Quantitative studies of the autoxidation of linoleate monomers sequestered in phosphatidylcholine bilayers. Absolute rate constants in bilayers

Abstract

The kinetics of autoxidation of linoleic acid in dimyristoylphosphatidylcholine (DMPC) bilayers were studied at 30 °C and pH 7 under 760 Torr O2. Reactions were initiated using either the lipid-soluble di-tert-butylhyponitrite (DBHN) or water-soluble azobis(2-amidinopropane)•HCl (ABAP). Rates of chain initiation, Ri, were measured with a lipid-soluble antioxidant, a-tocopherol, or a water-soluble one, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox). The slightly higher oxidizability obtained [Formula: see text] compared to methyl linoleate in chlorobenzene [Formula: see text] is attributed to a moderate polar solvent effect on ionized linoleate near the bilayer surface. A low initiator efficiency, e = 0.0895 for DBHN in DMPC, is attributed to the cage effect in the bilayer of high microviscosity. Similar autoxidation experiments on methyl linoleate in DMPC bilayers gave a lower oxidizability [Formula: see text], indicating that the ester is sequestered deeper in the hydrophobic region of DMPC than is ionized linoleate. Some absolute rate constants are determined using the rotating sector technique for linoleic acid in 0.50 M SDS micelles, and egg lecithin and dilinoleoylphosphatidylcholine (DLPC) bilayers. A hundredfold decrease in the termination rate constant, 2kt for DLPC bilayer compared to homogeneous solution is attributed to chain termination in a bilayer region of high polarity. A concomitant reduction (up to tenfold) in the propagation rate constant, kp, is attributed to diffusion of polar peroxyl radicals away from the oxidizable region of the bilayer.