Structural characterization of lipids using 193 nm ultraviolet photodissociation mass spectrometry

As their diversity of structures suggests, lipids have a wide range of functions from storing energy to participant signal transduction. Over the last decade, mass spectrometry has emerged as a powerful analytical tool for identification and quantitation of lipids. While tandem mass spectrometry affords a higher level of structural characterization than intact mass measurements alone, conventional ion activation methods often fail to elucidate subtle yet critical structural features. The research presented in this dissertation explores the utility of 193 nm ultraviolet photodissociation (UVPD), a fast and high-energy alternative ion activation method, for structural characterization of phosphatidylcholines and lipooligosaccharides. Phosphatidylcholines are the most abundant glycerophospholipid in mammalian cell membranes and therefore represent an important analytical target. Low-energy collision-based activation methods generate head group and acyl chain fragment ions that permit determination of the lipid class and acyl chain composition, respectively. Photodissociation at 193 nm UVPD similarly produced head group and acyl chain fragment ions, in addition to diagnostic pairs of fragment ions that afforded localization of double bonds within acyl chains. Implementation of UVPD in a shotgun lipidomic workflow enabled identification of double bond positional isomers in complex glycerophospholipid extracts that otherwise would have gone undetected with low-energy collision-based activation methods. Coupling of the UVPD method to a mass spectrometry imaging workflow further facilitated detection of double bond positional isomers in situ. Changes in the relative abundances of double bond positional isomers were correlated to specific tissue features. Lipooligosaccharides (LOS) are saccharolipids that are the main component of the outer membrane of Gram-negative bacteria. Determination of unknown lipooligosaccharides structures and modifications is crucial considering the relationship between LOS structures and the development of antibiotic resistance. A mass spectrometry method utilizing the MS [superscript n] capabilities of linear ion trap mass analyzer, 193 nm UVPD, and high resolution scanning in the Orbitrap mass analyzer facilitated detailed structural characterization of intact LOS structures in both shotgun and chromatographic workflows. The methods permitted both localization of non-stoichiometric modification and determination of unknown core oligosaccharide structures