phase equilibria of all individual lipids and several in vivo polar lipid mixtures from acyl chain modified membranes of Acholeplasma laidlawii were investigated with different NMR techniques. All dioleoyl (DO) polar lipids, except monoglucosyldiglyceride (MGDG), form lamellar liquid crystalline (L alpha) phases only. The phase diagram of DOMGDG reveals reversed cubic (III), reversed hexagonal (HII), and L alpha phases. In mixtures of DOMGDG and dioleoyldiglycosyldiglyceride (DODGDG), the formation of an III (or HII) phase is enhanced by DOMGDG and low hydration or high temperatures. For in vivo mixtures of all polar DO lipids, a transition from an L alpha to an III phase is promoted by low hydration or high temperatures (50 degrees C). The phospholipids are incorporated in this III phase. Likewise, III and HII phases are formed at similar temperatures in a series of in vivo mixtures with different extents of acyl chain unsaturation. However, their melting temperatures (Tm) vary in an expected manner. All cubic and hexagonal phases, except the III phase with DOMGDG, exist in equilibrium with excess water. The maximum hydration of MGDG and DGDG is similar and increases with acyl chain unsaturation but is substantially lower than that for, e.g., phosphatidylcholine. The translational diffusion of the lipids in the cubic phases is rapid, implying bicontinuous structures. However, their appearances in freeze-fracture electron microscope pictures are different. The III phase of DOMGDG belongs to the Ia3d space group.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase equilibria in four lysophosphatidylcholine/water systems. Exceptional behaviour of 1-palmitoyl-glycerophosphocholine

The phase equilibria in four lysophosphatidylcholine/water systems were investigated at different temperatures. Each of the 1-palmitoyl-, 1-stearoyl-, 1-oleoyl- and 1-linoleoyl-sn-glycero-3-phosphocholines was dispersed in heavy water at different concentrations. The phase structures were determined by 2H-, 14N- and 31P-NMR, polarization microscopy and low-angle X-ray diffraction. The phase diagrams of the oleoyl and linoleoyl systems were quite similar. At room temperature and with decreasing water content the isotropic micellar solution was followed by a hexagonal phase and then a cubic phase. Finally the lamellar phase appeared before the region of hydrated crystals. The same sequence of phases was observed in the stearoyl system at elevated temperatures. The palmitoyl system differed from the others: here a cubic phase followed after the micellar solution, then came a hexagonal phase and after this a lamellar phase. In general the lysophosphatidylcholines seem to behave similarly to the many soaps and detergents as they show the same sequence of isotropic micellar solution, hexagonal phase, lamellar phase with interspersed cubic phases. The presently established phase diagrams demonstrate that the major lysophosphatidylcholines which may be generated by phospholipase A2 in mammalian cell membranes, viz. 1-palmitoyl- and 1-stearoyl-glycerophosphocholines differ greatly in their packing properties. The extraordinary ability of 1-palmitoyl-glycerophosphocholine to form a cubic phase in equilibrium with a micellar solution is of particular interest with regard to the possible occurrence of cubic structures in biomembranes during the process of fusion.

Molecular organization in phases of lecithin-cholate-water as studied by nuclear magnetic resonance

The molecular organization in the liquid crystalline phases and the micellar solution phase has been investigated using numerous nuclear magnetic resonance techniques. A brief review of previous studies on the lamellar, hexagonal and cubic liquid crystalline phases is given. Mixed micelles were studied by measurements of 2H T1 and T2 nuclear magnetic resonance relaxation times of 2H-labeled phosphatidylcholine. Using simple and rough models, the size and shape of the micellar aggregate were estimated.

The interactions between monovalent ions and phosphatidyl cholines in aqueous bilayers

The interactions of lithium and sodium ions and water with phosphatidylcholine bilayers have been studied by means of 7Li, 23Na and 2H NMR quadrupole splittings. The experimental results are interpreted in terms of a simple three-site model with two anisotropic sites ('binding' sites) and one isotropic site ('free' ions and water molecules). The findings obtained for the zwitterionic model membrane are compared with previous investigations of lamellar phases composed of ionic and nonionic amphiphiles. It is shown that the data obtained are compatible with our previous suggestion [Lindblom, G., Persson, N.-O., and Arvidson G. (1976) Adv. Chem. Ser. 152, 121] that an increase in the salt content in the water layer induces a conformational change in the polar head group of phosphatidylcholine. Thus at high salt concentration the phosphocholine head group tends to be oriented perpendicular to the lipid bilayer surface. The study also shows that increasing the amount of salt leads to a squeezing out of water between the bilayers. This is interpreted in terms of a reduction of the repulsion forces between the bilayers.

Lipid phase structure in the regulation of lipid composition in Acholeplasma laidlawii membranes

In Acholeplasma laidlawii membranes the ratio between the dominating lipids, monoglucosyldiglyceride (MGDG) and diglucosyldiglyceride (DGDG), depends on temperature, configuration of incorporated fatty acids, and membrane cholesterol content, which affect the molecular geometry of the lipids. MGDG and DGDG have wedge- and rod-like molecular shapes, respectively, that are modifiable. The packing constraints of lipids in amphiphilic aggregates, i.e., the area of the hydrocarbon-water interface and the volume and length of the hydrocarbon chains, are important in determining the aggregate structure. Pure MGDG forms a reversed hexagonal- (HII) phase structure with different acyl chain contents, while DGDG forms a lamellar phase. Depending on the unsaturated acyl chain content in the lipids, an in vitro mixture of MGDG and DGDG forms lamellar or cubic phases at physiologic temperatures. A high degree of cis-unsaturation, large amounts of MGDG and high temperatures favor formation of the cubic phase. Addition of cholesterol corresponding to the maximal amount incorporable into A. laidlawii induces a transition from a lamellar or a cubic phase to a reversed hexagonal phase. Lipid mixtures containing only unsaturated acyl chains are more sensitive to the bilayer-destabilizing effect of cholesterol than are mixtures with equal amounts of saturated and unsaturated acyl chains. The lamellar phase is the only one compatible with a functional biological membrane. Consequently, the balance between lipids that form lamellar and other mesophase structures must keep within certain limits. The cubic and reversed hexagonal structures were discovered under conditions not existing in the living Acholeplasma cell. Thus, the response of A. laidlawii lipid metabolism to external and internal stimuli can be predicted on the basis of molecular shapes and is necessary to the maintenance of optimal membranes stability. The reduced capacity of Acholeplasma membranes to incorporate cholesterol is a consequence of this regulation.

Molecular organization in the liquid--crystalline phases of lecithin--sodium cholate-water systems studied by nuclear magnetic resonance

The molecular organization in the hexagonal and lamellar phases of the ternary systems lecithin--sodium cholate--water has been investigated by using a variety of nuclear magnetic resonance techniques. The main findings and conclusions are the following: (i) When calculated on a mole fraction basis, the phase equilibria are insensitive to changes in the alkyl chains of the lecithin. (ii) When incorporated into a lecithin bilayer, cholate exerts a strong perturbation on the lecithin alkyl chain order, giving a large decrease of the order parameters. (iii) This decrease of the order occurs since the average cross-sectional area per alkyl chain increases probably as a result of cholate placing itself flat on the bilayer surface. (iv) The diffusion of lecithin molecules is approximately equally rapid in the lamellar and hexagonal phases. (v) The hexagonal phase is formed by rodlike aggregates with the polar groups at the surface of the rods and with a continuous hydrocarbon core. The rods are not formed by stacking disklike mixed micelles. (vi) With the interpretations of the molecular packing and the phase structures, the observed phase equilibria are in good agreement with current theories of the factors that govern phase behavior in amphiphile--water systems.

Studies of chromophores in model membranes by polarized light-absorption spectroscopy. The orientation and binding of tetracaine and procaine

A method for studying the orientation and binding of chromophores in macroscopically aligned membranes by polarized light absorption spectroscopy is described. Here tetracaine and procaine solubilized in a lamellar phase of octanoyl-1-glyceride (monooctanoin) and water have been investigated. Tetracaine is found to be located in the lipid region with a preferential orientation of the molecular long axis parallel to the hydrocarbon chains. The orientation of procaine, mainly residing in the water region, is very small.

Effects of anesthetics on water permeability and lipid metabolism in Acholeplasma laidlawii membranes

The addition of tetracaine and diethyl ether to Acholeplasma laidlawii at concentrations commonly used for local anesthesia did not affect water permeability over the cell membrane, as measured by a pulsed magnetic field gradient NMR method. However, A. laidlawii changed its membrane lipid composition upon treatment with these anesthetics. Both tetracaine and diethyl ether addition resulted in a decrease in the molar ratio between the major membrane glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol. The ratio between saturated and unsaturated acyl chains did not change. The results are in accordance with our proposal that A. laidlawii regulates its lipid composition in order to maintain optimal packing stability in the membrane (Wieslander, A., Christiansson, A., Rilfors. L. and Lindblom, G. (1980) Biochemistry 19, 3650--3655). Introduction of anesthetics into the hydrophobic region of a bilayer is likely to affect the lipid packing. A membrane which contains lipids like monoglucosyldiacylglycerol, which forms a reversed hexagonal phase, will be destabilized unless the amounts of such lipids are reduced. The membrane concentration of anesthetics was estimated to one molecule per 12--15 lipid molecules. The fact that A. laidlawii regulates its lipid composition as a response to these concentrations, despite their negligible effect on water permeability, indicates a high sensitivity of this regulatory system.

The effect of cholesterol on the phase structure of glucolipids from Acholeplasma laidlawii membranes

1. The packing properties in lipid mixtures containing cholesterol and membrane glucolipids from Acholeplasma laidlawii are modified by varying the amounts of cholesterol, diacylmonoglucosylglycerol and diacyldiglucosylglycerol in the mixtures as well as the temperature and the degree of acyl chain unsaturation. These changes affect both the proportions of different lipids having dissimilar molecular geometries and the geometry of the lipid molecules themselves. 2. All mixtures containing glucolipids with equal amounts of palmitoyl and oleoyl chains formed a lamellar liquid-crystalline phase in the growth temperature range of A. laidlawii, while a reversed cubic liquid-crystalline phase dominated in mixtures containing dioleoyl glucolipids. These lipids formed a reversed hexagonal phase together with 27% cholesterol. Mixtures with lipid compositions occurring in the membranes of living Acholeplasma cells formed a lamellar liquid-crystalline phase. 3. Large amounts of cholesterol and diacylmonoglucosylglycerol, high temperatures and a high degree of cis unsaturation favoured the formation of cubic or hexagonal liquid-crystalline phase structures of the investigated lipid mixtures. Diacylmonoglucosylglycerol and cholesterol are both wedge-shaped. Temperature and cis unsaturation accentuate the wedge-shape properties of the glucolipid molecules. 4. The changes in the lipid composition of A. laidlawii membranes as a response to cholesterol incorporation can be explained by the geometry and packing characteristics of the sterol molecule and the concept of 'fluidity' does not need to be involved.

Effect of cholesterol in membranes. Pulsed nuclear magnetic resonance measurements of lipid lateral diffusion

Lateral diffusion coefficients of lipids in a bilayer can be measured directly in a macroscopically aligned sample by use of a pulsed NMR method with pulsed magnetic field gradients [Lindblom, G., & Wennerström, H. (1977) Biophys. Chem. 6, 167]. This technique has been utilized to investigate the influence of cholesterol on the lipid diffusion of egg yolk lecithin, palmitoyloleoyllecithin, and dioleoyllecithin. It is found that cholesterol has a very small effect on the phospholipid diffusion. On the other hand, cholesterol has a great influence on the molecular ordering in the bilayer and on the lipid phase structure. It is therefore suggested that cholesterol exerts its dominant effect on the lipid membrane stability.

Reversed cubic phase with membrane glucolipids from Acholeplasma laidlawii. 1H, 2H, and diffusion nuclear magnetic resonance measurements

Monoglucosyl diglyceride and diglucosyl diglyceride are the dominant lipids of the Acholeplasma laidlawii membrane. Diglucosyl diglyceride forms a lamellar liquid crystalline phase with water while monoglucosyl diglyceride forms a reversed hexagonal phase. Depending on the amounts of unsaturated acyl chains of the lipids, a mixture of monoglucosyl diglyceride and diglucosyl diglyceride forms lamellar or reversed cubic phases at physiological temperatures. A high degree of cis unsaturation favors formation of the cubic phase with increasing monoglucosyl diglyceride content. The structure of the cubic phase is composed of aggregates, where the lipids can diffuse over macroscopical distances. A structure containing close-packed spherical micelles is therefore ruled out, and the NMR diffusion data are compatible with other previously proposed cubic bicontinuous structures [Luzzati, V., & Spegt, P. A. (1967) Nature (London) 215, 701; Scriven, L. E. (1976) Nature (London) 263, 123; Lindblom, G., Larsson, K., Johansson, L. B.-A., Fontell, K., & Forsén, S. (1979) J. Am. Chem. Soc. 101, 5465]. Monoglucosyl diglyceride/diglucosyl diglyceride ratios forming cubic phases have not been observed in vivo. It is concluded that formation of the cubic phase is strongly dependent on the molecular shape of the lipids. The results are significant for the physiological regulation of the lipid composition in A. laidlawii membranes as well as for the function and organization of biological membranes in general.

Lipid bilayer stability in membranes. Regulation of lipid composition in Acholeplasma laidlawii as governed by molecular shape

The polar lipid composition in membranes of Acholeplasma laidlawii is extensively regulated as a response to environmental changes. In particular, the ratio between the dominating lipids monoglucosyldiglyceride and diglucosyldiglyceride is altered depending on temperature, configuration of incorporated fatty acids, and membrane cholesterol content. Synthesis of monoglucosyldiglyceride is stimulated by low temperature and saturated fatty acids but diminished by the presence of cholesterol. These factors are likely to affect the molecular geometry of the membrane lipids. Monoglucosyldiglyceride and diglucosyldiglyceride have wedge- and rodlike molecular shapes, respectively, that are modifiable to a certain extent. The packing constraints of lipids in amphilphilic aggregates, i.e., hydrocarbon-water interfacial area, hydrocarbon chain volume, and hydrocarbon chain length, are very important in determining the aggregate structure [Israelachvili, J. N. Mitchell, D. J., & Ninham, B. W. (1976) J. Chem. Soc., Faraday Trans. 272, 1525]. Pure monoglucosyldiglyceride forms a reversed hexagonal (HII) phase structure with different fatty acid contents, while diglucosyldiglyceride forms a lamellar phase. However, the only lipid structure compatible with a functional biological membrane is the lamellar phase. Consequently, the balance between lipids forming lamellar and other mesophase structures must keep within certain limits. Here we show that the response in A. liaidlawii lipid metabolism following external and internal stimuli can be predicted on the basis of molecular shapes and is necessary for the cell in order to maintain optimal membrane stability. Furthermore, the reduced capacity of Acholeplasma membranes to incorporate cholesterol is another consequence of this regulation, aiming at preservation of bilayer stability.

Orientation and mobility of molecules in membranes studied by polarized light spectroscopy

Biological membranes are composed of mainly lipids and proteins. The physical properties of the lipids, forming a bilayer structure, are of crucial importance for the living cell, since the plasma membrane is the guardian barrier towards the environment. Thus, the functioning cell needs a highly stable lipid bilayer, which depends on molecular packing and orientation properties of the various membrane components (Wieslanderet al.1980). The spatial arrangement of the membrane proteins incorporated in the lipid matrix plays an essential role for the different chemical processes occurring at or within the membrane. Information about molecular orientation and mobility is therefore necessary for unravelling the functional mechanisms of a biological membrane.

Electronic transitions in the isoalloxazine ring and orientation of flavins in model membranes studied by polarized light spectroscopy

The orientation of flavin mononucleotide (FMN) in model membranes and the directions of the transition moments of the first three bands in the electronic absorption spectrum of the oxidized form of the isoalloxazine ring have been determined by means of linear dichroism and polarized fluorescence spectroscopy. Measured counterclockwise relative to the axis connecting the two nitrogens in the central ring (considered positive when going in the direction from -CN less than to greater than or equal to N), these angles are 58 +/- 4 degrees (450-nm band), 97 +/- 3 degrees (350-nm band), and 119 +/- 2 degrees (260-nm band).

Water binding and phase structures for different Acholeplasma laidlawii membrane lipids studied by deuteron nuclear magnetic resonance and x-ray diffraction

Water binding capability and phase structures for different lipid species extracted from Acholeplasma laidlawii A membranes have been studied using deuteron nuclear magnetic resonance and low-angle X-ray diffraction. The dominating membrane lipids are monoglucosyldiglyceride and diglucosyldiglyceride and each of them takes up limited amounts of water (bound plus trapped), i.e., up to 13% (w/w), whereas the phospholipids and phosphoglycolipids have larger hydration capacities. Addition of magnesium and calcium ions, but not sodium ions, to the diglucosyldiglyceride increases the hydration capability. This increase is accompanied by the formation of a metastable liquid crystalline phase and a hysteresis effect for the transition temperature. Large differences in water deuteron quadrupole splitting were observed between mono- and diglucosyldiglyceride. Both 2H nuclear magnetic resonance and low-angle X-ray diffraction studies on lipids containing biosynthetically incorporated omega-d3-palmitic acid clearly indicate the existence of a reverse hexagonal phase structure for the monoglucosyldiglyceride and lamellar structures for the diglucosyldiglyceride and the other membrane lipids. The low hydration capability of the large diglucosyldiglyceride polar head is discussed in terms of polar head configuration. Both mono- and diglucosyldiglyceride have several physical properties similar to those of phosphatidylethanolamine.

Deuteron nuclear magnetic resonance studies of phase equilibria in a lecithin-water system

Water deuteron NMR spectra have been studied for the system dipalmitoyllecithin (DPL)-heavy water (D2O) at different compositions and temperatures. From an analysis of the spectra in terms of quadrupole splittings, a phase diagram has been constructed for the temperature range 25-60 degrees C and the composition range 4-15 mol of D2O/mol of DPL. Evidence is given that the "pretransition" observed by differential scanning calorimetry is caused by a crossing of a three-phase line. Strong support for a specific hydration of about 11 water molecules per lecithin molecule in the phase between the pretransition and main transition is also found.