6 results on '"Stephen W. C. Walker"'
Search Results
2. Measuring Electronic Spectra of Differential Mobility-Selected Ions in the Gas Phase
- Author
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Stephen W. C. Walker, W. Scott Hopkins, J. Larry Campbell, Mircea Guna, Ce Zhou, Patrick J. J. Carr, and Neville J. A. Coughlan
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Chemistry ,010401 analytical chemistry ,Photodissociation ,Analytical chemistry ,010402 general chemistry ,Mass spectrometry ,01 natural sciences ,Spectral line ,0104 chemical sciences ,Triple quadrupole mass spectrometer ,Ion ,Structural Biology ,Quadrupole ,Mass spectrum ,Spectroscopy - Abstract
We describe the modification of a commercially available tandem differential mobility mass spectrometer (DMS) that has been retrofitted to facilitate photodissociation (PD) of differential mobility-separated, mass-selected molecular ions. We first show that a mixture of protonated quinoline/isoquinoline (QH+/iQH+) can be separated using differential mobility spectrometry. Efficient separation is facilitated by addition of methanol to the DMS environment and increased residence time within the DMS. In action spectroscopy experiments, we gate each isomer using appropriate DMS settings, trap the ions in the third quadrupole of a triple quadrupole mass spectrometer, and irradiate them with tunable light from an optical parametric oscillator (OPO). The resulting mass spectra are recorded as the OPO wavelength is scanned, giving PD action spectra. We compare our PD spectra with previously recorded spectra for the same species and show that our instrument reproduces previous works faithfully.
- Published
- 2020
3. Determining molecular properties with differential mobility spectrometry and machine learning
- Author
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W. Scott Hopkins, Jarrod Psutka, Justin I. Montgomery, Chang Liu, Ahdia Anwar, J. C. Yves Le Blanc, Jeff Crouse, John S. Janiszewski, Gilles H. Goetz, Stephen W. C. Walker, and J. Larry Campbell
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0301 basic medicine ,Octanol ,Materials science ,Science ,General Physics and Astronomy ,Machine learning ,computer.software_genre ,Mass spectrometry ,01 natural sciences ,Article ,General Biochemistry, Genetics and Molecular Biology ,Polar surface area ,03 medical and health sciences ,chemistry.chemical_compound ,Phase (matter) ,Molecule ,lcsh:Science ,Cluster analysis ,Multidisciplinary ,Spectrometer ,Drug discovery ,business.industry ,010401 analytical chemistry ,General Chemistry ,0104 chemical sciences ,3. Good health ,030104 developmental biology ,chemistry ,lcsh:Q ,Artificial intelligence ,business ,computer - Abstract
The fast and accurate determination of molecular properties is highly desirable for many facets of chemical research, particularly in drug discovery where pre-clinical assays play an important role in paring down large sets of drug candidates. Here, we present the use of supervised machine learning to treat differential mobility spectrometry – mass spectrometry data for ten topological classes of drug candidates. We demonstrate that the gas-phase clustering behavior probed in our experiments can be used to predict the candidates’ condensed phase molecular properties, such as cell permeability, solubility, polar surface area, and water/octanol distribution coefficient. All of these measurements are performed in minutes and require mere nanograms of each drug examined. Moreover, by tuning gas temperature within the differential mobility spectrometer, one can fine tune the extent of ion-solvent clustering to separate subtly different molecular geometries and to discriminate molecules of very similar physicochemical properties., The fast and accurate determination of molecular properties is particularly crucial in drug discovery. Here, the authors employ supervised machine learning to treat differential mobility spectrometry – mass spectrometry data for ten classes of drug candidates and predict several condensed-phase properties.
- Published
- 2018
4. Separating and probing tautomers of protonated nucleobases using differential mobility spectrometry
- Author
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John S. Janizewski, Jarrod Psutka, W. Scott Hopkins, J. Larry Campbell, Stephen W. C. Walker, Ahdia Anwar, and Thorsten Dieckmann
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Chemistry ,Electrospray ionization ,010401 analytical chemistry ,Analytical chemistry ,Protonation ,010402 general chemistry ,Condensed Matter Physics ,Mass spectrometry ,01 natural sciences ,Tautomer ,0104 chemical sciences ,Nucleobase ,Ion ,Yield (chemistry) ,Hydrogen–deuterium exchange ,Physical and Theoretical Chemistry ,Instrumentation ,Spectroscopy - Abstract
The protonated nucleobases (C + H)+, (T + H)+, (U + H)+, (A + H)+, and (G + H)+ are investigated in a combined experimental and computational study using differential mobility spectrometry (DMS), mass spectrometry, and electronic structure calculations. DMS is used to isolate individual tautomeric forms for each protonated nucleobase prior to characterization with HDX or CID. The population distributions of each protonated nucleobase formed by electrospray ionization (ESI) are dominated by a single tautomeric form, as is predicted by our calculations. However, all nucleobases present additional tautomers upon ESI, with these minor contributions to the ensemble populations attributed to additional higher energy metastable species. In addition to the tautomer-derived species, additional ion signals in the DMS data are attributed to larger nucleobase-containing clusters, which fragment post-DMS to yield bare ion and fragment ion signals that are consistent with those expected for the bare protonated nucleobases. Contributions from larger clustered species are instead distinguished by monitoring DMS ion signal as declustering potential voltages are ramped.
- Published
- 2018
- Full Text
- View/download PDF
5. Characterizing the Tautomers of Protonated Aniline Using Differential Mobility Spectrometry and Mass Spectrometry
- Author
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Alison Mark, Bogdan Bogdanov, J. Larry Campbell, Stephen W. C. Walker, W. Scott Hopkins, and Brent R. Verbuyst
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education.field_of_study ,Electrospray ionization ,010401 analytical chemistry ,Population ,Protonation ,010402 general chemistry ,Mass spectrometry ,01 natural sciences ,Tautomer ,0104 chemical sciences ,Solvent ,chemistry.chemical_compound ,Aniline ,chemistry ,Computational chemistry ,Physical and Theoretical Chemistry ,education ,Acetonitrile - Abstract
The site of protonation for gas-phase aniline has been debated for many years, with many research groups contributing experimental and computational evidence for either the amino-protonated or the para-carbon-protonated tautomer as the gas-phase global minimum structure. Here, we employ differential mobility spectrometry (DMS) and mass spectrometry (MS) to separate and characterize the amino-protonated (N-protonated) and para-carbon-protonated ( p-protonated) tautomers of aniline. We demonstrate that upon electrospray ionization (ESI), aniline is protonated predominantly at the amino position. Similar analyses are conducted on another three isotopically labeled forms of aniline to confirm our structural assignments. We observe a significant reduction of the relative population of the p-protonated tautomer when a protic ESI solvent is employed (methanol/water) compared to when an aprotic solvent (acetonitrile) is employed. We also observe conversion of the p-protonated species into the N-protonated species upon clustering with protic solvent vapor post-DMS selection-a finding supported by previous experimental data acquired using DMS-MS.
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- 2017
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6. FTIR Matrix-Isolation Study of the Reaction Products of Vanadium Atoms with Propene: Observation of Allylvanadium Hydride As a Precursor to Sacrificial Hydrogenation of Propene
- Author
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Stephen W. C. Walker, Matthew G. K. Thompson, and J. Mark Parnis
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Chemistry ,Hydride ,Matrix isolation ,Vanadium ,chemistry.chemical_element ,Infrared spectroscopy ,Photochemistry ,Inorganic Chemistry ,Propene ,chemistry.chemical_compound ,Propane ,Physical and Theoretical Chemistry ,Fourier transform infrared spectroscopy ,Methyl group - Abstract
Vanadium atoms have been reacted with different partial pressures of propene in Ar under matrix-isolation conditions, and the products have been observed using Fourier transform infrared (FTIR) spectroscopy. Under dilute propene in Ar conditions, new features are observed in the IR spectra corresponding to a C-H insertion product, identified here as H-V-(η(3)-allyl). Use of d(3)-propene (CD(3)-CH═CH(2)) demonstrates that the initial V-atom insertion occurs at the methyl group of the propene molecule, and DFT calculations have been used to support the identity of the initial product. Upon increasing the partial pressure of propene, additional features corresponding to propane (C(3)H(8)) are observed, with the hydrogen-atom source for the observed hydrogenation demonstrated to be additional propene units. Analysis of a systematic increase in the partial pressure of propene in the system demonstrates that the yield of propane correlates with the decrease of the allyl product, demonstrating the H-V(allyl) species as a reactive intermediate in the overall hydrogenation process. An overall mechanism is proposed to rationalize the formation of the insertion product and ultimately the products of hydrogenation, which agrees with previous gas-phase and matrix-isolation work involving propene and the related system, ethene.
- Published
- 2011
- Full Text
- View/download PDF
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