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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.