Assessing presence of alternate lipid oxidation pathways from volatile products detected by gas chromatography

Recently a new reaction scheme that integrates alternate lipid oxidation pathways including addition, cyclization, and scission with traditional hydrogen abstraction was proposed to more completely account for products and kinetics of lipid oxidation. These pathways compete with hydrogen abstraction during autoxidation, and the balance between them changes with reaction conditions, altering product levels and distributions. Alternate pathways, if present, must force a new paradigm in analysis of lipid oxidation since conjugated dienes, hydroperoxides, and perhaps hexanal alone can no longer adequately reflect lipid oxidation. Thus, it is critical to verify the presence of alternate pathways and learn how they can be used to more fully elucidate mechanisms of lipid oxidation. Overall oxidation kinetics, individual kinetics, identifications, and distributions of volatile products formed during oxidation of methyl linoleate were tracked using three different gas chromatography (GC) methods -- static headspace (SHS), solid-phase micro-extraction (SPME), and thermal desorption (TD). Methyl linoleate was incubated neat at 25, 40, and 60 °C under varying conditions of oxygen and solvent to investigate ability of reaction environment to force shifts in reaction pathways. Incubations were limited to 20 days to focus specifically on mechanisms involved in early oxidation. Major early products pentane, hexanal, t-2-heptenal, t-2-octenal, and methyl octanoate were all detected by each GC method but in different amounts, distributions, and kinetics of formation. During early incubation, C-13 alkoxyl radical scission was the dominant pathway for generation of volatile products, with pentane as the overwhelming major product and hexanal at much lower levels even as the second largest peak. Levels and timing of appearance of remaining products were best explained by a process in which C-9 alkoxyl radicals internally rearranged to epoxides, moving radical sites down the acyl chain to C-11 in the process. Hydrolysis of the epoxides generates methyl octanoate. Oxidation at C-11, a secondary rather than initiating process, yields t-2-heptenal and t-2-octenal as products from β and α-scission, respectively. Results verify existence of multiple pathways in oxidation of methyl linoleate and demonstrate that these must be considered to explain the distribution and kinetics of oxidation products.Ph.D.Includes bibliographical referencesby Brandon A. Bogus