Lipid oxidation and gel to liquid-crystalline transition temperatures of synthetic polyunsaturated mixed-acid phosphatidylcholines.

Synthetic preparations of the polyunsaturated phosphatidylcholines 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) and 1-stearoyl-2-alpha-linolenoyl-sn-glycero-3-phosphocholine (SLnPC) were observed to undergo autooxidation sometimes during synthesis and also on storage. Oxidation was also induced by treatment of unoxidized SLPC with ultraviolet irradiation. Oxidation was estimated by thin layer chromatographic, fatty acid and ultraviolet spectral analyses. With limited oxidation, the gel to liquid-crystalline transition temperatures of aqueous dispersions of these lipids were seen to increase. Extensive oxidation led to a reduction in the enthalpies of the transitions. The increases in transition temperatures were consistent with the presence of conjugated double bonds, as shown by increased absorption at 230 nm, and trans double bonds, in the oxidized lipids leading to the creation of more rigid domains within the bilayer. Some of the changes in the transitions, especially the decreasing enthalpy after extensive oxidation, may have occurred because of the presence of small amounts of lysophosphatidylcholine and other oxidation intermediates or breakdown products seen by thin layer analysis. Thermograms of mixtures of unoxidized SLPC with amounts of lysostearoylPC found in the oxidized samples showed, however, that lysoPC likely did not contribute significantly to the increase in transition temperatures. Thin layer analysis suggested that the presence of cross-linked products could have contributed to the observed thermotropic properties.