Direct analysis in real time mass spectrometry (DART-MS) of ionic liquids.

Direct analysis in real time mass spectrometry (DART-MS) was used to analyze ionic liquids (ILs) containing either imidazolium or phosphonium cations combined with different types of inorganic and organic anions. Ionic liquids were directly inserted into the ionization source using a glass probe without dissolution into organic solvents. Mass spectra of the ILs were collected in both positive and negative mode with a linear ion-trap instrument. The intact cation of the compound was typically the dominant peak in positive mass spectra and cluster ion formation was present. Some individual anions were not readily observed in the negative mass spectra (based on the type of anion); however, the mass difference of adjacent cluster ions equal the mass of a complete IL and the anion mass could be verified by subtracting the known cation mass. The degree and intensity of the cluster ion formations was found to be dependent on the nature of the specific ILs as well as the DART temperature gas stream.

Optimization of direct analysis in real time (DART) linear ion trap parameters for the detection and quantitation of glucose.

Presented here are findings for the development and optimization of a simple, high-throughput, and rapid method for the analysis of glucose. Because the applications of glucose and other six-carbon sugars is a growing field of interest especially in the production of biofuels, an efficient and rapid method for their quantitation from lignocelluloses is necessary. Glucose was analyzed using direct analysis in real time (DART) ionization and formed adducts (along with fragmentation) were observed with a linear ion trap (LIT) mass spectrometer. Since DART can be considered a complex thermal desorption ionization process, an optimization study of the helium gas temperature and introduction into the ionization region was performed. It was observed these parameters have a significant effect on the overall signal intensity as well as the signal-to-noise ratios in DART mass spectra. Using these optimized parameters, a set of different glucose concentrations (ranging from 10 to 3000 µM) were analyzed and used to determine a linear dynamic range (with the use of an internal standard). The analysis of the samples was done with minimal sample preparation and found to be reproducible on different days.

Global shifts in protein sumoylation in response to electrophile and oxidative stress.

Human small ubiquitin-like modifier (sumo) proteins include sumo-1 and the less studied, nearly identical sumo-2 and sumo-3 proteins. Whereas the structurally related ubiquitin molecule targets proteins for degradation, sumo provides a distinct, yet poorly understood regulatory signal. Protein sumoylation is sensitive to diverse cellular stresses, yet the targets of sumoylation in stress are unknown. We studied protein sumoylation in HEK293 cells exposed to hydrogen peroxide, alkylating agents, and the lipid oxidation-derived electrophile 4-hydroxynonenal, which is an ubiquitous product of lipid oxidation associated with oxidative stress. Confocal immunofluorescence microscopy indicated that in unstressed cells sumo-1 targeted nuclear proteins, whereas sumo-2/3 targeted proteins in both nuclei and cytoplasm. Western blot analyses revealed changes in sumo-1 and sumo-2/3 targeting patterns with stress. We used immunoaffinity chromatography to harvest sumo-associated proteins from HA-sumo-1- and HA-sumo-3-expressing HEK293 cells both before and after treatment with 4-hydroxynonenal. Multidimensional liquid chromatography-tandem mass spectrometry analyses identified 54 HA-sumo-1-associated proteins and 38 HA-sumo-3-associated proteins. Major protein targets included RNA binding and processing proteins, transcription factors, metabolic enzymes, and cytoskeletal regulators. Treatment with 4-hydroxynonenal caused a near-complete redistribution of sumo-1 and sumo-3 to different protein targets, which included chaperones, antioxidant, and DNA damage signaling proteins. A 10-15% overlap of sumo-1 and sumo-3 targets before and after stress suggests that sumo proteins target distinct protein groups. The results suggest that reactive electrophiles not only directly modify proteins but also lead to indirect changes in endogenous protein modifications that regulate protein functions.

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films.

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH3+. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD3+ and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH4 (formal hydride ion addition) and C2H6 (formal methyl anion addition) from a hydrocarbon surface and CH3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH3CF3 (formal addition of trifluoromethyl anion).