Metal-Oxide Enrichment and Gas-Phase Characterization of Sulfopeptides using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry.

Abstract

Though not as well studied as phosphopeptides, sulfopeptides are important for many biological processes, including proper endocrine function and extracellular signaling. The discovery of sulfopeptides dates back to the 1920s; however, their enrichment and characterization have only recently become of broader interest. With a limited toolbox for analyzing sulfopeptides, we employ several chemistries to develop robust enrichment and characterization methods. At the heart of each method lies Fourier transform ion cyclotron resonance mass spectrometry, a gas-phase detection method with the power to differentiate even the slightest mass differences, such as phosphate vs. sulfonate. First, Lewis acid-base characteristics inherent to transition metal oxides are examined for the selective interaction and enrichment of sulfopeptides in the presence of mixtures of competing poly-oxyanions. Careful control of the binding and elution pH with an optimized amount of sulfopeptide loaded onto the metal oxide surface can enhance enrichment selectivity of sulfopeptides up to 97% relative abundance compared to as low as 4% prior to enrichment. Second, the utility of gas-phase activation methods for structural characterization of sulfopeptides is investigated. The sulfonate post-translational modification (PTM) is extremely labile at low pH, high temperature, and during gaseous collisional activation. This fragility has challenged researchers to discover new techniques for analysis of intact sulfonated biomolecules. As recently as 2011, authors have accepted that the sulfonate modification is lost during mass spectrometric analysis. We have found that a combination of different activation techniques can elucidate sulfopeptide sequence while keeping the labile sulfonate residue intact, allowing for unambiguous localization of this PTM. In particular, negative ion electron capture dissociation was found to yield >50% fragmentation efficiency with complete sulfonate retention. Finally, ideas are explored for improving fragmentation efficiency in electron detachment dissociation, which commonly leads to extensive neutral loss from carboxylic acids, precluding efficient backbone fragmentation and subsequent structural elucidation. To block carbon dioxide loss, chemical derivatization and anion adduction were employed. We found that chloride adduction to acidic peptides improves the fragmentation efficiency and provides nearly complete sequence coverage for several peptides. In addition, N-acetylation was shown to alter observed fragmentation pathways, presumably through changes in peptide gas-phase structures.