J. Michael Nilles

Zwitterion formation in hydrated amino acid, dipole bound anions: How many water molecules are required?

While the naturally occurring amino acids are not zwitterions in the vapor phase, they are in aqueous solutions, implying that water plays an important role in inducing zwitterion formation. Together, these observations inspire the question, “How many water molecules are required to induce zwitterion formation in a given amino acid molecule?” In this paper, we address this question in the context of mass spectrometric and size-selected photoelectron spectroscopic studies of hydrated amino acid anions. We utilize the facts that zwitterions possess very large dipole moments, and that excess electrons can bind to strong dipole fields to form dipole bound anions, which in turn display distinctive and recognizible photoelectron spectral signatures. The appearance of dipole-bound photoelectron spectra of hydrated amino acid anions, beginning at a given hydration number, thus signals the onset of greatly enhanced dipole moments there and, by implication, of zwitterion formation. We find that five water molecules are needed to transform glycine into its zwitterion, while four each are required for phenylalanine and tryptophan. Since the excess electron may also make a contribution to zwitterion stabilization, these numbers are lower limits for how many water molecules are needed to induce zwitterion formation in these amino acids when no extra (net) charges are involved.

Photoelectron spectroscopy of chromium-doped silicon cluster anions

The photoelectron spectra of chromium-doped silicon cluster anions, CrSi-(n), were measured over the size range, n=8-12. Their vertical detachment energies were measured to be 2.71, 2.88, 2.87, 2.95, and 3.18 eV, respectively. Our results support theoretical calculations by Khanna, Rao, and Jena [Phys. Rev. Lett. 89, 016803 (2002)] which found CrSi12 to be an enhanced stability (magic) cluster with its chromium atom encapsulated inside a silicon cage and with its magnetic moment completely quenched by the effects of the surrounding cage.

Barrier-free intermolecular proton transfer induced by excess electron attachment to the complex of alanine with uracil

The photoelectron spectrum of the uracil-alanine anionic complex (UA)(-) has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6 and 2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)(-) anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second-order Møller-Plesset level of theory with 6-31++G(*) (*) basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61, and 0.57 eV. The electron hole in complexes of uracil with alanine is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)(-) occupies a pi(*) orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as a neutral radical of hydrogenated uracil solvated by a deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine, and together they indicate that the BFPT process is not very sensitive to the nature of the amino acids hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons.

Solvent-induced stabilization of the naphthalene anion by water molecules: A negative cluster ion photoelectron spectroscopic study

We show that (a) only a single water molecule is needed to stabilize the naphthalene anion, (b) the EAa of naphthalene is −0.20 eV, in agreement with determinations by electron transmission spectroscopy, (c) the energetics are consistent with the number of waters required to stabilize the naphthalene anion, and (d) the excess electron is located on the naphthalene moiety of Nph1−(H2O)n.

Photoelectron spectroscopy of naphthalene cluster anions

Mass spectrometric and anion photoelectron spectroscopic studies of homogeneous naphthalene cluster anions, (Nph)n=2–7−, were conducted to characterize the nature of their anionic cores. The smallest stable species in this series was found to be the naphthalene dimer anion. The vertical detachment energies of naphthalene clusters, n=2–7, were determined and found to increase smoothly with cluster size. By extrapolation, the vertical detachment energy of the isolated naphthalene molecule was found to be −0.18 eV, in agreement with its adiabatic electron affinity value from literature. The strong similarity between the spectral profiles of (Nph)2− and (Nph)1−(H2O)1 implied that (Nph)2− possesses a solvated monomeric anion core. All of the naphthalene cluster anions studied here were interpreted as having monomer anion cores.

Barrier-free proton transfer in anionic complex of thymine with glycine

We report the photoelectron spectrum of the thymine–glycine anionic complex (TG−) recorded with low energy photons (2.540 eV). The spectrum reveals a broad feature with a maximum between 1.6–1.9 eV. The measured electron vertical detachment energy is too large to be attributed to a complex in which an anion of intact thymine is solvated by glycine, or vice versa. The experimental data are paralleled by electronic structure calculations carried out at the density functional theory level with 6-31++G** basis sets and the B3LYP and MPW1K exchange–correlation functionals. The critical structures are further examined at the second order Moller–Plesset level of theory. The results of calculations indicate that the excess electron attachment to the complex induces an intermolecular barrier-free proton transfer from the carboxylic group of glycine to the O8 atom of thymine. As a result, the four most stable structures of the thymine–glycine anionic complex can be characterized as a neutral radical of hydrogenated thymine solvated by an anion of deprotonated glycine. The calculated vertical electron detachment energies for the four most stable anionic complexes lie in a range 1.6–1.9 eV, in excellent agreement with the maximum of the photoelectron peak.

Thermal separation to facilitate Direct Analysis in Real Time (DART) of mixtures

We explore a thermal separation technique for use with Direct Analysis in Real Time (DART). By applying gas temperature ramping, we are able to disburse a mixture of compounds in time. The three components were selected to create a challenging mixture that would not likely be discerned solely using exact mass capabilities. While the thermal separation technique is of low resolution, it preserves the inherent rapid, non-contact, ambient characteristics of the ion source.

Photoelectron Spectroscopic and Computational Study of Hydrated Pyrimidine Anions

The stabilization of the pyrimidine anion by the addition of water molecules is studied experimentally using photoelectron spectroscopy of mass-selected hydrated pyrimidine clusters and computationally using quantum-mechanical electronic structure theory. Although the pyrimidine molecular anion is not observed experimentally, the addition of a single water molecule is sufficient to impart a positive electron affinity. The sequential hydration data have been used to extrapolate to -0.22 eV for the electron affinity of neutral pyrimidine, which agrees very well with previous observations. These results for pyrimidine are consistent with previous studies of the hydrated cluster anions of uridine, cytidine, thymine, adenine, uracil, and naphthalene. This commonality suggests a universal effect of sequential hydration on the electron affinity of similar molecules.

Direct Analysis of Aerosolized Chemical Warfare Simulants Captured on a Modified Glass-Based Substrate by “Paper-Spray” Ionization

Paper spray ionization mass spectrometry offers a rapid alternative platform requiring no sample preparation. Aerosolized chemical warfare agent (CWA) simulants trimethyl phosphate, dimethyl methylphosphonate, and diisopropyl methylphosphonate were captured by passing air through a glass fiber filter disk within a disposable paper spray cartridge. CWA simulants were aerosolized at varying concentrations using an in-house built aerosol chamber. A custom 3D-printed holder was designed and built to facilitate the aerosol capture onto the paper spray cartridges. The air flow through each of the collection devices was maintained equally to ensure the same volume of air sampled across methods. Each approach yielded linear calibration curves with R2 values between 0.98-0.99 for each compound and similar limits of detection in terms of disbursed aerosol concentration. While the glass fiber filter disk has a higher capture efficiency (≈40%), the paper spray method produces analogous results even with a lower capture efficiency (≈1%). Improvements were made to include glass fiber filters as the substrate within the paper spray cartridge consumable. Glass fiber filters were then treated with ammonium sulfate to decrease chemical interaction with the simulants. This allowed for improved direct aerosol capture efficiency (>40%). Ultimately, the limits of detection were reduced to levels comparable to current worker population limits of 1 × 10-6 mg/m3.