Effect of head-group structure and counterion condensation on phase equilibria in anionic phospholipid-water systems studied by deuterium, sodium-23, and phosphorus-31 NMR and x-ray diffraction

Abstract

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.

Destabilization of the Colicin E9 Endonuclease Domain by Interaction with Negatively Charged Phospholipids

“…In the presence of DOPG vesicles at a lipid:protein ratio of 150, the stability of the E9 DNase is severely compromised to the extent that a sigmoidal transition is not observable in this temperature range. The transition temperatures for the lipids used in this study are Ϫ18 and Ϫ20°C for DOPG and DOPC, respectively, with the effect of temperature upon membrane fluidity reported to be small and continuous in the temperature range used (25,26). We infer from this that the thermal destabilization of the E9 DNase observed with DOPG is unlikely to be due to changes in bilayer fluidity.…”

Wild-type Escherichia coli Cells Regulate the Membrane Lipid Composition in a “Window” between Gel and Non-lamellar Structures

“…Another argument can be given; suppose that E. coli synthesized phosphatidylcholine (PC) or PG instead of PE. The tendency to form non-lamellar phases under physiological conditions would then have been completely lost (Lindblom et al, 1991;Sjölund et al, 1989), and there would be practically no need for a regulation of the acyl chain structure; calculations using the acyl chain composition obtained in the present study at 37°C show that the T m value of PC and PG would be 30°C below the growth temperature (Silvius, 1982). Most probably, it is the presence of PE in the E. coli membranes that necessitates the careful regulation of the acyl chain structure.…”

“…Phase Equilibria of Individual E. coli Lipids-Two of the major lipids in E. coli, the anionic lipids PG and DPG, form only an L ␣ phase up to 55°C at water contents between 20 and 95 weight % and with the lengths and degrees of unsaturation of the acyl chains that are relevant to the present study (Lindblom et al, 1991). Thus, although the shape of the lipid molecules, and the values of the spontaneous radii of curvature for monolayers formed by these phospholipids, will change with longer and more unsaturated acyl chains, the change is not large enough to make these molecules form non-bilayer aggregates under physiological conditions.…”

“…4). However, 20 weight % water is below the maximum hydration for PE as well as for PG and DPG (Lindblom et al, 1991) having the acyl chain compositions in question, and this condition reduces the T LH value . When the lipid extracts were hydrated with 50 weight % water, a cubic phase was formed instead of an H II phase; as mentioned previously, cubic phases of membrane lipids often form between L ␣ and H II phases, and this explains why the cubic phase is formed at lower temperatures than the H II phase.…”

Lipid dependence and activity control of phosphatidylserine synthase from Escherichia coli

“…At higher mole fractions, the ability of the two anionic lipids to perturb the structure of the mixed micelles may be more important. Since Ptd 2 Gro has a greater tendency than PtdGro to form reversed hexagonal phases [27], the former lipid probably has a stronger tendency to change a globular micelle structure into a more planar-like shape, thus increasing the packing density in the aggregates. This effect decreases the ability of PtdSer synthase to penetrate into the hydrophobic environment of the aggregates and can explain why the V max of PtdSer synthase is lower for Ptd 2 Gro than for PtdGro.…”