Multifunctional Reactive MALDI Matrix Enabling High-Lateral Resolution Dual Polarity MS Imaging and Lipid C═C Position-Resolved MS² Imaging

Abstract

Local lipid variations in tissues are readily revealed with mass spectrometry imaging (MSI) methods, and the resulting lipid distributions serve as bioanalytical signatures to reveal cell-or tissue-specific lipids. Comprehensive MSI lipid mapping requires measurements in both ion polarities. Additionally, structural lipid characterization is necessary to link the lipid structure to lipid function. Whereas some structural elements of lipids are readily derived from high-resolution mass spectrometry (MS) and tandem-MS (MS n ), the localization of CC double bonds (DBs) requires specialized fragmentation and/or functionalization methods. In this work, we identify a multifunctional matrix-assisted laser desorption/ionization (MALDI) matrix for spatially resolved lipidomics investigations that reacts with lipids in Paterno−Buchi (PB) reactions during laser irradiation facilitating DB-position assignment and allows dual-polarity high-resolution MALDI-MSI and MALDI MS 2 I studies. By screening 12 compounds for improved ionization efficiency in positive-/negative-ion mode and the functionalization yield compared to the previously introduced reactive MALDI matrix benzophenone, 2-benzoylpyridine (BzPy) is identified as the best candidate. The new matrix enables DB localization of authentic standards belonging to 12 lipid classes and helps to assign 133/58 lipid features in positive-/negative-ion mode from mouse cerebellum tissue. The analytical capabilities of BzPy as a multifunctional MALDI-MSI matrix are demonstrated by imaging endogenous and PB-functionalized lipids in mouse kidney sections with 7 μm lateral resolution in both ion modes. Tracking diagnostic lipid DB-position fragment ions in mouse pancreatic tissue with down to 10 μm pixel size allows us to identify the islets of Langerhans associated with lipid isomer upregulation and depletion.

Advanced tandem mass spectrometry in metabolomics and lipidomics—methods and applications

“…The resulting oxetanes allow to spatially discern DB position isomers as shown in Fig. 11 [152]. Reactive MALDI-MS 2 I of protonated PC 34:1 from mouse pancreas tissue with 10 μm lateral resolution reveals the presence of two distinct DB position isomers.…”

“…The authors developed a compound that serves as MALDI matrix but also reacts with most neurotransmitters installing a permanent charge and enabling compound identification via MS n . The concept of reactive MALDI matrices for structure diagnostic MSI investigations has further been developed by Heiles and co-workers [151,152]. Instead of reacting compounds with the matrix on-tissue, the authors identified benzophenone-based compounds that function as MALDI matrices but react on-demand, during UV laser irradiation, with DBs of lipids in a PB reaction.…”

“…Therefore, a combination of CID with specialized tandem MS methodologies, even when they have inferior analytic performance compared to CID at the moment, will broaden the spectrum of structures available from untargeted metabolomic and lipidomic studies. [152], copyright 2020 American Chemical Society Furthermore, developments of new and some established advanced tandem MS tools or corresponding mass spectrometers will most likely decrease the gap in performance between CID and other activation strategies.…”

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

“…Benzophenone (BPh) derivatives undergo PB reactions and at the same time show good absorption behavior as needed for the MALDI MS process. This was used by the group of Heiles for the on‐tissue derivatization and detection of unsaturated phospholipids (PLs) [99] and allowed to visualize 12 lipid classes from mouse kidney and pancreas tissue through MALDI MSI [100] …”

Quantification and molecular imaging of fatty acid isomers from complex biological samples by mass spectrometry

“…Visualization of FA isomers across tissue samples provides critical information to better understand their region-specic functions. It is noted that the Paternò-Büchi reaction, 15,17,36 ozone-induced dissociation (OzID), 16 ozonolysis 37 and m-CPBA epoxidation 29 have been adopted for MS-based mapping of lipid isomers on tissues, however, challenges still remain including low yields, side reactions, and interference from excess on-tissue derivatization reagent. Therefore, the development of novel methodologies for spatial mapping and quantication of FA isomers within biological samples is still in great demand.…”