KINETIC-ANALYSIS OF FREE-RADICAL REACTIONS IN THE LOW-TEMPERATURE AUTOXIDATION OF TRIGLYCERIDES

The kinetics of the low-temperature autoxidation of triglycerides has been investigated by electron spin resonance spectroscopy. After initial radical production, four reaction stages are found in the overall autoxidation of unsaturated lipids: (1) formation of peroxyl radicals by addition of molecular oxygen to the initial carbon radicals, (2) consumption of oxygen in the autoxidation cycle, (3) decay of the lipid peroxyl radical into allylic and pentadienyl radicals, and (4) recombination of the carbon-centered radicals. Kinetic analyses reveal that peroxidation of the initial carbon-centered radicals (stage 1) is controlled by O2 migration which has an apparent activation energy of 24 kJ/mol in unsaturated lipids. The autoxidation cycle (stage 2) is dependent on the nature of the parent lipid matrices. Our analysis gives the activation energies of 9 ± 2, 34 ± 8, and 88 ± 11 kJ/mol for the decay of the peroxyl radical into allylic and pentadienyl radicals (stage 3) in trilinolenin, trilinolein, and triolein, respectively. The very low activation energy found for radical decay of the trilinolenin peroxyl radical suggests this process proceeds easily through intra- or interchain hydrogen abstraction and cyclization at low temperatures (<140 K). In stage 4 the allylic and pentadienyl radicals recombine at temperatures near the softening points with an apparent activation energy of ca. 40 kJ/mol. Peroxyl radical decay in saturated lipids follows second-order kinetics with an apparent activation energy of ca. 50 kJ/mol. We find that, for polyunsaturated lipids, even at quite low temperatures (120 K), the autoxidation process occurs readily and must be considered in the storage of biological samples. © 1990 American Chemical Society.