A Caged In-Source Laser-Cleavable MALDI Matrix with High Vacuum Stability for Extended MALDI-MS Imaging.

Insufficient vacuum stability of matrix chemicals is a major limitation in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) of large tissue sample cohorts. Here, we designed and synthesized the photo-cleavable caged molecule 4,5-dimethoxy-2-nitrobenzyl-2,5-dihydroxyacetophenone (DMNB-2,5-DHAP) and employed it for lipid MALDI-MSI of mouse brain tissue sections. DMNB-2,5-DHAP is vacuum-stable in a high vacuum MALDI ion source for at least 72 h. Investigation of the uncaging process suggested that the built-in laser (355 nm) in the MALDI ion source promoted the in situ generation of 2,5-DHAP. A caging group is used for the first time in designing a MALDI matrix that is vacuum-stable, uncaged upon laser irradiation during the measurement process, and that boosts lipid ion intensity with MALDI-2 laser-induced postionization.

New Derivatization Reagent for Detection of free Thiol-groups in Metabolites and Proteins in Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging.

Several diseases are associated with disturbed redox signaling and altered metabolism of sulfur-containing metabolites and proteins. Importantly, oxidative degradation of fresh-frozen tissues begins within the normal time scale of MALDI MSI sample preparation. As a result, analytical methods that preserve the redox state of the tissue are urgently needed for refined studies of the underlying mechanisms. Nevertheless, no derivatization strategy for free sulfhydryl groups in tissue is known for MALDI MSI. Here, we report the first derivatization reagent, (E)-2-cyano-N-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethyl)-3-(4-hydroxyphenyl)acrylamide (CHC-Mal), for selective detection of free thiols using MALDI MSI. We performed in situ derivatization of free thiol groups from thiol-containing metabolites such as glutathione and cysteine and reduced proteins such as insulin and imaged their spatial distribution in porcine and mouse xenograft tissue. Derivatization of thiol-containing metabolites with CHC-Mal for MALDI MSI was also possible when using aged tissue in the presence of excess reducing agents. Importantly, CHC-Mal-derivatized low mass-metabolites could be detected without the use of a conventional MALDI matrix.

Highly Stable trans-Cyclooctene Amino Acids for Live-Cell Labeling.

trans-Cyclooctene groups incorporated into proteins via non-canonical amino acids (ncAAs) are emerging as specific handles for bioorthogonal chemistry. Here, we present a highly improved synthetic access to the axially and the equatorially linked trans-cyclooct-2-ene isomers (1 a,b). We further show that the axially connected isomer has a half-life about 10 times higher than the equatorial isomer and reacts with tetrazines much faster, as determined by stopped-flow experiments. The improved properties resulted in different labeling performance of the insulin receptor on the surface of intact cells.