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.

Kinetics of lipid peroxidation in compartmentalized systems initiated by a water-soluble free radical source.

Kinetic rate laws arising from theoretical expectations for the oxidation of lipids initiated by water-soluble free radicals in compartmentalized systems under different experimental conditions are deduced. In particular, the predictions for the kinetic reaction orders in: (a) intra-particle oxidizable compound concentration (at fixed number of particles and particle size), alpha; (b) number of particles or analytical lipid concentration (at fixed intra-particle concentration and particle size), beta and (c) initiator, gamma, are obtained. The reaction orders beta and gamma are determined by the fraction of initiator derived radicals captured by the particles (f) and the mean number of chain carrying radicals per particle () when the system reaches the steady state condition. Predicted orders in initiator range from 0 ( = 0.5) to 0.5 (f-->1; > > 1), while the order in number of particles ranges between 0.5 (f-->1; > > 1) and 1. These predictions are tested by measuring the kinetic law for the oxidation of SUV's egg yolk phosphatidylcholine vesicles initiated by the thermal decomposition of ABAP. The results indicate that, under the conditions employed, beta = 0.68 +/- 0.05 and gamma = 0.46 +/- 0.04. These values are close to those expected for a system in which > > 1 and the efficiency of capture is relatively high. This last condition is confirmed by estimating the efficiency of capture from a comparison of induction times elicited by similar concentrations of Trolox and alpha-tocopherol.