Photo- and thermal-oxidation studies on methyl and phenyl linoleate: anti-oxidant behaviour and rates of reaction

Photo-peroxidation of methyl and phenyl linoleate in methanol solutions at 25 degrees C, in the presence of methylene blue or 5,10,15,20-tetra(4-pyrid yl)-porphyrin (TPP) as sensitisers of singlet oxygen, was found to proceed at more than 30 times the rate of the same polyunsaturated fatty acid (PUFA ) ester species undergoing thermal-peroxidation in the bulk phase at 50 deg rees C. The addition of anti-oxidants such as butylated hydroxyanisole (BHA ) and butylated hydroxytoluene (BHT) quench the thermal-oxidation effective ly but appear to only partially inhibit the photosensitised peroxidation re actions. The kinetics of the overall peroxidation reactions were followed b y ultraviolet spectroscopy, measurements of hydroperoxide concentration and by high performance liquid chromatography (HPLC). The photo-peroxidation r eaction proceeds more rapidly in chloroform solution as the lifetime of sin glet oxygen is shown to be over ten times longer in chloroform than methano l. The initial fast reaction kinetics of the photo-peroxidation reactions w ere evaluated using a pulsed laser technique to show that singlet oxygen re acts competitively with both the anti-oxidants and the polyunsaturated fatt y acid ester. Second order kinetic rate constants (in the range 10(5)-10(7) dm(3) mol(-1) s(-1)) were evaluated for the reactivity of singlet oxygen w ith a range of anti-oxidants and a singlet oxygen quencher, and the results used to explain the effect of anti-oxidants at different concentrations on the rate of the linoleate photo-peroxidation reaction. (C) 2000 Elsevier S cience Ireland Ltd. All rights reserved.