Isomerization of conjugated linolenic acids during methylation.

Conjugated linolenic acids (CLnA) need to be converted into their methyl esters when they are analyzed by gas liquid chromatography (GLC) or high performance liquid chromatography (HPLC). We found that methylation under different conditions could cause substantial isomerization of CLnA. The present study was therefore to optimize the acid-catalyzed or base-catalyzed methylation conditions in order to minimize the artifact derived from isomerization. It demonstrated clearly that isomerization was temperature and time-dependent if methylation was conducted by acid catalysis. For the two acid-catalyzed methylation reagents, BF3/methanol caused greater isomerization than H2SO4/methanol. It was found that using H2SO4/methanol as a reagent at 40 degrees C for 10 min was most appropriate to avoid isomerization when free CLnA was methylated. In contrast, base-catalyzed methylation in NaOMe/methanol at 40 degrees C for 10 min could minimize the isomerization of CLnA in triacylglycerol form.