cis-trans isomerization of monounsaturated fatty acid residues in phospholipids by thiyl radicals

Thiyl radicals reversibly attack the double bonds of methyl oleate and dioleoyl phosphatidyl choline (DOPC), thus producing methyl elaidate and the corresponding phospholipids containing trans-fatty acid residues in high yield. These processes are radical chain reactions with relatively long chain lengths. The rate constant for the beta-elimination of a thiyl radical from the adduct radical has been estimated to be 6 x 10(6) s(-1) at ambient temperature. The cis-trans isomerization of fatty acid residues in DOPC vesicles (multilamellar vesicles and large unilamellar vesicles made by the extrusion technique) by a thiyl radical, generated from biologically relevant thiols, has also been studied in detail. The presence of 0.2 mM oxygen does not influence the effectiveness of cis-trans isomerization in both homogeneous solution and lipid vesicles. This process, which does not cause lipid degradation but permanent modification of the membrane constituents, ultimately influences the barrier properties and functions of biological membranes.