Absolute Rate Constants of Elementary Reaction Steps in Radiation-Induced Peroxidation of Unsaturated Fatty Acids

Initial radiation-chemical yields of peroxidation in the homologous series of unsaturated free fatty acids were measured in neat substrates by steady-state irradiation with 60 Co γ rays to obtain baseline values of oxidizability pertaining to a pure lipid moiety at 293 K for oleic (OlH), linoleic (LiH), linolenic (LinH), and arachidonic (ArH) acid. Rate constants of individual reaction steps of peroxidation, propagation and termination, were also measured in the same compounds by intermittent irradiation with 60 Co gamma rays by means of a specially built apparatus. The values of these rate constants were found to double as the number of methylenic carbons increased by one so that the relative values of oxidizabilities in the series LiH/LinH/ArH/docosapentaenoic acid (DPA)/docosahexaenoic acid (DHA) increased as 1::2:2:4. Literature data on oxidizabilities and the pertaining rate constants were subject to scaling with respect to temperature and solvent effects. Consistent baseline values of these quantities were obtained, indicating that an adequate understanding of the applied corrections was reached. Radiation-induced peroxidation inhibited by a reference antioxidant α-tocopherol gave radiation-chemical yield of alkyl free radicals G (L • ) = 0.52 ± 0.16 μmol/J in all fatty acids studied. Temperature dependence of the literature data on k p / k inh, corrected for solvent polarity effect on k p, together with our own measurements of k inh gave Arrhenius activation parameters for the inhibition reaction in pure lipid moiety. The activation energy of the reaction, 4.5 kJ/mol, large negative entropy, −138 J/mol K, and pre-exponential factor 10 6 support the view that the mechanism of α-toc action in nonpolar media may involve both charge and proton transfer.