Thiyl radical-induced cis/trans -isomerization of methyl linoleate in methanol and of linoleic acid residues in liposomes

Purpose: To investigate the role of a thiol-containing biologically active compound in lipid peroxidation of membranes. Materials and methods: Thiyl radicals were generated from 3-(2-mercaptoethyl)quinazoline-2,4(1H,3H)-dione (MECH) using pulse radiolysis and gamma -radiolysis in aqueous and alcoholic solutions saturated with N2O. The products were analysed by 1H NMR and by HPLC. Results: The thiyl radicals abstract bisallylic hydrogens from {cis-9,cis-12}-methyl linoleate, yielding a pentadienyl radical. In the absence of oxygen, a thiyl radical-induced cis/trans-isomerization leads to linoleic-type isomers. These chain-type isomerization reactions can occur with the long living pentadienyl radical, followed by a 'repair' reaction of the attached thiol, and with the thiyl radical adduct with a double bond of the fatty acid residue. Conclusions: The results show that the mechanism of cis/trans isomerization is an integral part of the thiyl radical attack on polyunsaturated fatty acids in homogeneous solutions and in bilayers.