Hydroperoxide characterisation as a signature of the micelle/monomer balance in radiation-induced peroxidation of arachidonate.

Archidonate peroxidation has been studied using HO. radicals radiolytically generated as initiators of this process. Irradiated aqueous solutions of arachidonate (between 0.01 and 25 mM at pH 10.5) have been characterised by means of conjugated dienes measurement (234 nm-absorption spectroscopy) and hydroperoxide detection (high-performance liquid chromatography coupled with a chemiluminescence detection). Radiation-induced peroxidation of arachidonate gives a different trend of peroxide products, depending on the degree of substrate interaction; endoperoxide and hydro-endoperoxide being favored at low concentrations (monomer/oligomer) and monohydroperoxide at high concentrations ( micellar form). The experimental ratios G((Hydro2))/G((Hydro1)) increase significantly only for arachidonate concentrations higher than 1 mM, i. e. in micellar medium. However, between 0.1 and 1 mM in arachidonate, G-values (for conjugated dienes, Hydro2 and Hydro1) remain nearly constant, meaning that the physical arrangement of the solution changes: Aggregation occurs. The experimental yields of conjugated dienes formation indicated that G(Dienes) > G(HO) for [arachidonate] 2.5 mM, indicating that a chain propagation process had occurred. Radiolytic yields and structural identification (HPLC-MS analysis) of peroxidation products allowed us to propose a mechanism for the formation of both hydroperoxides.