Evidence for L· against LOO· being spin-trapped by 4-POBN during the reaction of Fe²⁺-induced lipid peroxidation

Abstract

The character of a water soluble spin trap-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN) in the peroxidation of linoleic acid induced by Fe²⁺ has been studied. An ESR spectrum with six lines (a _N=1.58 mT,a _H=0.26 mT) was formed after Fe²⁺ and 4-POBN were added to linoleic acid solution. Its intensity depends on the concentration of Fe²⁺ and the incubation time of the system. It increased when the reaction solution was flushed with nitrogen, comparing with the reaction solution exposed to air at the same concentration of Fe²⁺, 4-POBN and linoleic acid. And it decreased greatly when the system was bubbled with oxygen. At the same time, the concentration of Fe²⁺ did not decrease distinctly with the existence of oxygen detected colorimetrically using phenanthroline, by ultraviolet spectruphotometer. When N-t-butyl-phenylnitrone (PBN) was used instead of 4-POBN, a similar spectrum (a _N=1.61 mT,a _H=0.32 mT) was obtained. The hyperfine coupling constants (hfcs) of the signal are very similar to that of the adduct formed from alkyl radical with PBN. According to above results it is reasonable to conclude that the trapped radical should be assigned as a carbon-centered species (i.e. L·), not the oxygen-derived peroxyl one (i.e. LOO·). The adduct of 4-POBN and L· can survive as long as two hours.