Mass spectrometric evaluation of the tert.-butyldimethylsilyl derivatives of monohydroxyeicosatetraenoic acids and leukotrienes.

The tert.-butyldimethylsilyl (t-BDMS) derivatives of the hydroxyl and the carboxyl groups on the monohydroxyeicosatetraenoic acids (HETEs) 5-, 12- and 15-hydroxyeicosatetraenoic acid (5-HETE, 12-HETE and 15-HETE, respectively) as well as leukotriene B4 (LTB4), 20-carboxy-LTB4 (20-COOH-LTB4) and 20-hydroxy-LTB4 (20-OH-LTB4) have been prepared. The derivatization reagents N-methyl-N-tert.-butyldimethylsilyltrifluoroacetamide (MTBSTFA) and tert.-butyldimethylsilylimidazole (t-BDMSIM) were utilized and the derivatization yields evaluated by gas chromatographic analysis. While the derivatization of 12-HETE and 15-HETE was achieved with good yield using MTBSTFA, the 5-hydroxylated compounds could only be derivatized satisfactorily with t-BDMSIM. The t-BDMS ester derivatives of monohydroxy fatty acids and 20-COOH-LTB4 were found quite susceptible to hydrolysis upon attempted isolation. The mass spectrometric fragmentation patterns of the HETEs, LTB4, 20-COOH-LTB4 and 20-OH-LTB4 and their reduced analogues as the t-BDMS ester and ether derivatives were examined. The fragmentation of the unsaturated compounds was mainly influenced by the double bonds while, on the contrary, the saturated compounds showed intense high-mass ions derived from elimination of a tert.-butyl radical. Furthermore, it was observed that the t-BDMS ester moiety of the unsaturated compounds exerted certain influence upon the fragmentation pattern. With regard to earlier results obtained with the methyl ester t-BDMS ether derivative, it was demonstrated that the use of the t-BDMS ester derivative resulted in decreased intensities of high-mass ions with a few exceptions. Taking into account the mass spectrometric fragmentation and the poor hydrolytic stability of the t-BDMS ester derivatives of some of the studied compounds, the use of these derivatives was not found to constitute a major advantage over the use of the methyl ester derivative. However, a favourable fragmentation pattern with a very intense high-mass ion was obtained for 15-HETE and the levels of this compound were determined in human lung tissue samples.