Spatial mapping of phosphatidylcholine sn-positional isomers using CID of divalent metal complexes in imaging mass spectrometry

Abstract

Phosphatidylcholines (PCs) are the main components of cellular membranes. The high degree of structural heterogeneity leads to significant variations in PC functions and complicates structural characterization. For example, the complex mixtures of lipid structures create challenges when analyzing and identifying these compounds directly from tissue in matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry experiments. Phosphatidylcholine (PCs) are preferentially ionized in the positive ion mode in MALDI imaging mass spectrometry. However, low-energy collision induced dissociation (CID) of protonated PCs largely only results in cleavages of the phosphocholine headgroup, with little to no information obtained about the fatty acyl chain identities and positions. Alternatively, metal cationization of lipids is known to generate increased structural information upon CID, but metal coordination has been less studied. Herein, we highlight the use of divalent metal-ligand complexes to produce new ion types for CID analysis in MALDI imaging mass spectrometry. CID of the new [PC + M + ligand]⁺ ion type (where M is a divalent metal) eliminates the headgroup loss fragmentation channel and opens new fragmentation channels at the fatty acyl chain positions. The gas-phase fragmentation behavior of [PC + M + ligand]⁺ ion type is characterized using multiple divalent metals and ligands. The fatty acyl chain product ions are then used to relatively quantify sn-positional isomers. Furthermore, this method is integrated into an imaging mass spectrometry workflow to enable the spatial mapping of PC sn-positional isomers in rat brain and glioblastoma tissues, revealing differential distributions of the sn-positional isomers.