REMOTE-CHARGE-SITE DECOMPOSITION REACTIONS OF GAS-PHASE IONS: MECHANISTIC CONSIDERATIONS AND ANALYTICAL APPLICATIONS.

摘要

The analysis and characterization of lipids is a challenging problem as lipids occur naturally as complex mixtures of structurally similar compounds. Mass spectrometry methods have proven useful for analysis of these compounds; however, current methods require derivatization for fatty acids and degradation followed by derivatization for analysis of complex lipids. Hence, improved methods are needed.;Carboxylates that bear structural modifications also undergo this remote-charge-site fragmentation, but the process is interrupted at the site of the modification. The nature of the interruption serves to identify the type of structural modification, and correlation of daughter ions in the spectrum with carbon atoms of the alkyl chain beginning with the highest mass daughter ion and the alkyl terminus of the acid chain permits assignment of location of the modification on the acid chain.;FAB in combination with tandem mass spectrometry methods is also useful for characterizing intact complex phospholipids. Phosphatidylcholines, phosphatidylethanolamine, phosphatidylglycerol, phosphatidic acid, and cardiolipin yield characteristic high mass negative ions as well as carboxylate anions from the esterified acid chains. The high mass ions are useful for general characterization of the molecule. Selection and collisional activation of the acid chains permit specific structural characterization of the carboxylate components of the complex lipid.;The use of fast atom bombardment (FAB) in combination with tandem mass spectrometry, as explored in this research, offers the potential for specific structural characterization of free fatty acids and intact complex lipids from both pure samples and mixtures. Carboxylate anions, which are readily desorbed into the gas phase by FAB, do not fragment in the desorption process. However, if the (M - H)('-) anions are selected and collisionally activated, they undergo a highly specific 1,4-elimination of H(,2) that results in the losses of the elements of CH(,4), C(,2)H(,6), C(,3)H(,8), etc. These C(,n)H(,2n+2) losses begin at the alkyl terminus and progress along the entire alkyl chain. The study of deuterium labeled saturated acids has established this mechanism.