Kinetics of peroxidation of linoleic acid incorporated into DPPC vesicles initiated by the thermal decomposition of 2,2'-azobis(2-amidinopropane) dihydrochloride.

In a previous work [Chem. Phys. Lipids 2000 104, 49], we have derived the following rate law for the oxidation of lipids in compartmentalized systems: R(T)=(k(1)/k(t))(0.5) k(p) [In](0.5) c(0.5) [LH], where, R(T) is the total rate of oxidation, k(1) is the rate constant for the production of free radicals, k(t) and k(p) are the intra-particle rate constants for the termination and propagation sets, respectively, [In] is the concentration of a water-soluble initiator, c is the concentration of particles, and [LH] is the intra-particle concentration of oxidable lipid. In the present work, we have investigated on the applicability of the proposed kinetic rate law for a system where it takes place the oxidation of a reactive lipid incorporated into an inert matrix. With this purpose, we have measured the rate of oxidation of linoleic acid incorporated into dipalmitoylphosphatidylcholine vesicles initiated by the thermal decomposition of 2,2'-azobis(2-amidinopropane) dihydrochloride as a function of the initiator, particles, and intra-particle LH concentrations. The experimentally determined kinetic orders obtained were 0.54+/-0.02, 0.48+/-0.05 and 0.83+/-0.04 for the dependence of the oxidation rate with initiator, particles, and LH intra-particle concentrations, respectively, in agreement with those theoretically predicted. The lower value obtained for the kinetic order in LH is attributed to a change in k(t) with the increase in oxidable lipid intra-particle concentration. The main point to be emphazised from the results here obtained is that the kinetic rate law for the oxidation of lipids in compartmentalized systems can be significantly different than that observed when to the oxidation takes place in homogeneous solution.