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Free radical chain reactions in radiation-induced peroxydation of neat and adsorbed oleic acid

Reliable data on the absolute values of the rate constants of individual reaction steps in the peroxidation of unsaturated fatty acids are scarce. Relatively more attention was paid to polyunsaturated fatty acids (LH) as compared to oleic acid (OlH) due to the availability of suitable analytical methods. Our previous investigation of radiation-induced peroxidation of neat unsaturated fatty acids include the series: oleic, linoleic, linolenic and arachidonic acid[1]. In this paper we report on radiation-induced peroxidation of oleic acid adsorbed on silica gel in comparison with peroxidation of bulk oleic acid. The measured radiation chemical yields of oleic acid hydroperoxides, G(OlOOH), were inversely proportional to the square root of dose rate P in both systems, indicating a chain mechanism with bimolecular termination of propagating radicals: where kp and kt are propagation and termination rate constants, respectively, and G(Ol˙) is the radiation chemical yield of initiated alkyl radicals (Ol˙). G(Ol˙) was determined by the inhibitor method using a-tocopherol (a-toc) and measuring inhibition dose, Dinh : G(Ol˙) = 2[a-toc]/Dinh From these measurements the respective oxidizabilities ( ) and kinetic chain lengths n = G(OlOOH)/ G(Ol˙) could be calculated. However, individual values of kp and kt could not be determined from steady-state radiolysis data alone. Therefore a variant of the rotating sector method was applied, using a specially built apparatus for intermittent irradiation. Lifetime of peroxyl radical t(OlOO˙) was thus obtained, which was related to kt: t(OlOO˙) = (ktG(Ol˙)P)-1/2 Absolute rate constants of all individual reaction steps could then be calculated. The proximity of molecules adsorbed on silica gel caused the acceleration of both propagation and termination reactions, increasing kp and kt values, but on the other hand, the kinetic chain length was shortened in the heterogeneous system. Due to restricted mobility, kinh = 5.8´ 103dm3mol-1s-1 was two orders of magnitude lower than in neat fatty acids.

Ionizing radiation; oleic acid; peroxidation; free radicals; kinetics

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