Krister Fontell

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.

Liquid Crystallinity in Lipid-Water Systems

Abstract Following a short review of the three most common liquid crystalline structures occurring in lipid-water systems, viz. the lamellar and the two hexagonal structures, attention is focused on the occurrence of cubic structures in such systems and on the various proposals for the arrangement on the molecular level. It is especially stressed that the tacitly assumed structure of globular aggregates, either of the “oil in water” or “water in oil” type, is often difficult to reconcile with the interpretations of the experimentally obtained X-ray diffraction findings. The different structure proposals are discussed, and the connection between cubic phases of model systems and of systems of biological origin proper are stressed. Due to their optical anisotropy the occurrence of cubic phases in lipid-water systems has often been overlooked.

A cubic protein-monoolein-water phase.

A cubic monoacylglycerol-protein-water phase has been identified by low-angle X-ray diffraction, and the main features of the ternary phase diagram monoolein/lysozyme/water are presented. The thermal stability of the protein in the lipid-protein cubic phase has been examined by differential scanning calorimetry. According to the physical properties of the phase it is proposed that the protein molecules are located in the water medium, i.e. in the water channel systems of the cubic structure earlier suggested. The ability of various proteins to form this cubic phase has been studied, and it was found that the formation of this phase is favoured by an isoelectric point (pI) far from pH 7 in a salt-free solution, thus by high electrostatic repulsive forces.

Liquid crystallinity in systems of magnesium and calcium surfactants: phase diagrams and phase structures in binary aqueous systems of magnesium and calcium di-2-ethylhexylsulfosuccinate

The two binary systems of water and calcium or magnesium di-2-ethylhexylsulfosuccinate were investigated in the temperature range 298–423 K (25–150°C) concerning phase behavior, in particular, and also as regards to water binding and thermodynamics of phase transitions. The results are compared with those of the corresponding sodium system (Aerosol OT). In all three systems, the sequence of homogeneous phases with increasing surfactant concentration is: isotropic solution (L1, lamellar (D), cubic (I2), and reversed hexagonal (F) liquid crystalline phases. The single-phase regions are separated by two-phase regions. The Ca2+ and Mg2+ systems give closely the same phase diagrams which differ considerably in two respects from the sodium system phase diagram. First, the aqueous solubility is lower by orders of magnitude with the divalent counterions. Second, the lamellar phase can swell and incorporate water to a much larger extent with Na+ than with Ca2+ or Mg2+ (difference in molar ratio of water to surfactant ion by more than a factor of 10). The gross features of the phase behavior are in qualitative agreement with electrostatic theory. Minor differences between Ca2+ and Mg2+ as regards water content of phases and thermal stability of phases are discussed in terms of ion specificity effects arising from differences in hydration, and the hydration aspects were also investigated directly from 2H nuclear magnetic resonance quadrupole splittings.

Some aspects on the cubic phases in surfactant and surfactant-like lipid systems

Abstract Cubic liquid crystalline phases are abundant in systems of surfactants and surfactant-like lipids. Characteristic for them is that there is a three-dimensional crystalline long-range order while the order on atomic level is liquid-like. The cubic phases occur in quite different compositional regions and belong to quite different space groups, both primitive, body-centered and face-centered. Several space groups in the International System are considered, in the first place Nos. 218, 223, 224, 227, 229 and 230. The existence of cubic phases is not always recognised due to optical isotropy and stiffness. There are often difficulties in obtaining the cubic phases in pure state. Membrane lipids participate actively in the functions of living cells and it is probable that the formation of cubic phases here have a role.

Flow properties of lamellar liquid crystalline lipid-water systems

Abstract A model lipid-water lamellar liquid crystalline system has been studied experimentally and theoretically. The lamellar phase of the system CTAB-hexan-1-ol-water was investigated in a Couette-type rotary viscometer at 25°C. Newtonian flow at low rates of strain and power law flow, stress ∼ (strain rate) 1 2 , at higher rates of strain was observed. The flow coefficients as a function of increasing water content decreased rapidly to a minimum at ∼50% water content and then increased. Hysteresis effects associated with nonsteady flow and structure reformation were studied. Theoretically the lamellar structure was modeled as flexible layers of water in a liquid hydrocarbon chain environment. Flow was associated with cooperative changes of conformation of flexible layers subject to frictional forces in the liquid layers. Equations of steady flow, τ = η D (low strain rates) and τ = ξD 1 z (intermediate strain rates), were derived with z representing a cooperative coordination number. It is concluded that the steady flow of a lamellar phase of a lipid-water system is characterized by Newtonian flow at low strain rates and cooperative flow typical of a twofold cooperative coordination at higher strain rates. The flow coefficients are dependent on the thickness of the water layer and on the effect of the bilayer interaction on the mobility in the liquid hydrocarbon chain layer.

Phase diagrams and NMR studies of some ternary sodium deoxycholate-surfactant-water systems

Abstract The isotropic solution phase of sodium deoxycholate and water may dissolve large amounts of another surfactant, anionic, cationic, or nonionic. If the other surfactant forms liquid crystalline phases, those may incorporate rather large amounts of the bile salt and, in addition, new liquid crystalline phases may be formed. In the present study the “guest” surfactant has been either sodium diethylhexylsulfosuccinate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, or Triton X-100. The phase diagrams for the ternary systems (pseudo- for the ionic surfactants) have been determined and the structures of the various phases have been studied by polarizing microscopy, NMR, and low-angle X-ray diffraction.

On structural relations between lipid mesophases and isotropic reversed micellar (L2) solutions

Abstract The structural relations in lipid systems between liquid crystalline and isotropic liquid L2 phases have been studied by low-angle X-ray diffraction for two-model systems, viz. sodium octanoate/decan1-of/water and soybean lecithin/soybean oil/water. A transition between the two states may be obtained either by a change in temperature or in composition. The low-angle X-ray scattering curve for the liquid L2 solution shows a maximum with the same angular position as the innermost reflection given by a coexisting liquid crystalline phase, and the integrated intensities for the scattering maxima are almost the same. The relative amounts of ordered structural units are thus about the same on both sides of the transition. A close resemblance in the structures of the reversed micellar aggregates in the L2 solution and the liquid crystalline phase is thus proposed. If the liquid crystalline phase possesses a reversed hexagonal structure its long-range two-dimensional order will be replaced by a “medium” range order in the liquid phase but the latter will basically have the same hexagonal arrangement with the difference that the polar rod aggregates are short. If the liquid crystalline phase has a lamellar structure there will be a reduction in the size of the bilayer domains and an increase in their curvature.

The Structure of a Lyotropic Liquid Crystalline Phase that Orients in a Magnetic Field

Abstract The structure of a lyotropic liquid crystalline phase with positive diamagnetic anisotropy (type I), that spontaneously orients in a magnetic field has been studied by means of water NMR quadrupole splittings, NMR diffusion and polarized absorption spectroscopy. It is concluded that this phase is built up of long rodlike aggregates. A preliminary study of a sample with negative diamagnetic anisotropy (type II) shows that this phase probably consists of lamellar aggregates. It is suggested that these phases are suitable as orientation matrices for studies of chromophores with polarized light spectroscopy.

Phase equilibria and formation of vesicles of dioleoylphosphatidylcholine in glycerol / water mixtures

The lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) forms a lamellar liquid crystalline phase (L alpha) in arbitrary mixtures of glycerol and water. The phase has been characterized by means of X-ray diffraction, 31P-NMR spectroscopy and differential scanning calorimetry (DSC). In the L alpha state, and for DOPC concentrations greater than 50% (w/w), the thickness of the lipid bilayer decreases, while the area of the polar head group increases with increasing glycerol concentration. The phase transition from gel to L alpha state occurs in the range of 240 to 260 K. Contrary to a previous (McDaniel, R.V., McIntosh, T.J. and Simon, S.A. (1983) Biochim. Biophys. Acta 731, 97) study of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) we find that in the gel state, the thickness of the DOPC lipid bilayer is greater than that in the L alpha state. This suggests that in the gel state, the lipid acyl chains of DOPC are in extended configuration. The lamellar phase reaches its maximum swelling at about 50% (w/w) of DOPC. At lower DOPC concentrations a two-phase system is formed where the lamellar phase exists in equilibrium with excess of solvent. Unilamellar vesicles can be prepared from a diluted suspension of the lamellar phase either by using the sonicator or extruder technique. We show this by means of 31P-NMR, EPR and fluorescence spectroscopy. The mean radius of the vesicles, prepared by a sonicator, has been determined at different glycerol/water mixtures. It is found to decrease continuously from 100 A at 100% water to a minimum of 75 A at about 50% water in the solvent mixture. By further decreasing the water content in the solution, the radius rapidly increases, and a mean radius of 450 A is estimated at a water content of 10%. The rotational relaxation times of a fluorescent probe and two EPR spin probes, solubilized in DOPC vesicles, have been measured at different glycerol/water mixtures. It is found that the rotational rates are always much slower in the systems containing glycerol.