Fatty acids conjugated dienes value

Measurement of Unsaturation. The presence of double bonds in a fatty acid side chain can be detected chemically or through use of instmmentation. Iodine value (IV) (74) is a measure of extent of the reaction of iodine with double bonds the higher the IV, the more unsaturated the oil. IV may also be calculated from fatty acid composition. The cis—trans configuration of double bonds may be deterrnined by infrared (59) or nmr spectroscopy. Naturally occurring oils have methylene-intermpted double bonds that do not absorb in the uv however, conjugated dienes maybe deterrnined in an appropriate solvent at 233 nm. 

The ene-yne CM of fatty acid-derived terminal alkenes with several alkyne derivatives was shown by Bruneau et al. [75], These reactions, which led to renewable conjugated dienes, were performed in a one-pot two-step procedure. In the first step, the ethenolysis of methyl oleate was performed in the presence of the first-generation Hoveyda-Grubbs catalyst (2.5 mol%) using dimethyl carbonate as solvent at room temperature. After completion of the ethenolysis (90% conversion), C4 (1 mol%) and the corresponding alkyne (0.5 equivalents with respect to olefins) were added and the reaction was run at 40°C for 2 h (Scheme 9). The desired dienes were thus obtained in high yields close to the maximum theoretical value (50%). Moreover, in order to maximize the formation of functional dienes, the same reaction sequence was applied to the diester obtained by SM of methyl oleate. In this way, the yield of functional dienes was increased up to 90% depending on the... 

Conjugated dienoic fatty acids have a 234 nm chromophore (e = 24,-000), and their formation can be followed continuously in optically clear mixtures or discontinuously by solvent extraction (Surrey, 1964). The method is specific and is particularly suitable for purified enzymes at high pH where fatty acids form soap solutions. In crude extracts, the method has limited value because of interfering absorptions and, more importantly, secondary reactions of the unstable hydroperoxide produets (see later) resulting in chromophore destruction (Berkeley and Galliard, 1976). It should be noted that even some purified LOX preparations contain lipoperoxidase activity (Grosch et al., 1972). Thus the diene absorption method should be used with caution and, preferably, checked for stoichiometry. 

The second approach consisted in the separation of CLA isomers as the p-methoxyphenacyl esters with dichloromethane/hexane/acetonitrile mixtures as the mobile phase (14). In this instance, only a single chromatographic column was required. Because detection was by the absorbance of the p-methoxyphenacyl moiety at 270 nm, aU fatty acids were detected and quantified, not simply the conjugated dienes. Finally, good resolution of CLA as the free acids has recently been reported, with hexane/acetonitrile/acetic acid mixtures as the mobile phase with detection of the conjugated double bonds at 234 nm (15). This procedure may be of special value for commercial CLA samples supplied as the free adds because no derivatization step is required. 

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