30] Detection of conjugated dienes by second derivative ultraviolet spectrophotometry

Abstract

This chapter discusses the detection of conjugated dienes by second derivative ultraviolet spectrophotometry. Conjugated dienes (CD) refers to two double bonds separated by a single bond. This structure is unusual in polyunsaturated fatty acids (PUFA). It is generally accepted that the occurrence of conjugated dienes in lipids means autoxidation of lipids. In fact, because of the divinylmethane structure, PUFA are particularly susceptible to hydrogen abstraction by free radical attack, becoming themselves free radical intermediates. This results in the rearrangement of the double bond to conjugated dienes and, in the presence of O2, the formation of fatty acid hydroperoxides. The conjugated diene moiety is a strong chromophore that can be detected spectrophotometrically. When present in fatty acids they show a characteristic absorption in the UV region at around 234 nm. However, detection and quantitation of conjugated dienes in mixtures of peroxidized and nonperoxidized lipids, by means of simple UV spectrophotometry, is complicated by the end absorption exhibited by naturally occurring and nonperoxidized lipids.