Pathways of arachidonic acid peroxyl radical reactions and product formation with guanine radicals

Peroxyl radicals were derived from the one-electron oxidation of polyunsaturated fatty acids by sulfate radicals that were generated by the photodissociation of peroxodisulfate anions in air-equilibrated aqueous solutions. Reactions of these peroxyl and neutral guanine radicals, also generated by oxidation with sulfate radicals, were investigated by laser kinetic spectroscopy, and the guanine oxidation products were identified by HPLC and mass spectrometry methods. Sulfate radicals rapidly oxidize arachidonic (ArAc), linoleic (LnAc), and palmitoleic (PmAc) acids with similar rate constants, (2-4) × 10⁹ M^-1 s^-1. The C-centered radicals derived from the oxidation of ArAc and LnAc include nonconjugated R _n^• (∼80%) and conjugated bis-allylic R _ba^• (∼20%) radicals. The latter were detectable in the absence of oxygen by their prominent, narrow absorption band at 280 nm. The R_n^• radicals of ArAc (containing three bis-allylic sites) transform to the R_ba^• radicals via an intramolecular H-atom abstraction [rate constant (7.5 ± 0.7) × 10⁴ s^-1]. In contrast, the R_n^• radicals of LnAc that contain only one bis-allylic site do not transform intramolecularly to the R_ba^• radicals. In the case of PmAc, which contains only one double bond, the R_ba^• radicals are not observed. The R_n^• radicals of PmAc rapidly combine with oxygen with a rate constant of (3.8 ± 0.4) × 10⁹ M^-1 s^-1. The R_ba ^• radicals of ArAc are less reactive and react with oxygen with a rate constant of (2.2 ± 0.2) × 10⁸ M^-1 s^_1. The ArAc peroxyl radicals formed spontaneously eliminate superoxide radical anions [rate constant = (3.4 ± 0.3) × 10 ⁴ M^-1 s^-1]. The stable oxidative lesions derived from the 2′,3′,5′-tri-O-acetylguanosine or 2′,3′,5′-tri-O-acetyl-8-oxo-7,8-dihydroguanosine radicals and their subsequent reactions with ArAc peroxyl radicals were also investigated. The major products found were the 2,5-diamino-4H-imidazolone (Iz), dehydroguanidinohydantoin (Gh_ox), and diastereomeric spiroiminodihydantoin (Sp) nucleosides from 2′,3′,5′-tri-O- acetylguanosine and the Gh_ox and Sp nucleosides from 2′,3′,5′-tri-O-acetyl-8-oxo-7,8-dihydroguanosine. In air-saturated aqueous solutions, covalent alkylated guanine adducts were not detected.