Mechanism of thiyl radical-catalyzed isomerization of unsaturated fatty acid residues in homogeneous solution and in liposomes

NMR spectroscopy and gas chromatography were used on methanolic solutions o f fatty acid methyl esters and on small bilayer liposomes to study the radi cal-induced denaturation of the fatty acid residues from the natural cis-co nfiguration into trans-isomers. To analyze the mechanism of the thiyl radic al-catalyzed lipid isomerization, we compared the effects of thiols on olei c and linoleic fatty acid residues using pulse radiolysis, gamma-radiolysis and chemolysis (AAPH) to generate thiyl radicals. The isomerization step t akes place within the adduct of the thiyl radical to an olefinic group of u nsaturated fatty acids, but not within the pentadienyl radical. The stabili ty of the adduct can be described by an equilibrium constant of (12 +/- 5) mol(-1) dm(3). The isomerization rate depends on the structure of the thiol . However, the resulting isomeric equilibrium (transfraction: 81%) does not depend on the structure of the thiyl radical or the organization of the li pids. Quantum chemical calculations were performed to estimate the barriers for rotation, the geometry and the enthalpy difference between cis- and tr ans-thiyl radical adducts. (C) 2000 Elsevier Science B.V. All rights reserv ed.