Autoxidation of Biological Molecules. 2. The Autoxidation of a Model Membrane. A Comparison of the Autoxidation of Egg Lecithin Phosphatidylcholine in Water and in Chlorobenzene

Article abstract of DOI:10.1021/ja00411a036

The kinetics of autoxidation of egg lecithin phosphatidylcholine in homogeneous solution in chlorobenzene and as a bilayer dispersion in 0.1 M aqueous NaCl has been studied at 30 deg C under 760 torr of O2.The autoxidations were initiated by the thermal decomposition of di-tert-butyl hyponitrile.The efficiency of chain initiation, e, was determined by the induction period method using α-tocopherol as the chain-breaking antioxidant.In chlorobenzene e was ca. 0.66 but in the aqueous dispersion e was only ca. 0.091.The reduced efficiency of initiation in the bilayeris attributed to a reduction in the fraction of tert-butoxyls which escape from the solvent cage, and this in turn is due to the fact that the bilayer has a high microviscosity.The rate of autoxidation of the egg lecithin in chlorobenzene is proportional to the lecithin concentration and to the square root of the rate of chain initiation, and is virtually independent of the oxygen pressure, which means that this autoxidation follows the usual kinetic rate law.In the aqueous dispersion the concentration of egg lecithin in the bilayer cannot be altered, but since the rate of autoxidation is proportional to the square root of the rate of chain initiation and is virtually independent of the oxygen pressure, the usual kinetic rate law would also appear to be followed.The oxidizability of egg lecithin in chlorobenzene is 0.61 M^-1/2 s^-1/2, and in the aqueous dispension it is 0.0165 M^-1/2 s^-1/2.The reduction in oxidizability in the bilayer is attributed to the diffusion of the peroxyl radical center, which is a polar moiety, out of the autoxidizable, nonpolar, interior region of the bilayer and into the nonautoxidizable, polar surface region.As a consequence, chain progogation will be retarded and chain termination will be accelerated.