However, the powerful method of ECD requires at least two positive charges (because capture of one electron reduces the total charge by one and mass spectrometers cannot detect neutrals) and, thus, analysis of acidic biomolecules, which show improved ionization in negative ion mode, is challenging. We have explored the utility of divalent metals as charge carriers in ECD and found that positive ion mode ionization efficiency improves for sulfonated peptides and acidic glycans from glycoproteins. In addition, ECD of metal-adducted sulfonated species proceeds with partial or complete retention of the highly gas-phase labile sulfonate, thereby allowing determination of its location. ECD of metal-adducted glycans provides intriguing fragmentation patterns with highly sought after sugar cross-ring fragmentation (which can provide linkage information) being dominant in several cases. EDD, which operates in negative ion mode, has lower fragmentation efficiency than ECD but also appears valuable for glycan analysis: We have shown that EDD provides structural information that is complementary to that obtained from ECD, CAD, and IRMPD, including additional cross-ring cleavages.
In more recent efforts, we have utilized chloride adducts to extend the utility of EDD (which requires at least two negative charges) and begun to explore electron-induced dissociation (EID) of singly charged analytes. Similar to metal-assisted ECD, chloride-assisted EDD results in complementary fragmentation pathways for carbohydrates, thereby generating more extensive glycan structural information. EID of manganese-adducted fatty acids allows double bond locations to be determined. Finally, we have discovered that negatively charged biomolecular ions can capture electrons (rather than lose electrons such as in EDD, or undergo electronic excitation such as in EID). This phenomenon, which we termed negative ion electron capture dissociation (niECD), results in charged-increased species that undergo dissociation analogous to that in ECD of cationic species. niECD is exciting for several reasons, e.g.: 1) negative ion mode analysis yields improved ionization efficiency for many important acidic molecules, including phosphorylated and sulfonated peptides. 2) FT-ICR MS detection efficiency is proportional to charge and thus niECD results in improved detection limits. 3) niECD is compatible with singly charged ions and thus allows coupling with matrix-assisted laser desorption/ionization (MALDI).
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