No abstract

Glycans have an immense number of biological activities, necessitating increased efforts to characterize glycan structures. Mass spectrometry has been coupled to electrospray ionization (ESI) to characterize carbohydrates. While the gas-phase structures of glycan− and carbohydrate−metal adducts have been characterized, several questions persist concerning the mechanism of transfer of carbohydrates from ESI droplets into the gas phase. Using various computational methods, including molecular dynamics, steered molecular dynamics, and density functional theory calculations, we present a mechanistic investigation on the evaporation of solvent from nanosized droplets, formation of carbohydrate−metal adducts, and their subsequent release into the gas phase. We relate the computational results to mass spectra of melezitose, a model carbohydrate, and its permethylated derivative. Our results confirm two mechanisms for the release of carbohydrate−ion adducts from solvated droplets. Native (unmodified) carbohydrates are ionized via the charged residue model, while the permethylated derivative is ionized via the ion evaporation model. For both mechanisms, the monomer carbohydrate−metal adduct is the dominant species observed. This work illustrates that the ionization mechanisms are dictated by interactions between the carbohydrate and solvent, and coordination of the carbohydrate with the metal ion. Thus, these results provide insight into the molecular interactions that govern the mechanism of release.

Characterizing glycans is analytically challenging since glycans are heterogeneous, branched polymers with different three-dimensional conformations. Hydrogen/deuterium exchange-mass spectrometry (HDX-MS) has been used to analyze native conformations and dynamics of biomolecules by measuring the mass increase of analytes as labile protons are replaced with deuterium following exposure to deuterated solvents. The rate of exchange is dependent on the chemical functional group, the presence of hydrogen bonds, pH, temperature, charge, and solvent accessibility. HDX-MS of carbohydrates is challenging due to the rapid exchange rate of hydroxyls. Here, we describe an observed HDX reaction associated with residual solvent vapors saturating electrospray sources. When undeuterated melezitose was infused after infusing DO, samples with up to 73% deuterium exchange were detected. This residual solvent HDX was observed for both carbohydrates and peptides in multiple instruments and dependent on sample infusion rate, infusion time, and deuterium content of the solvent. This residual solvent HDX was observed over several minutes of sample analysis and persisted long enough to alter the measured deuterium labeling and possibly change the interpretation of the results. This work illustrates that residual solvent HDX competes with in-solution HDX for rapidly exchanging functional groups. Thus, we propose conditions to minimize this effect, specifically for top-down, in-electrospray ionization, and quench-flow HDX experiments. Graphical Abstract ᅟ.

Carbohydrates play key roles in facilitating cellular functions, yet characterizing their structures is analytically challenging due to the presence of epimers, regioisomers, and stereoisomers. In-electrospray-hydrogen/deuterium exchange-mass spectrometry (in-ESI HDX-MS) is a rapid HDX method that samples solvated carbohydrates with minimal instrument modification. When applied to proteins, HDX is often measured after multiple time points to sample the dynamics of structures. Herein, we alter the HDX reaction time by modifying the spray-solvent conductivity, which changes the initial size of ESI droplets, and thus, the droplet lifetimes. We show that this change in droplet lifetime alters the magnitude of HDX for carbohydrate-metal adducts. Furthermore, we illustrate how monitoring HDX at multiple time points enables three trisaccharide isomers (melezitose, maltotriose, and isomaltotriose) to be distinguished. This work illustrates the feasibility of this method for characterizing solvated carbohydrates, including isomeric species which differ only by linkage.

The reactions of a monomeric borole and a dimeric borole with 2,3‐dimethyl‐1,3‐butadiene and 1,3‐cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3‐Dimethyl‐1,3‐butadiene reacts to give diverse products resulting from a cycloaddition process with the B−C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3‐cyclohexadiene, a [4+2] process is observed in which 1,3‐cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.

No abstract

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.