Lipid hydroperoxides conjugated dienes value

The conjugated diene hydroperoxides produced from polyunsaturated lipids can be determined quantitatively by their strong absorption maximum at 234 nm (molecular extinction of 25,000-29000 for a solution of 1 mole hydroperoxides/1 methanol). A weighed sample is simply diluted in methanol (for esters) or in isooctane (for triacylglycerols) and the absorbance at 234 nm measured spectrophotometrically. Aqueous emulsions can also be measured directly by dissolving in methanol even if the solutions becomes slightly cloudy, provided a suitable blank is used with the same diluted unoxidized sample. The conjugated diene value of lipids can be determined by their UV absorbance at 234 nm and expressed in /imol hydroperoxides/g sample. 

The radiolytic yields of formation of hydroperoxides (G(hydro)) are determined from the slope ofthe initial tangent ofthe curves (concentration versus radiation dose, as in Fig. 4). An important lipid concentration dependency is observed in micellar medium where, above the cmc, the yields of oxidized products formation increase steeply with PUFA concentration. This means that a chain-mechanism is involved in the formation of hydroperoxides and conjugated dienes. As an example, in the case of a 25 mM linolenate concentration (Fig. 4), the G-values are 20.0 x 10 mol J" and 9.3 x 10" mol J" for hydroperoxides and conjugated dienes, respectively. These yields are considerably larger than G.oh = 2.8 x 10" mol J since the hydroxyl radicals are initiators ofthe oxidation chain mechanism. 

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