Aminated cinnamic acid analogs as dual polarity matrices for high spatial resolution MALDI imaging mass spectrometry

Abstract

Background: We have developed a new class of dual polarity molecules for matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) capable of acquiring 5 μm pixel sizes with high sensitivity toward polar lipids and metabolites. Aminated cinnamic acid analogs (ACAAs) are vacuum stable, have high extinction coefficients at 355 nm, are highly sensitive to polar lipids, have low toxicity, and are affordable. Current molecules used for high spatial resolution MALDI IMS of polar lipids have shown great success, but are plagued with issues such as low sensitivity at high spatial resolution, vacuum instability, and/or high toxicity. Results: ACAAs were evaluated as MALDI matrices, testing them for vacuum stability, absorption at 355 nm, crystal size, sensitivity, and molecular coverage. Among them, 4-aminocinnamic acid (ACA) and 4-(dimethylamino)cinnamic acid (DMACA) were found to perform better than conventional MALDI matrices for lipid IMS experiments. ACA generated fewer in-source fragments due to its high extinction coefficient at 355 nm. This leads to better discernment of thermally labile molecules such as gangliosides compared to typical ‘soft’ ionization matrices like DHA using murine brain tissue. On the other hand, DMACA showed better optical properties than ACA, giving it higher sensitivity from many lipid classes, such as phospholipids and sulfatides. DMACA outperformed DAN and DHA at their individually optimized laser power at small pixel sizes (≤10 μm). DMACA also allows for lower laser power to be used without compromising sensitivity, which reduced the laser spot size at the sample surface from ∼6 μm to ∼4.5 μm without hardware modifications. Significance: Improved sensitivity and absorption efficiency at 355 nm allow for 5 μm pixel size MALDI IMS without oversampling while maintaining high S/N on commercial mass spectrometry platforms. Performing MALDI experiments at reduced laser energies minimizes tissue damage, enabling advanced multimodal MALDI IMS studies to be performed on single tissue sections. Comparisons and optimized MALDI IMS methods were performed on murine tissues and human kidney samples as part of the Human Biomolecular Atlas Program.