Structural simulation of free radical damage in a model membrane system: a small-angle X-ray diffraction study

Membrane injury is an important factor contributing to cellular death following environmental stress. Recent evidence suggests that in plants, changes in membrane phase properties after exposure to a lethal environmental stress may be a result of lipid deesterification mediated by free radicals. To test this hypothesis, the effects of primary free radicals, produced by the radiolysis of water in the absence of oxygen, upon the structure of a dipalmitoylphosphatidylcholine (DPPC) model membrane system were studied. Small-angle X-ray diffraction and differential scanning calorimetry were used to examine DPPC bilayers exposed to increasing doses of gamma radiation. The electron density distribution of the bilayer did not change with increasing irradiation, but the thickness of the water layer between adjacent bilayers increased significantly. The upper limit of the broadened gel to liquid-crystal transition was increased by 4 to 10 Cdeg by the radiation treatment. Compositional analysis of the DPPC bilayer after irradiation indicated that a substantial portion of the phospholipid had been deesterified. Lysophosphatidylcholine was not detected. The changes in structural properties of the bilayer were simulated when the sodium salt of palmitic acid was incorporated into the DPPC bilayers but not by the addition of the acid itself. A mixture of lysophosphatidylcholine, sodium palmitate and DPPC did not simulate the structural changes. The free radicals generated by irradiation promoted deesterification at both the 1 and 2 positions of the glycerol backbone of DPPC, and the resultant accumulation of fatty acid salt in the bilayer altered its structural properties.