AM1 CALCULATIONS ON REACTIVE OXYGEN SPECIES .2. KINETICS OF LIPID-PEROXIDATION BASED UPON APPROXIMATE RATE CONSTANTS FROM SCALED ENERGY BARRIERS

Absolute reaction rate theory has been used to examine the processes i nvolved in the autoxidation of lipid molecules. The AM1 semiempirical molecular-orbital procedure was employed to calculate energy barriers and evaluate preexponential terms of the expressions for rate constant s. Improved estimates of the activation energies were obtained by scal ing AM1-CI barriers using empirical thermochemical data. Simplified mo dels of reactants were employed to reduce computational burdens while retaining essential features of the system. As a means of assessing ho w well calculated rate constants reproduce the experimental findings, a chemical kinetics procedure was used to monitor the course of the ox idative chain reaction in various computational experiments. The resul ts qualitatively reproduce experimental findings with regard to the na ture of the hydroperoxide products and the relative rate of reaction a t different sites in the lipid chain. Unexpected processes, such as re versible tetroxide formation and addition to unsaturated molecules, ar e found to control peroxy-radical concentration. Peroxy addition to li pid double bonds also provides a mechanism for the production df unsat urated hydroperoxides exhibiting cis-trans isomerization relative to t he parent lipid.