Evidence of H/D Exchange within Metal-Adducted Carbohydrates after Ion/Ion-Dissociation Reactions.

Tandem mass spectrometry (MS/MS) using fragmentation has become one of the most effective methods for gaining sequence and structural information on biomolecules. Ion/ion reactions are competitive reactions, where either proton transfer (PT) or electron transfer (ET) can occur from interactions between multiply charged cations and singly charged anions. Utilizing ion/ion reactions with fluoranthene has offered a unique method of fragment formation for the structural elucidation of biomolecules. Fluoranthene is considered an ideal anion reagent because it selectively causes electron-transfer dissociation (ETD) and minimizes PT when interacting with peptides. However, limited investigations have sought to understand how fluoranthene─the primary, commercially available anion reagent─interacts with other biomolecules. Here, we apply deuterium labeling to investigate ion/ion reaction mechanisms between fluoranthene and divalent, metal-adducted carbohydrates (Ca2+, Mg2+, Co2+, and Ni2+). Deuterium labeling of carbohydrates allowed us to observe evidence of hydrogen/deuterium exchange (HDX) occurring after ion/ion dissociation reactions. The extent of deuterium loss is dependent on several factors, including the physical properties of the metal ion and the fragment structure. Based on the deuterium labeling data, we have proposed ETD, PTD, and intermolecular PT─also described as HDX─mechanisms. This research provides a fundamental perspective of ion/ion and ion/molecule reaction mechanisms and illustrates properties that impact ion/ion and ion/molecule reactions for carbohydrates. Together, this could improve the capability to distinguish complex and heterogeneous biomolecules, such as carbohydrates.

Metal adduction in mass spectrometric analyses of carbohydrates and glycoconjugates.

Glycans, carbohydrates, and glycoconjugates are involved in many crucial biological processes, such as disease development, immune responses, and cell-cell recognition. Glycans and carbohydrates are known for the large number of isomeric features associated with their structures, making analysis challenging compared with other biomolecules. Mass spectrometry has become the primary method of structural characterization for carbohydrates, glycans, and glycoconjugates. Metal adduction is especially important for the mass spectrometric analysis of carbohydrates and glycans. Metal-ion adduction to carbohydrates and glycoconjugates affects ion formation and the three-dimensional, gas-phase structures. Herein, we discuss how metal-ion adduction impacts ionization, ion mobility, ion activation and dissociation, and hydrogen/deuterium exchange for carbohydrates and glycoconjugates. We also compare the use of different metals for these various techniques and highlight the value in using metals as charge carriers for these analyses. Finally, we provide recommendations for selecting a metal for analysis of carbohydrate adducts and describe areas for continued research.

Formation of Carbohydrate-Metal Adducts from Solvent Mixtures during Electrospray: A Molecular Dynamics and ESI-MS Study.

Electrospray ionization (ESI) is frequently used to produce gas-phase ions for mass spectrometry (MS)-based techniques. The composition of solvents used in ESI-MS is often manipulated to enhance analyte ionization, including for carbohydrates. Moreover, to characterize analyte structures, ESI has been coupled to hydrogen/deuterium exchange, ion mobility, and tandem MS. Therefore, it is important to understand how solvent composition affects the structure of carbohydrates during and after ESI. In this work, we use molecular dynamics to simulate the desolvation of ESI droplets containing a model carbohydrate and observe the formation of carbohydrate adducts with metal ions. Molecular-level details on the effects of formulating mixtures of water, methanol, and acetonitrile to achieve enhanced ionization are presented. We complement our simulations with ESI-MS experiments. We report that when sprayed from aqueous mixtures containing volatile solvents, carbohydrates ionize to form metal-ion adducts rapidly due to rapid solvent evaporation rather than changes in the ionization mechanism. We find that when sprayed from solvent mixtures, carbohydrates are primarily solvated by water due to the migration of more volatile solvents to the surface of the droplet. Ultimately, the structure of the carbohydrate varies depending on its solvent environment, as inter- and intramolecular interactions are affected. We propose that solvents with 25% or more water may be used to enhance the ionization of carbohydrates with minimal effect on the structure during and after ESI.