Thiyl radical induced isomerization of unsaturated fatty acids: determination of equilibrium constants

Thiyl radical-induced isomerization of polyunsaturated fatty acids (PUFAs) have been studied in homogeneous solution and in liposomes. Four one-trans isomers of arachidonic acid have been assigned with the help of 13C NMR spectroscopy. At a dose of 132 Gy, the trans fraction amounts to 9.2 ± 1.2% in each of the four isomers. Therefore, all the four double bonds are equally susceptible to isomerization, which can be achieved by means of gamma radiolysis or chemolysis (AAPH) using both lipophilic and hydrophilic thiols. The equilibrium is characterised by a cis/trans ratio of 19 : 81, far away from the composition of the natural fatty acids (cis fraction 100%). However, compared to the linoleate isomerization in the homogeneous solution, we observed a preferential formation of trans-trans isomers if linoleate is incorporated in the bilayer of liposomes. This difference might be explained by the better fitting of the all-trans isomer into the parallel-aligned acyl chains. The isomerization step takes place within an adduct of the thiyl radical to an olefinic bond. Using a competition method, the numerical value of the equilibrium constant for the adduct formation was determined by pulse radiolysis to be (15 ± 5) dm³ mol^-1. This value does not depend on the number of double bonds and holds for all fatty acids under investigation.