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)

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The phase equilibria in four lysophosphatidylcholine/water systems were investigated at different temperatures. Each of the 1 -palmitoyl-, 1-stearoyl-, 1 -0leoyl-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 ternpcrature 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 l-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 bioinembranes during the process of fusion.Lysophosphatidylcholines are natural constituents of many biological membranes. They usually occur in small amounts but have nevertheless attracted much interest, mainly because of their membrane-perturbing properties which make them both cytolytic and fusogenic (for a review see [I]). In vivo lysophosphatidylcholines may play an important role in secretory processes involving membrane fusion such as the release of histamine [2] and neurotransmitters [3] and the gastric secretion of hydrochloric acid (Olaisson, H., Mirdh, S., and Arvidson, G., unpublished work). The presence of a calcium-dependent phospholipase Az provides the possibility for a controlled formation of lysophosphatidylcholine in these secretory membranes. In vitro lysophosphatidylcholines and their structural analogues have proved to be of particular value for gentle solubilization of easily denatured membrane proteins [4].How lysophosphatidylcholines interact with cell membranes is still not very well understood. This may in part be due to the lack of information on their physico-chemical properties. The main features of the phase diagram and the phase structures of a lysophosphatidylcholine/water system have been obtained by X-ray diffraction studies [ S ] . However, the lysophosphatidylcholine used in these experiments was CorrcJspondence to G . Arvidson, Institutionen for Medieinsk och Fysiolog...

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-2H2O, DOPS-2H2O, DOPG-2H2O, and DPG-2H2O were investigated with 2H, 23Na, and 31P NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (HII) phase up to about 35 wt % water and a lamellar liquid-crystalline (L alpha) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 A2 in the HII phase (10-40 wt % water) and 69 A2 in the L alpha phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L alpha phase at temperatures between 25 and 55 degrees C. At temperatures above 55 degrees C, DPG with 20 and 30 wt % water formed a mixture of L alpha, HII, and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L alpha phase and at least one nonlamellar liquid-crystalline phase at 25 and 35 degrees C, and a pure HII phase at 45 degrees C and higher temperatures. At water concentrations above about 50 wt % the 23Na quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be greater than 95%. The 2H and 23Na NMR quadrupole splittings of 2H2O and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L alpha phase in the order DOPA approximately DPG greater than DOPS greater than DOPG.

Phosphatidylethanolamine(PE) was isolated from membranes of Bacillus megaterium. The organism was grown at 20°C and 55°C. The phase equilibria in PE/water systems were studied by *H and 31P nuclear magnetic resonance, and by polarized light microscopy. PE isolated from B. megaterium grown at 20°C forms a lamellar liquid crystalline phase at the growth temperature, and at low water contents a cubic liquid crystalline phase at 58°C. The ratio iso/ante-iso acyl chains was 0.3 in this lipid. PE isolated from this organism grown at 55°C forms only a lamellar liquid crystalline phase up to at least 65°C. In this lipid the ratio iso/ante-iso acyl chains was 3.2. Lipid polymorphism Cubic phase Phosphatidylethanolamine Bacillus megaterium NMR Branched acyl chain