A Sulfoxide-Based Isobaric Labelling Reagent for Accurate Quantitative Mass Spectrometry.

Modern proteomics requires reagents for exact quantification of peptides in complex mixtures. Peptide labelling is most typically achieved with isobaric tags that consist of a balancer and a reporter part that separate in the gas phase. An ingenious distribution of stable isotopes provides multiple reagents with identical molecular weight but a different mass of the reporter groups, allowing relative quantification of multiple samples in one measurement. Here we report a new isobaric labelling reagent, where the balancer and the reporter are linked by a sulfoxide group, which, based on the sulfoxide pyrolysis, leads to easy and asymmetric cleavage at low fragmentation energy. The fragmentation of our new design is significantly improved, yielding more intense complementary ion signals, allowing complementary ion cluster analysis as well.

An Endoperoxide Reactivity-Based FRET Probe for Ratiometric Fluorescence Imaging of Labile Iron Pools in Living Cells.

Iron is essential for sustaining life, as its ability to cycle between multiple oxidation states is critical for catalyzing chemical transformations in biological systems. However, without proper regulation, this same redox capacity can trigger oxidative stress events that contribute to aging along with diseases ranging from cancer to cardiovascular and neurodegenerative disorders. Despite its importance, methods for monitoring biological iron bound weakly to cellular ligands-the labile iron pool-to generate a response that preserves spatial and temporal information remain limited, owing to the potent fluorescence quenching ability of iron. We report the design, synthesis, and biological evaluation of FRET Iron Probe 1 (FIP-1), a reactivity-based probe that enables ratiometric fluorescence imaging of labile iron pools in living systems. Inspired by antimalarial natural products and related therapeutics, FIP-1 links two fluorophores (fluorescein and Cy3) through an Fe(II)-cleavable endoperoxide bridge, where Fe(II)-triggered peroxide cleavage leads to a decrease in fluorescence resonance energy transfer (FRET) from the fluorescein donor to Cy3 acceptor by splitting these two dyes into separate fragments. FIP-1 responds to Fe(II) in aqueous buffer with selectivity over competing metal ions and is capable of detecting changes in labile iron pools within living cells with iron supplementation and/or depletion. Moreover, application of FIP-1 to a model of ferroptosis reveals a change in labile iron pools during this form of cell death, providing a starting point to study iron signaling in living systems.

Fragmentation-Rearrangement of Peptide Backbones Mediated by the Air Pollutant NO2 (.).

The fragmentation-rearrangement of peptide backbones mediated by nitrogen dioxide, NO2 (.) , was explored using di-, tri-, and tetrapeptides 8-18 as model systems. The reaction, which is initiated through nonradical N-nitrosation of the peptide bond, shortens the peptide chain by the expulsion of one amino acid moiety with simultaneous fusion of the remaining molecular termini through formation of a new peptide bond. The relative rate of the fragmentation-rearrangement depends on the nature of the amino acids and decreases with increasing steric bulk at the α carbon in the order Gly>Ala>Val. Peptides that possessed consecutive aromatic side chains only gave products that resulted from nitrosation of the sterically less congested N-terminal amide. Such backbone fragmentation-rearrangement occurs under physiologically relevant conditions and could be an important reaction pathway for peptides, in which sections without readily oxidizable side chains are exposed to the air pollutant NO2 (.) . In addition to NO2 (.) -induced radical oxidation processes, this outcome shows that ionic reaction pathways, in particular nitrosation, should be factored in when assessing NO2 (.) reactivity in biological systems.