Washington University School of Medicine TITLE:   Into the Mechanisms of Fragment Formation of Lipids: Multiple-Stage Tandem Mass Spectrometric Studies on Complex Lipids By Fong-Fu Hsu Washington University School of Medicine St. Louis, MO 63110

3. The fragmentation processes involving the a-hydrogen are deterred by the substitution of the fatty acyl moieties with alkyl, alkenyl, or hydroxyl groups. The substitution results in a distinct product-ion spectrum for each PC class, including diacyl-, plasmanyl-, plasmenyl-, and lyso-PC isomers. The preferential formation of the [M+H-R2CH=C=O]+ ion over the [M+H-R1CH=C=O]+ ion for the [M+H]+ species is also attributable to the fact that the a-hydrogens of the fatty acyl at sn-2 are more labile. 4. The involvement of a-hydrogen in the losses of the fatty acid substituents occurred to all the phospholipid classes including TAG, PS (as shown), and other PPL such as PA, PG, PI, PE, and the more complex PPL such as CL, resulting in structural characterization of complex lipids. However, the lithiated (or sodiated) adduct ions are often formed poorly by ESI (except PC). In contrast, ions in the form of [M – H]- ions are abundant for all the PPL class (except PC).

Acknowledgements: Center of Mass Spectrometry Resource in Washington University School of Medicine is supported by US Public Health Service Grants P41-RR-00954, R37-DK-34388, P60-DK-20579, and P01-HL-57278 and P30-DK56341.