Proportion of geometrical hydroperoxide isomers generated by radical oxidation of methyl linoleate in homogeneous solution and in aqueous emulsion

Abstract

The proportion of geometrical hydroperoxide isomers generated by aerobic oxidation of methyl linoleate (18:2 Me) in either aqueous emulsion consisting of Tris-HCl buffer (pH 7.4) or in a homogeneous dichloromethane solution was determined to understand the mechanism of lipid oxidation in different reaction systems. Four geometrical isomers were generated after oxidation of 18:2 Me in dichloromethane: methyl 13-hydroperoxy-cis-9, trans-11-octadecadienoate, methyl 13-hydroperoxy-trans-9,trans-11-octadecadienoate, methyl 9-hydroperoxy-trans-10,cis-12-octadecadienoate, and methyl 9-hydroperoxy-trans-10, trans-12-octadecadienoate in the ratios of 1:4:1:4, respectively. The ratios between each isomer did not change until the peroxide value (PV) increased to 58 meq/kg. Oxidation of 18:2 Me in aqueous emulsion yielded the same geometrical isomers of hydroperoxide. However, the ratios were different: 3:2:3:2 until the PV increased to 110 meq/kg. Predominant (60%) formation of trans,trans hydroperoxide isomers was obtained in the oxidation of a mixture of 18:2 Me and methyl laurate (12:0 Me). These results are interpreted to reflect the importance of the concentration of hydrogen atom-donating equivalents to the kinetic preference for different products. The high effective concentration of hydrogen donors in the oxidation of 18:2 Me in emulsions favored the formation of the less stable cis,trans isomers. The lower concentration of hydrogen donor in the dichloromethane solution effectively slowed hydrogen donation and led to the strong preference for the more stable trans,trans isomers. This interpretation was further tested by preparing emulsions of 18:2 Me and 12:0 Me to dilute concentration of hydrogen-donating species using the nonhydrogen-donating 12:0 Me. Consistent with the proposed hypothesis, the proportion of trans,trans isomers increased as a result of 12:0 Me addition.