J. M. Collins

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.

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.

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.

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.

Structures and Mechanisms of Phase Transitions in Surfactant Mixtures: Systems Which Induce the Ribbon Phase

Abstract Phase transitions in lyotropic liquid crystal systems made from potassium palmitate/potassium laurate/water (KP/KL/H2O) and potassium palmitate/benzoyl alcohol/water (KP/BZA/H2O) were characterized by time resolved x-ray diffraction done at the SERC Daresbury (U.K.) Synchrotron Laboratory. Hysteretic phase behavior was observed for some of the KP/KL/H2O mixtures but not for any of the KP/BZA/H2O mixtures studied. In addition, the transition between the rectangular and ribbon phases in these systems is described using x-ray diffraction. General trends related to the observation of phase transitions in these systems are discussed. The ribbon phase appears to be more stable in the KP/BZA/H2O system since it retains its structure after the coexisting lamellar phase has transformed into a liquid crystalline bilayer for a larger temperature range than for KP/KL/H2O.

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.

Ripple Phase Stability in Lipid Systems that Form Interdigitated Bilayers

Abstract Interdigitated bilayer phases have been unambiguously shown to occur in phospholipid systems. The presence of the interdigitated bilayer phase is influenced by the character of the lipids present in the bilayer and the structure of the solvent at the bilayer-solvent interface. Phase structures and transitions involving interdigitated phases are examined using real time x-ray diffraction. The synthetic lipids 1,2 O-dihexadecylphosphatidylcholinc (DHPC) in water and dipalmitoylphosphatidylcholine (DPPC) in ethanol/water or 1 M KSCN have been previously shown to form an interdigitated gel state bilayer phase. The dynamic phase sequence observed for DHPC in water was found to be: The dynamic phase sequence observed for DPPC in ethanol/water or 1 M KSCN was found to not contain a ripple (P β) phase. It can be concluded that changes in the ordering of the solvent structure at the bilayer-solvent interface can prevent the dynamic formation of the rippled bilayer phase.

Modulation of Phospholipid Phase Structures and Transitions Induced by Oxysterols

Abstract Admixtures of various oxidized sterol compounds and 1-palmitoyl, 2-oleoyl phosphatidylethanolamine (POPE) in buffer, or 1,2 O-dihexadecylphosphatidylcholine (DHPC) in water were examined using real time x-ray diffraction. The formation of non-lamellar POPE phases was enhanced by the presence of 5 mole% cholesterol, 7-ketocholesterol, or 20α-hydroxycholesterol, but not by the presence of 5 mole% 5α-cholestane-3p,5,6β-triol. The presence of 5 mole% cholesterol, 7-ketocholesterol or 5α-cholestane-3β,5,6β-triol was sufficient to eliminate the DHPC interdigitated gel bilayer phase over its usual temperature range and to replace it with a gel state ripple phase. Sterol-phospholipid interactions thus resulted in a direct conversion from the sub gel bilayer phase to the gel state ripple phase for DHPC mixtures with 5 mole% cholesterol, 7-ketocholesterol, or 5α-cholestane-3β,5,6β-triol rather than the sequence of interdigitated sub gel bilayer phase to interdigitated gel state bilayer phase to a final sta...

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.

Phase transition characterization in low hydration phases of dilauroylphosphatidylcholine

Abstract Phase transitions in slightly hydrated samples of dilauroylphosphatidylcholine (DLPC) were studied using fixed temperature and real time X-ray diffraction. Only a single transition was observed involving, in all cases, the crystalline ( L c ) and liquid crystalline ( L α ) bilayer phases. Real time X-ray diffraction measurements indicated that this transition proceeded via a two-state or first-order transition mechanism involving both mesophases and acyl chain packings. Transition temperatures, structural parameters, and transition mechanisms obtained from real time X-ray diffraction measurements of DLPC bilayers in limited water samples were the same as those derived from fixed temperature X-ray experiments comparing samples of the same lipid/water concentration using stepwise temperature changes. These results differ from previous reports on slightly hydrated phospholipids, which show different transition mechanisms for the gel to liquid crystalline phase transition, which depended on whether real time or static X-ray measurements were used.