Characterization of Phosphatidylcholine Oxidation Products by MALDI MS^<i>n</i>

Phospholipid oxidation has been implicated in the pathogenesis and progression of numerous age-related and neurodegenerative diseases. Despite these implications, this broad class of biomolecules remains poorly characterized. In this work, the fragmentation patterns of [M + H]⁺ and [M + Na]⁺ ions of intact phosphatidylcholine oxidation products (OxPCs) were characterized by matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI MS^<i>n</i>, <i>n</i> = 2, 3, and 4). MS² of both the [M + H]⁺ and [M + Na]⁺ ions of short-chain OxPCs yielded product ions related to the PC headgroup and the fatty acid substituents. MS³ of the [M + Na – N­(CH₃)₃]⁺ ions yielded fragmentation indicative of the OxPC modification; specifically, a product ion corresponding to the neutral loss of CO₂ (NL of 44) was observed for OxPCs containing a terminal carboxylic acid rather than an aldehyde. Furthermore, MS⁴ of the [M + Na – HPO₄(CH₂)₂N­(CH₃)₃]⁺ ions resulted in fragmentation pathways dependent on the <i>sn</i>-2 fatty acid chain length and type of functional group(s). Specifically, CHO-containing OxPCs with palmitic acid esterified to the <i>sn</i>-1 position of the glycerol backbone yielded a NL of 254, 2 u less than the nominal mass of palmitic acid, whereas the analogous terminal COOH-containing OxPCs demonstrated a NL of 256. Finally, the presence of a γ-ketone relative to the terminal carboxyl group resulted in C–C bond cleavages along the <i>sn</i>-2 substituent, providing diagnostic product ions for keto-containing OxPCs. This work illustrates the enhanced selectivity afforded by MS^<i>n</i> on the linear ion trap and develops a method for the identification of individual products of PC oxidation

MALDI Mass Spectrometry Imaging in Microscope Mode with Infrared Lasers: Bypassing the Diffraction Limits

This letter demonstrates the use of infrared matrix-assisted laser desorption/ionization coupled with microscope mode mass spectrometry imaging. It is aimed to explore the use of intrinsic water in tissue as a matrix for imaging at spatial resolutions below the diffraction limit of the employed IR optics. Stigmatic ion optics with a magnification factor of ∼70 were used to project the spatial distribution of produced ions onto a detector while separating ions with different mass-to-charge ratios using a time-of-flight mass spectrometer. A pixelated detector was used to simultaneously record arrival time and impact position. A previously described dried-droplet sample system of 2,5-dihydroxybenzoic acid (DHB) and 5 peptides covered by a copper grid for defined surface structure was used to benchmark the light- and ion-optical setup for spatial resolution and mass spectrometric performance. A spatial resolving power of 9.8 μm, well below the optical limit of diffraction (14 μm for the given setup), was established. After, frozen cryo-sections from a biological model system were measured by exploiting the endogenous water content as a matrix. Principal component analysis enabled a clear distinction between distinct tissue regions identified by both light microscopy and MS imaging