Phase separation in short-chain lecithin/gel-state long-chain lecithin aggregates.
Small bilayer particles form spontaneously from gel-state long-chain phospholipids such as dipalmitoylphosphatidylcholine and 0.2 mol fraction short-chain lecithins (e.g., diheptanoyl-phosphatidylcholine). When the particles are incubated at temperatures greater than the Tm of the long-chain phosphatidylcholine (PC), the particles rapidly fuse (from 90-A to greater than or equal to 5000-A radius); this transition is reversible. A possible explanation for this behavior involves patching or phase separation of the short-chain component within the gel-state particle and randomization of both lipid species above Tm. Differential scanning calorimetry, 1H T1 values of proteodiheptanoyl-PC in diheptanoyl-PC-d26/dipalmitoyl-PC-d62 matrices of varying deuterium content, solid-state 2H NMR spectroscopy as a function of temperature, and fluorescence pyrene excimer-to-monomer ratios as a function of mole fraction diheptanoyl-PC provide evidence that such phase separation must occur. These results are used to construct a phase diagram for the diheptanoyl-PC/dipalmitoyl-PC system, to propose detailed geometric models for the different lipid particles involved, and to understand phospholipase kinetics toward the different aggregates.
Bian
J R
JR
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167.
Roberts
M F
MF
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism*; D002152:Calorimetry, Differential Scanning; D005453:Fluorescence; D006859:Hydrogen; D007700:Kinetics; D008051:Lipid Bilayers / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D010713:Phosphatidylcholines / Q000378:metabolism*; D010740:Phospholipases / Q000378:metabolism; D012995:Solubility; D013816:Thermodynamics; D014867:Water / Q000378:metabolism
D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D014867:Water; D015060:1,2-Dipalmitoylphosphatidylcholine; C038470:1,2-heptanoylphosphatidylcholine; D006859:Hydrogen; D010740:Phospholipases