Formation and Reactions of Alkyl, Allyl, Biallylic, and Peroxyl Radicals from Unsaturated Fatty Acids in Micellar and Monomeric Aqueous Solutions

Abstract

Pulsed irradiation experiments with spectrophotometric and conductivity detection were performed on aqueous solutions of unsaturated fatty acids in monomeric form and as rod-shaped micelles. The initial •OH and O•- radicals produce allylic and biallylic free radicals as well as •OH adducts. The reactions of these radicals and their corresponding peroxyl radicals were investigated. The rate constant for the reaction of •OH with linoleate is 1 × 1010 L mol-1 s-1 for the monomeric form and 3.7 × 108 L mol-1 s-1 for the rod-shaped micelle, with a sharp decrease occurring as increasing linoleate content nears its critical micelle concentration. The rate constant of O•- radicals is lower, k = 1.6 × 109 L mol-1 s-1 in dilute solutions, decreasing to 1.0 × 108 L mol-1 s-1 at 10 mmol L-1 linoleate, where rod-shaped micelles start forming. Though •OH radicals form •OH adducts, alkyl, allyl, and biallylic radicals, O•-, radicals form a selective group of allylic and biallylic radicals. The biallylic radicals are also produced as a result of abstraction of hydrogen atoms from the linoleate by the linoleate •OH adducts and by alkyl radicals; this process is enhanced in the rod-shaped micelles. The yield of biallylic radicals increases with increasing micellation and with decreasing dose rate. The peroxyl radicals derived from the OH adducts of linoleate, linolenate, and arachidonate undergo unimolecular elimination of hyroperoxide (HO2•) or superoxide (O2•-). The yield of HO2•/O2•- increases with the increasing number of double bonds.