Linoleic Acid Hydroperoxide Reacts with Hypochlorous Acid, Generating Peroxyl Radical Intermediates and Singlet Molecular Oxygen

The reaction of hypochlorous acid (HOCl) with hydrogen peroxide is known to generate stoichiometric amounts of singlet molecular oxygen [O₂(¹Δg)]. This study shows that HOCl can also react with linoleic acid hydroperoxide (LAOOH), generating O₂(¹(Δg) with a yield of 13 ± 2% at physiological pH. Characteristic light emission at 1,270 nm, corresponding to O₂(¹Δg) monomolecular decay, was observed when HOCl was reacted with LAOOH or with liposomes containing phosphatidylcholine hydroperoxides, but not with cumene hydroperoxide or tert-butyl hydroperoxide. The generation of O₂(¹Δg) was confirmed by the acquisition of the spectrum of the light emitted in the near-infrared region showing a band with maximum intensity at 1,270 nm and by the observation of the enhancing effect of deuterium oxide and the quenching effect of sodium azide. Mechanistic studies using $^{18}O-labeled$ linoleic acid hydroperoxide $(LA^{18}O^{18}OH)$ showed that its reaction with HOCl yields $^{18}O-labeled$ O₂(¹Δg) [$^{18}O_{2}(^{1}\Deltag)$], demonstrating that the oxygen atoms in O₂(¹Δg) are derived from the hydroperoxide group. Direct analysis of radical intermediates in the reaction of LAOOH with HOCl by continuous-flow electron paramagnetic resonance spectroscopy showed a doublet signal with a g-value of 2.014 and a hyperfine coupling constant from the α-hydrogen of $a_{H} = 4.3$ G, indicating the formation of peroxyl radicals. Taken together, our results clearly demonstrate that HOCl reacts with biologically relevant lipid hydroperoxides, generating O₂(¹Δg). In addition, the detection of $^{18}O_{2}(^{1}\Deltag)$ and peroxyl radicals strongly supports the involvement of a Russell mechanism in the generation of O₂(¹Δg).