Release of Carbohydrate–Metal Adducts from Electrospray Droplets: Insight into Glycan Ionization by Electrospray

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.

Ionization of glycans from alkali metal salt-impregnated paper

“…This suggests that stronger affinities of the alkali metal ions to glycans lead to higher signals. It is coincident with that reported in reference [22].…”

Formation of Gaseous Proteins via the Ion Evaporation Model (IEM) in Electrospray Mass Spectrometry

“…The prevailing current view is that the IEM applies to small preformed ions, while the CRM describes the behavior of large globular species such as protein ions generated by native ESI. ,,,, However, the IEM vs CRM debate is far from settled, − and there are unresolved issues that deserve closer attention. For example, the distinction between “small” and “large” analytes is contentious.…”

“…21,38−40 In recent years, molecular dynamics (MD) simulations of charged nanodroplets have emerged as an important tool for exploring ESI mechanisms. 21,28,32,41 For example, it has become possible to obtain atomistic insights into the IEM ejection of small ions. 19,21,42,43 Simulations on compact folded proteins, 21,44 nucleic acid duplexes, 45 and peptides 46,47 revealed CRM behavior, while unfolded proteins were shown to follow the CEM.…”

Atomistic Insights into the Formation of Nonspecific Protein Complexes during Electrospray Ionization

“…Such IEM ejection of low MW ions is a common occurrence. 34,40,41,85,87 One of these IEM events is highlighted in Figure 3A for t = 0.5 ns. The two ubiquitin molecules remained inside the droplet until all of the water had evaporated.…”

“…We and others ,,,,,− previously used MD simulations to gain insights into ESI mechanisms. The current work marks the first time that this approach was applied to nonspecific protein clustering, focusing on the two proteins introduced above.…”