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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.

We present a computational modeling study examining ion transport dynamics of aqueous electrolytes under severely confined conditions. Ionic current and solvent transport through carbon nanotubes in an external electric field are studied using all atom molecular dynamics simulations. Specifically, we have examined the behavior of sodium and chloride ions in nanotubes of different radii to assess the influence of confinement on the ionic current. We find a linear relationship between the current computed and potential applied for the wider nanotubes; however, there is a significant departure from linearity when the tube diameter becomes comparable to the size of the solvated ion. For the smallest tubes studied, the energy penalty to access the pore interior is too great for most ions, leading to minimal current. We provide analyses of the energy barriers associated with ion entry as well as the hydration shell properties, which supports the absence of ionic current in the smallest carbon nanotubes. ■ INTRODUCTIONThe exceptional properties of carbon nanotubes 1 (CNTs) have garnered significant attention in recent years. The ability to tune the size and material properties of CNTs makes them attractive components for nanodevices. Broad investigations have led to a number of practical applications ranging from novel subnanometer supercapacitors, 2 hydroelectric voltage generation, power converters, 3 transformative technologies for chemical separations and desalinations of water, 4 to a variety of biomedical applications such as nanoscale pipes, 5 devices to mimic fluidic transport in biological channels, 6 water pumps, 7 carbon nanotube membranes in microfluidics to control electro-osmotic flow, 8 electrophoretic transport of RNA 9 and DNA, 10 drug delivery, 11,12 etc. In particular, there have been a number of computational studies on water transport through CNTs that provide fundamental insights into confined water behavior and explore the extent that environment can alter this behavior. Examples include research on water conduction through the hydrophobic CNTs, 13 solvent kinetics of filling and emptying of CNTs, 14 water flux through modified and unmodified CNTs, 15 and water ordering in the nanotubes. 16,17 However, in many physical systems, the fluids of interest are not pure water, but solutions whose collective behavior under confinement can be drastically different. A number of factors contribute to the discrepancies, and detailed computational studies are often required to obtain a clear understanding of the observed physical and chemical phenomena. 18−20 In this work, we have concentrated on ion transport behavior in a hydrophobic environment, a pristine CNT. We utilize a set of seven different armchair carbon nanotubes 40 Å in length with radii ranging from ∼4.1−8.2 Å to provide varying degrees of confinement. 21 The CNTs have been solvated in a water box with a 1 M concentration of NaCl. The simulated system, comprised of a carbon nanotube, ions, and the water box, is shown as Figure 1. Detailed syste...

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We have examined the structure of water and aqueous solutions in carbon nanotubes using molecular dynamics simulations. We find confinement changes the structure of water as well as the interactions between ions and their solvation shells. The density and orientation of water at the nanotube walls are strongly dependent on the surface charge and cations/anions present at the interfaces. Decreasing the nanotube diameter alters the ion hydration properties as well as hydrogen bonding structure and formation dynamics. The results indicate that fluid structure and hydrogen bond characteristics in nano-channels can be tuned through modification of tube charge and with ion selection.

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.

Glycans are prominent in biological processes. Glycan structural characterizations can have significant implications for disease progression, e.g. cancer metastasis.1 Glycans are challenging to analyze with traditional chemical techniques due to a lack in diversity of functional groups; thus, computational modeling of glycans can help bridge this gap in knowledge regarding glycan structures. However, it is necessary that the computational models adequately represent carbohydrate structures and align with experimental results. Experimentally, mass spectrometry (MS) is one technique that is used for glycan characterization. In MS analyses, glycans are often analyzed as ion adducts. A group of computational methods and basis sets have been used to compare the energies of a subset of monosaccharides2, but this work has not been expanded to the ion adducts detected with MS, nor has it included experimental comparison. Herein, we optimized the geometry of both anomers of fucose and glucose in neutral, protonated, ammoniated, and sodiated forms using Density Functional Theory with the B3LYP or B3PW91 methods and 15 basis sets. Three basis sets (cc‐pVQZ, GEN, and GENECP) were unsuccessful in running calculations to completion. For the remaining computations, we examined values from the output files for variations between methods and basis sets. If computational methods yield accurate structures, the calculated values for molecular parameters, e.g. dipole moment, should be similar. Yet, if there are incorrect approximations in the basis set, the calculated electron density around the molecule will be incorrect and alter the values of the parameters using that specific functional. Greater variation occurred in the average dipole moments calculated for all methods and basis sets of fucose (standard deviation (SD) = 1.5) compared to glucose (SD = 0.68). The addition of ion adducts increased error in dipole moment calculations compared to the neutral molecules. The variation in these results suggests that some methods and basis sets are not reliable when modelling carbohydrates. Currently, molecular dynamics runs using GROMACS are being done to obtain over 50 different coordinate sets of each system for input into MOBCAL.3 MOBCAL will be used to obtain theoretical collisional cross sections (CCS) which will be compared with experimentally measured CCS by ion mobility MS. These results will illustrate which computational methods yield accurate carbohydrate structures, allowing future application of this work to address the biological utility of glycans.Support or Funding InformationBaylor University Office of the Vice Provost for Research and the Undergraduate Research and Scholarly Achievement Small Grant Program.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.