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Besides protein identification via mass spectrometric methods, protein and peptide quantification has become more and more important in order to tackle biological questions. Methods like differential gel electrophoresis or enzyme-linked immunosorbent assays have been used to assess protein concentrations, while stable isotope labeling methods are also well established in quantitative proteomics. Recently, we developed metalcoded affinity tagging (MeCAT) as an alternative for accurate and sensitive quantification of peptides and
proteins. In addition to absolute quantification via inductively coupled plasma mass spectrometry, MeCAT also enables sequence analysis via electrospray ionization tandem mass spectrometry. In the current study, we developed a new labeling approach utilizing an iodoacetamide MeCAT reagent (MeCAT-IA). The MeCAT-IA approach shows distinct advantages over the previously used MeCAT with maleinimide reactivity such as higher labelling efficiency and the lack of diastereomer formation during labeling. Here, we present a careful characterization of this new method focusing on the labeling process, which yields complete tagging with an excess of reagent of 1.6 to 1, less complex chromatographic behavior, and fragmentation characteristics of the tagged peptides using the iodoacetamide MeCAT reagent.
Mass spectrometry is applied as a tool for the elucidation of molecular structures. This premises that gas-phase structures reflect the original geometry of the analytes, while it requires a thorough understanding and investigation of the forces controlling and affecting the gas-phase structures. However, only little is known about conformational changes of oligonucleotides in the gas phase. In this study, a series of multiply charged DNA oligonucleotides (n¼15–40) has been subjected to a comprehensive tandem mass spectrometric study to unravel transitions between different ionic gas-phase structures. The nucleobase sequence and the chain length were varied to gain insights into their influence on the geometrical oligonucleotide organization. Altogether, 23 oligonucleotides were analyzed using collision-induced fragmentation. All sequences showed comparable correlation regarding the characteristic collision energy. This value that is also a measure for stability, strongly correlates with the net charge density of the precursor ions. With decreasing charge of the oligonucleotides, an increase in the fragmentation energy was observed. At a distinct charge density, a deviation from linearity was observed for all studied species, indicating a structural reorganization. To corroborate the proposed geometrical change, collisional cross-sections of the oligonucleotides at different charge states were determined using ion mobility-mass spectrometry. The results clearly indicate that an increase in charge density and thus Coulomb repulsion results in the transition from a folded, compact form to elongated structures of the precursor ions. Our data show this structural transition to depend mainly on the charge density, whereas sequence and size do not have an influence.