W. Tamura-Lis

The influence of ion species on phosphatidylcholine bilayer structure and packing

The effects of various monovalent cations and anions on the bilayer packing and structure of dipalmitoylphosphatidylcholine were studied using X-ray diffraction and differential scanning calorimetry. It was observed from the X-ray diffraction studies that monovalent salts, in general, have no effect on bilayer packing. The results of DSC studies on metal chloride systems are consistent with the interpretation that cations in general and Li+ in particular bind to DPPC bilayers. The effect of potassium salts on pre- and main-transition temperatures suggest that anions, such as Acetate-, also significantly bind to DPPC head groups.

Thermodynamic properties of acyl chain and mesophase transitions for phospholipids in KSCN

The presence of a one molar solution containing KSCN was shown to induce an interdigitated gel phase in dipalmitoylphosphatidylcholine which can be deduced by examining the lipid thermodynamic phase transition parameters. Specifically, the sub- and pre-transitions can no longer be observed even by low resolution calorimetry. The main transition was completely reversible, however, with scan repetition. We concluded that the presence of 1 M KSCN induces an interdigitated bilayr phase in saturated chain phosphatidylcholines but not saturated chain phosphatidylethanolamines. The lamellar to non-lamellar phase transition was also examined in dielaideoylphosphatidylethanolamines. The presence of 1 M KSCN raised the Lα to H11 phase transition temperature indicating that the lamellar phase has been stabilized. The presence of KSCN thus affects the packing within the bilayer such that it is the ‘preferred’ thermodynamic phase.

The influence of oxygenated sterol compounds on dipalmitoylphosphatidylcholine bilayer structure and packing

Fourier Transform Infra-red and Raman Spectroscopies indicate that 7 alpha-hydroxycholesterol and 7-ketocholesterol have a diminished capacity to condense (increase the packing order of) fluid-state dipalmitoylphosphatidylcholine (DPPC) acyl chains when compared with the effects of cholesterol and the other oxidized sterols studied. DPPC head groups were also more ordered by 7-ketocholesterol over the temperature range 10 degrees - 70 degrees C. Primary effects of these sterols appear to be associated with the hydrophillic regions of the DPPC bilayer, although packing arrangements with acyl chains are also involved. Phosphate and acyl chain ester groups were observed to possess a packing order which was invariant which indicates that these may be the target groups in the interaction with 7-ketocholesterol. A surprising observation was the synergistic amplification of the effects of 7-ketocholesterol by the presence of cholesterol in the DPPC bilayer.

Ca2+ Induced Phase Separations in Phospholipid Mixtures

We have probed the character of the observed phase separation in mixtures of phosphatidylcholines (PC) and/or phosphatidylethanolamines (PE) in the presence of CaCl2 solutions. Egg yolk phosphatidylethanolamine (EYPE) and a 1:1 molar ratio of dioleoylphosphatidylcholine/dioleoylphosphatidylethanolamine (DOPC/DOPE) were observed to undergo phase separation in CaCl2 solutions, as was previously observed for egg yolk phosphatidylcholine (EYPC) (L.J. Lis et al. Biochemistry, 20 (1981) 1771-1777). However, the mixed chain lipid, palmitoyloleoyl-PC, yielded only a single phase in water or CaCl2 solution. We hypothesize that two lipid species are necessary for the observed phase separation to occur, but that the separation itself is not a function of the individual lipid species, but of the mixture.

Ethanol-Phosphatidylcholine Interactions: A Real Time X-Ray Diffraction Study

Abstract Dipalmitoylphosphatidylcholine has been previously shown to have a biphasic response to the presence of ethanol-water solutions as a function of ethanol concentration (E.S. Rowe, Biochim. Biophys. Acta 22 (1985) 3299-3305). Subsequently, Simon and Mclntosh (Biochim. Biophys. Acta 773 (1984), 169-172) showed that DPPC bilayers formed an interdigitated multilamellar phase when the ethanol concentration was greater than 50 mg ethanol/ml water. We have used scanning calorimetry and real time x-ray diffraction to examine DPPC bilayers hydrated with 10 and 150 mg ethanol per ml water. When the lower concentration ethanol solution was present, DPPC bilayers formed subgel, gel, rippled, and liquid crystalline bilayer phases as the sample temperature was increased continuously. However, when the higher ethanol solution was used, DPPC bilayers formed subgel, gel (interdigitated), and liquid crystalline bilayer phases when driven by a temperature scan. In both cases, the subgel phase was not directly revers...

The Influence of Ca2+ on the Subgel Phase and Transitions of Dipalmitoylphosphatidylcholine

Abstract The influence of low molarity (10-50 mM) CaCl2 solutions on the phases and transitions of dipalmitoylphosphatidylcholine was determined using calorimetry and x-ray diffraction. In all cases, a subgel phase was induced before characterization was initiated. The presence of 20-50 mM CaCl2 was shown to have little influence on the pre- and main-phase transitions in DPPC bilayers. However, as the CaCl2 solution concentration is increased, the enthalpy of the subgel to gel bilayer phase transition decreases until no calorimetric endotherm was observed for DPPC in 50 mM CaCl2. An observed broadening of the pre-transition for DPPC in 10 mM CaCl2 can be correlated with a second order phase transition mechanism involving a continuous change in acyl chain packing with no change in mesophase spacing.

Time-resolved X-ray diffraction measurements of phosphatidylcholine-phosphatidylethanolamine mixtures: effect of phosphatidylethanolamine acyl chain length

Abstract Phase transitions in 1:1 molar mixtures of DLPE:DPPC, DMPE:DPPC, DPPE:DPPC, and DSPE: DPPC have been examined using time-resolved X-ray diffraction techniques. A model of the gel-to-liquid crystal phase transition observed upon heating for these lipid mixtures in which the mixing of domains of gel and liquid crystal phase lipids is the rate-limiting step can be formulated. Upon cooling, it is inferred that small domains of gel state lipid are nucleated from the Lα phase. The miscibility of components and domains is also related to the types of proposed phase diagrams.

Interactive forces between sphingomyelin bilayers

Abstract The net repulsive force as a function of bilayer separation was measured for sphingomyelin in water and in 30 m M CaCl 2 . The decay of the hydration repulsive force, 6.27 ± 0.23 A, and the decay of the repulsive electrostatic force, 16 ± 1 A, for sphingomyelin bilayers were found to be consistent with force decays previously observed for phosphatidylcholine bilayers. Bilayer interactive forces appear to be somewhat influenced by changing the backbone moiety of the choline-containing lipid from glycerol to sphingosine. The influence of just the lipid head groups on the van der Waals attractive force was less clear since the van der Waals force is a result of influences from both the head group and acyl chain regions of the lipid in addition to the aqueous environment.

Structure and Morphology of Dipalmitoylphosphatidylcholine/7-keto Cholesterol Mixtures

Abstract Lipid bilayers containing dipalmitoylphosphatidylcholine (DPPC) and 7-ketocholesterol or 19-hydroxycholesterol were examined in water using X-ray diffraction and scanning calorimetry. In contrast to DPPC/cholesterol bilayers, the presence of low contents of 7-ketocholesterol did not produce two DPPC lamellar phases. The swelling properties of the DPPC/7-ketocholesterol bilayers were also different from DPPC/cholesterol bilayers. However, the substitutions of 19-hydroxycholesterol as the oxygenated sterol compound incorporated into the DPPC bilayer produced a more complicated phase relationship. Thus the interaction of oxygenated sterol compounds with DPPC bilayers is compound specific.

Phase characterization of phospholipids dispersed in a variety of nonaqueous solvents

Abstract The phases and phase sequences observed for dipalmitoylphosphatidylcholine and dielaideoylphos-phatidylethanolamine in a variety of solvents were examined using calorimetry and static and dynamic X-ray diffraction techniques. The substitution of water by solvents such as ethylammonium nitrate and propylene carbonate favors the formation of nonlamellar phases in these systems. Solvents such as ethylene glycol, dimethyl sulfoxide, and pyrrolidone favor the formation of lamellar phases, although changes in the acyl chain packing and even the transition temperatures of the phases were observed. A general conclusion can be reached for relationships between the hydrogen bonding nature of the solvents and the stability of the lamellar vs nonlamellar lipid phase.