Oxidation of linoleic acid by copper(II) complexes: effects of ligand

The oxidation of linoleic acid (LH) by cupric (Cu(II)) complexes was investigated by absorption spectroscopy and high performance liquid chromatography (HPLC). The following Cu(II) complexes were used: Cu(II)(BC)(2) (BC: bathocuproinedisulfonic acid), Cu(II)(CyHH)(2) (CyHH: cyclo(L-histidyl-L-histidyl)), Cu(II)(OP)(2) (OP: o-phenanthroline), Cu(II)(HGG) (HGG: L-histidylplycylglycine), and Cu(II) (en), (en: ethylenediamine). The absorbance at 234 nm observed during Cu(II)-catalyzed oxidation of LH increased with incubation time, reached a maximum level, and subsequently decreased. HPLC chromatograms of oxidation products indicated the appearance of peaks corresponding to four isomers of linoleic acid hydroperoxide (LOOH) and subsequent complete decomposition of LOOH. The time to reach the maximal value of absorbance at 234 nm within 24 h was in the following order: Cu(II)(BC)(2) > Cu(II)(CyHH)(2) = Cu(II)(OP)(2) > Cu(II) (en)(2) > Cu(II) (HGG). These observations suggested that Cu(II)(BC)(2) can produce LOOH from linoleic acid more rapidly than Cu(II)(HGG) and decompose the LOOH generated more easily. The absorbance at 234 nm observed in the reaction of Cu(II)(HGG) or Cu(II)(en)(2) with LH in the presence of H2O2 increased with incubation time more rapidly than in the absence of H2O2. Hydroxyl radical generation from the reaction of Cu(II) (HGG) or Cu(II) (en)(2) with H2O2 may initiate the oxidation of LH. In contrast, the addition of H2O2 inhibited the oxidation of LH by Cu(II) (BC)(2). (C) 1999 Elsevier Science Inc. All rights reserved.