UV-initiated autoxidation of methyl linoleate in micelles studied by optical absorption.

The UV-induced dissociation of lipid hydroperoxides, naturally present in methyl linoleate and methyl linoelaidate, was monitored by absorption spectroscopy. The decay of hydroperoxides in homogeneous solution, for a fluence rate of 0.82 mW.cm-2 at 240 nm, followed an exponential course with an average rate constant of k = (2.0 +/- 0.1) . 10(-3) s-1. Addition of alpha-tocopherol led to an increase in the magnitude of k until it reached a limiting value of (3.4 +/- 0.2) . 10(-3)s-1. When methyl linoleate (containing adventitous traces of hydroperoxides) was incorporated in SDS micelles, sustained UV irradiation caused initially an increase in the amounts of lipid hydroperoxides, due to linoleate autoxidation, followed by an exponential decrease due to photodissociation. From the initial rate of hydroperoxide formation (due to linoleate autoxidation) and the rate constant of the subsequent hydroperoxide decay, an oxidizibility of 0.61 . 10(-2) (Ms)-1/2 of methyl linoleate in micelles was deduced. It is suggested that UV irradiation and optical absorption afford an easy and reliable means for studying autoxidation of polyunsaturated fatty acids.