AUTOXIDATION OF LINOLEIC-ACID METHYL-ESTER AT LOW-TEMPERATURES STUDIED BY ELECTRON-SPIN-RESONANCE

Autoxidation was initiated in frozen linoleic acid methyl ester by UV irradiation. The subsequent steps in autoxidation were controlled by raising the temperature above certain critical values. The pentadienyl radical, formed by UV irradiation at 77 or 90 K, was stabilized in traps of two different kinds. Above 100 K the peroxidation step was released, and the pentadienyl radicals in traps of the first kind were converted into peroxyradicals. This was a pseudo first order reaction with an activation energy of 4.4 kcal.mole-1. The remaining fraction of pentadienyl radicals, in the second kind of traps, was not accessible to O2 and vanished gradually with increasing temperature. Above 145 K the termination step of autoxidation was activated, in which the number of peroxyradicals decayed with time to the power 0.30. The value of this factor, which was independent of temperature, may either indicate that the peroxyradicals have restricted mobility in the matrix, or that the termination step is a fourth order reaction. When vitamin E was present, the decay of the peroxy radicals was accompanied by the appearance of the tocopheroxyl radical, which is expected from the antioxidant activity of vitamin E. Furthermore, in samples with vitamin E, the fraction of pentadienyl radicals in traps of the first kind, which could be converted into peroxy radicals, was smaller compared to samples deficient in vitamin E.