Autoxidation of phosphatidylcholines initiated with dicumylhyponitrite in solution and in bilayers

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Abstract
Dicumylhyponitrite (DCHN) was used as a thermal initiator of autoxidation of unsaturated phosphatidylcholines in o-dichlorobenzene and in mixed bilayers of dilinoleoylphosphatidylcholine (DLPC) with dimyristoylphosphatidylcholine (DMPC). Quantitative studies were made of the products, α,α-dimethylbenzyl alcohol (DMBA), acetophenone (AP), and dicumylperoxide (DCP) from DCHN. The ratio DMBA/Δ2DCHN (37 ± 6%) indicates the effectiveness of cumyloxy in hydrogen abstraction from the phospholipid in solution. The overall efficiency (e) of DCHN determined by the inhibitor method with α-tocopherol (α-T) was 84 ± 5%, close to the overall ratio (DMBA + AP)/Δ2DCHN (90 ± 6%), and indicates either significant initiation through the methylperoxy radical in o-dichlorobenzene solution or that α-tocopherol traps cumyloxy radical before β-scission occurs. The thermal decomposition rate constant of DCHN is greatly reduced in a medium of high viscosity; in DLPC to one-quarter, in Nujol to one-sixth that in isooctane and this is interpreted in terms of a one-bond scission of DCHN. In DLPC + DMPC mixed bilayers the effectiveness of the cumyloxy radical to initiate reaction varied from 30 to 19% depending on the amount of DLPC and in this medium there is little or no initiation by methylperoxy according to product analysis and α-T inhibition studies. In mixed bilayers, the rate of oxidation was directly related to the [DLPC] indicating that the classical rate law applies. The oxidizability of DLPC in mixed bilayers (0.046 M−1/2 s−1/2) is comparable to that of neat DLPC.