Ring formation and hydration effects in electron attachment to misonidazole

This research was funded by CZECH SCIENCE FOUNDATION grant number 19-01159S; Czech Ministry of Education Youth and Sports via OP RDE Grant no. CZ.02.2.69/0.0/16_027/0008355; S.D. acknowledges funding from the FWF, Vienna (P30332).We study the reactivity of misonidazole with low-energy electrons in a water environment combining experiment and theoretical modelling. The environment is modelled by sequential hydration of misonidazole clusters in vacuum. The well-defined experimental conditions enable computational modeling of the observed reactions. While the NO- 2 dissociative electron attachment channel is suppressed, as also observed previously for other molecules, the OH- channel remains open. Such behavior is enabled by the high hydration energy of OH- and ring formation in the neutral radical co-fragment. These observations help to understand the mechanism of bio-reductive drug action. Electron-induced formation of covalent bonds is then important not only for biological processes but may find applications also in technology.publishersversionpublishe

Sodium doping and reactivity in pure and mixed ice nanoparticles

Doping of clusters by sodium atoms and subsequent photoionization (NaPI) is used as a fragmentation-free cluster ionization method. Here we investigate different clusters using NaPI and electron ionization (EI) with a reflectron time-of-flight mass spectrometer (RTOF). The mass spectra of the same clusters ionized by NaPI and EI reveal significant differences which point to Na reactivity in the clusters. First, we discuss mixed XM·(H2O)N (X = HNO3, N2O) clusters where reactions between Na and molecules X leads to the “cluster invisibility” for the NaPI method. Second, mixed (NH3)M·(H2O)N clusters are observed by both methods, but they reveal different cluster compositions, and the mass spectra suggest that neither the EI nor the NaPI spectrum corresponds exactly to the neutral cluster distribution. Finally, we discuss the reactions of Na in pure water clusters as a function of the number of Na atoms doped into the clusters. In summary, we present experimental evidence that the NaPI method in the present cases does not reveal the size and composition of the neutral clusters. A detailed understanding of Na reactivity in the clusters is needed for its application as a fragmentation-free cluster ionization method. Besides, we introduce the combination of NaPI and EI as a new tool to investigate the sodium reactivity in clusters and aerosol particles

Proton transfer from pinene stabilizes water clusters

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Low-Energy Electron Induced Reactions in Metronidazole at Different Solvation Conditions

Metronidazole belongs to the class of nitroimidazole molecules and has been considered as a potential radiosensitizer for radiation therapy. During the irradiation of biological tissue, secondary electrons are released that may interact with molecules of the surrounding environment. Here, we present a study of electron attachment to metronidazole that aims to investigate possible reactions in the molecule upon anion formation. Another purpose is to elucidate the effect of microhydration on electron-induced reactions in metronidazole. We use two crossed electron/molecular beam devices with the mass-spectrometric analysis of formed anions. The experiments are supported by quantum chemical calculations on thermodynamic properties such as electron affinities and thresholds of anion formation. For the single molecule, as well as the microhydrated condition, we observe the parent radical anion as the most abundant product anion upon electron attachment. A variety of fragment anions are observed for the isolated molecule, with NO2− as the most abundant fragment species. NO2− and all other fragment anions except weakly abundant OH− are quenched upon microhydration. The relative abundances suggest the parent radical anion of metronidazole as a biologically relevant species after the physicochemical stage of radiation damage. We also conclude from the present results that metronidazole is highly susceptible to low-energy electrons

Clustering of Uracil Molecules on Ice Nanoparticles

We generate a molecular beam of ice nanoparticles (H₂O)_<i>N</i>, <i>N̅</i> ≈ 130–220, which picks up several individual gas phase uracil (U) or 5-bromouracil (BrU) molecules. The mass spectra of the doped nanoparticles prove that the uracil and bromouracil molecules coagulate to clusters on the ice nanoparticles. Calculations of U and BrU monomers and dimers on the ice nanoparticles provide theoretical support for the cluster formation. The (U)_<i>m</i>H⁺ and (BrU)_<i>m</i>H⁺ intensity dependencies on <i>m</i> extracted from the mass spectra suggest a smaller tendency of BrU to coagulate compared to U, which is substantiated by a lower mobility of bromouracil on the ice surface. The hydrated U_<i>m</i>·(H₂O)_<i>n</i>H⁺ series are also reported and discussed. On the basis of comparison with the previous experiments, we suggest that the observed propensity for aggregation on ice nanoparticles is a more general trend for biomolecules forming strong hydrogen bonds. This, together with their mobility, leads to their coagulation on ice nanoparticles which is an important aspect for astrochemistry

Biomolecule Analogues 2‑Hydroxypyridine and 2‑Pyridone Base Pairing on Ice Nanoparticles

Ice nanoparticles (H₂O)_<i>N</i>, <i>N</i> ≈ 450 generated in a molecular beam experiment pick up individual gas phase molecules of 2-hydroxypyridine and 2-pyridone (HP) evaporated in a pickup cell at temperatures between 298 and 343 K. The mass spectra of the doped nanoparticles show evidence for generation of clusters of adsorbed molecules (HP)_<i>n</i> up to <i>n</i> = 8. The clusters are ionized either by 70 eV electrons or by two photons at 315 nm (3.94 eV). The two ionization methods yield different spectra, and their comparison provides an insight into the neutral cluster composition, ionization and intracluster ion–molecule reactions, and cluster fragmentation. Quite a few molecules were reported <i>not to coagulate</i> on ice nanoparticles previously. The (HP)_<i>n</i> cluster generation on ice nanoparticles represents the first evidence for coagulating of molecules and cluster formation on free ice nanoparticles. For comparison, we investigate the coagulation of HP molecules picked up on large clusters Ar_<i>N</i>, <i>N</i> ≈ 205, and also (HP)_<i>n</i> clusters generated in supersonic expansions with Ar buffer gas. This comparison points to a propensity for the (HP)₂ dimer generation on ice nanoparticles. This shows the feasibility of base pairing for model of biological molecules on free ice nanoparticles. This result is important for hypotheses of the biomolecule synthesis on ice grains in the space. We support our findings by theoretical calculations that show, among others, the HP dimer structures on water clusters

Reactivity of Hydrated Electron in Finite Size System: Sodium Pickup on Mixed N₂O–Water Nanoparticles

We investigate the reactivity of hydrated electron generated by alkali metal deposition on small water particles with nitrous oxide dopant by means of mass spectrometry and ab initio molecular dynamics simulations. The mixed nitrous oxide/water clusters were generated in a molecular beam and doped with Na atoms in a pickup experiment, and investigated by mass spectrometry using two different ionization schemes: an electron ionization (EI), and UV photoionization after the Na doping (NaPI). The NaPI is a soft-ionization nondestructive method, especially for water clusters provided that a hydrated electron <i>e</i>_s^– is formed in the cluster. The missing signal for the doped clusters indicates that the hydrated electron is not present in the N₂O containing clusters. The simulations reveal that the hydrated electron is formed, but it immediately reacts with N₂O, forming first N₂O^– radical anion, later O^–, and finally an OH^• and OH^– pair

Photochemistry of Nitrophenol Molecules and Clusters: Intra- vs Intermolecular Hydrogen Bond Dynamics

We investigate both experimentally and theoretically the structure and photodynamics of nitrophenol molecules and clusters, addressing the question how the molecular photodynamics can be controlled by specific inter- and intramolecular interactions. Using quantum chemical calculations, we demonstrate the structural and energetic differences between clusters of 2-nitrophenol and 4-nitrophenol, using phenol as a reference system. The calculated structures are supported by mass spectrometry. The mass spectra of 2-nitrophenol clusters provide an evidence for a stacked structure compared to a strong O–H···O hydrogen bonding for 4-nitrophenol aggregates. We further investigate the photodynamics of nitrophenol molecules and clusters by means of velocity map imaging of the H-fragment generated upon 243 nm photodissociation. The experiments are complemented by <i>ab initio</i> calculations which demonstrate distinct photophysics of phenol, 2-nitrophenol, 4-nitrophenol. The measured H-fragment kinetic energy distributions (KEDs) from 2-nitrophenol molecules are compared to the KEDs from phenol. The comparison points to the intramolecular O–H···O hydrogen bond in 2‑nitrophenol, stimulating fast internal conversion into the ground electronic state. This reaction channel is marked by exclusive appearance of slow statistical hydrogen fragments in 2-nitrophenol, which contrasts with fast hydrogen atoms observed for phenol. The photodissociation of 2-nitrophenol clusters yields a fraction of H-fragments with higher kinetic energies than the isolated molecules. These fragments originate from the caging effect in the clusters leading to multiphoton dissociation of molecules excited by the previous photons. We also propose a new <i>ab initio</i> based value for the O–H bond dissociation enthalpy in 2-nitrophenol (4.25 eV), which is in excellent agreement with the maximum measured H-fragment kinetic energy

Lack of Aggregation of Molecules on Ice Nanoparticles

Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)­aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH₃Cl, CH₃CH₂CH₂Cl, C₆H₅Cl, CH₄, and C₆H₆) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters