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Quasi-Free Electron-Mediated Radiation Sensitizaion by C5-Halopyrimidines

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by Jun Ma, Teseer Bahry, Sergey A. Denisov, Amitava Adhikary, Mehran Mostafavi

J. Phys. Chem. A 2021, 125, 36, 7967–7975; doi.org/10.1021/acs.jpca.1c05974

Substitution of the thymidine moiety in DNA by C5-substituted halogenated thymidine analogues causes significant augmentation of radiation damage in living cells. However, the molecular pathway involved in such radiosensitization process has not been clearly elucidated to date in solution at room temperature. So far, low-energy electrons (LEEs; 0–20 eV) under vacuum condition and solvated electrons (esol–) in solution are shown to produce the σ-type C5-centered pyrimidine base radical through dissociative electron attachment involving carbon–halogen bond breakage. Formation of this σ-type radical and its subsequent reactions are proposed to cause cellular radiosensitization. Here, we report time-resolved measurements at room temperature, showing that a radiation-produced quasi-free electron (eqf–) in solution promptly breaks the C5-halogen bond in halopyrimidines forming the σ-type C5 radical via an excited transient anion radical. These results demonstrate the importance of ultrafast reactions of eqf–, which are extremely important in chemistry, physics, and biology, including tumor radiochemotherapy.

Reaction Mechanisms of Fluoroethylene Carbonate Degradation, an Additive of Lithium-Ion Batteries, Unraveled by Radiation Chemistry

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by Marin Puget, Viacheslav Shcherbakov, Sergey A. Denisov, Philippe Moreau, Jean-Pierre Dognon, Mehran Mostafavi and Sophie LE CAER

Chemistry – A European Journal 2021; doi.org/10.1002/chem.202100562

Numerous additives are used in electrolytes of lithium‐ion batteries, especially for the formation of efficient solid electrolyte interphase at the surface of the electrodes. The understanding of the degradation processes of these compounds is thus important. They can be obtained through radiolysis. In the case of fluoroethylene carbonate (FEC), picosecond pulse radiolysis experiments evidenced the formation of FEC●‐ . This radical is stabilized in neat FEC, whereas the ring opens to form more stable radical anions when FEC is a solute in other solvents, as confirmed by quantum chemistry calculations. In neat FEC, pre‐solvated electrons primarily undergo attachment compared to solvation. At long timescales, produced gases (H2, CO, and CO2 ) were quantified. A reaction scheme for both the oxidizing and reducing pathways at stake in irradiated FEC was proposed. This work evidences that the nature of the primary species formed in FEC depends on the amount of FEC in the solution.

Selective Oxidation of Transient Organic Radicals in the Presence of Gold Nanoparticles

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by Viacheslav Shcherbakov, Sergey A. Denisov and Mehran Mostafavi

Nanomaterials 2021, 11, 727; doi.org/10.3390/nano11030727

The ability of gold nanoparticles (AuNPs) to catalyze reactions involving radicals is poorly studied. However, AuNPs are used in applications where chemical reactions involving transient radicals occur. Herein, we investigate AuNPs’ catalytic effect on 2-propanol oxidation and acetanilide hydroxylation in aqueous solutions under ionizing radiation at room temperature. In both cases, the presence of AuNPs led to selective oxidation of organic radicals, significantly changing the products’ composition and ratio. Based on these observations, we stress how AuNPs’ catalytic activity can affect the correctness of reactive oxygen species concentration determination utilizing organic dyes. We also provide a discussion on the role of AuNPs’ catalytic activity in the radiosensitization effect actively studied for radiotherapy.

Real-Time Observation of Solvation Dynamics of Electron in Actinide Extraction Binary Solutions of Water and n-Tributyl Phosphate

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by Teseer Bahry, Sergey A. Denisov, Philippe Moisy, Jun Ma, Mehran Mostafavi

J. Phys. Chem. B 2021, 125, 15, 3843–3849; doi.org/10.1021/acs.jpcb.0c10831

The excess electron in solution is a highly reactive radical involved in various radiation-induced reactions. Its solvation state critically determines the subsequent pathway and rate of transfer. For instance, water plays a dominating role in the electron-induced dealkylation of n-tributyl phosphate in actinide extraction processing. However, the underlying electron solvation processes in such systems are lacking. Herein, we directly observed the solvation dynamics of electrons in H-bonded water and n-tributyl phosphate (TBP) binary solutions with a mole fraction of water (Xw) varying from 0.05 to 0.51 under ambient conditions. Following the evolution of the absorption spectrum of trapped electrons (not fully solvated) with picosecond resolution, we show that electrons statistically distributed would undergo preferential solvation within water molecules extracted in TBP. We determine the time scale of excess electron full solvation from the deconvoluted transient absorption–kinetical data. The process of solvent reorganization accelerates by increasing the water molar fraction, and the rate of this process is 2 orders of magnitude slower compared to bulk water. We assigned the solvation process to hydrogen network reorientation induced by a negative charge of the excess electron that strongly depends on the local water environment. Our findings suggest that water significantly stabilizes the electron in a deeper potential than the pure TBP case. In its new state, the electron is likely to inhibit the dealkylation of extractants in actinide separation.

Confined Water Radiolysis in Aluminosilicate Nanotubes: The Importance of Charge Separation Effects

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by Marie-Claire PIGNIE,   Viacheslav Shcherbakov,   Thibault Charpentier,   Mélanie Moskura,   Cedric Carteret,   Sergey A. Denisov,   M. Mostafavi,   Antoine Thill  and  Sophie LE CAER  

Nanoscale 2021, 13, 3092-3105; https://doi.org/10.1039/D0NR08948F

Imogolite nanotubes are potentially promising co-photocatalysts because they are predicted to have curvature-induced, efficient electron-hole pair separation. This prediction has however not yet been experimentally proven. Here, we investigated the behavior upon irradiation of these inorganic nanotubes as a function of their water content to understand the fate of the generated electrons and holes. Two types of aluminosilicate nanotubes were studied: one was hydrophilic on its external and internal surfaces (IMO-OH) and the other had a hydrophobic internal cavity due to Si-CH3 bonds (IMO-CH3), with the external surface remaining hydrophilic. Picosecond pulse radiolysis experiments demonstrated that the electrons are efficiently driven outward. For imogolite samples with very few external water molecules (around 1% of the total mass), quasi-free electrons were formed. They were able to attach to a water molecule, generating a water radical anion, which ultimately led to dihydrogen. When more external water molecules were present, solvated electrons, precursors of dihydrogen, were formed. In contrast, holes moved towards the internal surface of the tubes. They mainly led to the formation of dihydrogen and of methane in irradiated IMO-CH3. The attachment of the quasi-free electron to water was a very efficient process and accounted for the high dihydrogen production at low relative humidity values. When the water content increased, electron solvation dominated over attachment to water molecules. Electron solvation led to dihydrogen production, albeit to a lesser extent than quasi-free electrons. Our experiments demonstrated the spontaneous curvature-induced charge separation in these inorganic nanotubes, making them very interesting potential co-photocatalysts.

Oxidation of Silver Cyanide Ag(CN)2– by the OH Radical: From Ab Initio Calculation to Molecular Simulation and to Experiment

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by Céline Léonard, Frédéric Le Quéré, Daniel Adjei, Sergey A. Denisov, Mehran Mostafavi and Pierre Archirel

J. Phys. Chem. A 2020, 124, 51, 10787–10798; doi.org/10.1021/acs.jpca.0c08038

We investigate the oxidation of silver cyanide  in water by the OH radical in order to compare this complex with the free cation Ag+ and to measure the influence of the ligands. High-level ab initio calculations of the model species  enable the calibration of molecular simulations and the prediction of the oxidized species:  and its absorption spectrum, with an intense band at 292 nm and a weaker one at 390 nm. Pulse radiolysis measurements of the oxidation of  by the OH radical in water yields a transient species with a broad, intense band at 290 nm and a weaker band at 410 nm at short times after the pulse and a blue shift of the spectrum at longer times. The prediction of the simulations, that the oxidized complex  is formed, is confirmed by thermochemistry. Our calculations also suggest that the formation of the OH-adduct is possible only in very basic solution and that the blue shift observed at long times after the pulse is due to disproportionation of the oxidized complex. We also perform molecular simulations of the oxidation of free Ag+ cations by the OH radical. The results are compared to that of the literature and to the results obtained with the  complex.

On the Primary Water Radicals’ Production in the Presence of Gold Nanoparticles: Electron Pulse Radiolysis Study

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by Viacheslav Shcherbakov, Sergey A. Denisov  and Mehran Mostafavi 

Nanomaterials 2020, 10, 2478; https://doi.org/10.3390/nano10122478

Gold nanoparticles are known to cause a radiosensitizing effect, which is a promising way to improve radiation therapy. However, the radiosensitization mechanism is not yet fully understood. It is currently assumed that gold nanoparticles can influence various physical, chemical, and biological processes. Pulse radiolysis is a powerful tool that can examine one of the proposed effects of gold nanoparticles, such as increased free radical production. In this work, we shed light on the consequence of ionizing radiation interaction with gold nanoparticles by direct measurements of solvated electrons using the pulse radiolysis technique. We found that at a therapeutically relevant gold concentration (<3 mM atomic gold, <600 μg × cm−3), the presence of gold nanoparticles in solution does not induce higher primary radicals’ formation. This result contradicts some hypotheses about free radical formation in the presence of gold nanoparticles under ionizing radiation previously reported in the literature.

Hot‐Electron Photodynamics in Silver‐Containing BEA‐Type Nanozeolite Studied by Femtosecond Transient Absorption Spectroscopy

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by Farah Kawtharani, Svetlana Mintova, Richard Retoux, Mehran Mostafavi, Guy Buntinx and Vincent De Waele

CHEMPHYSCHEM 2020, 21, 2634-2643; doi.org/10.1002/cphc.202000822

Silver cations were introduced in nanosized BEA‐type zeolite containing organic template by ion‐exchange followed by chemical reduction towards preparation of photoactive materials (Ag0‐BEA). The stabilization of highly dispersed Ag0 nanoparticles with a size of 1–2 nm in the BEA zeolite was revealed. The transient optical response of the Ag‐BEA samples upon photoexcitation at 400 nm was studied by femtosecond absorption. The photodynamic of the hot electrons was found to depend on the sample preparation. The lifetime of the hot electrons in the Ag−BEA samples containing small Ag nanoparticles (1–2 nm) is significantly shortened in comparison to bear Ag nanoparticles with a size of 10 nm. While for the larger Ag nanoparticles, the energy absorbed in the conduction band is decaying by electron‐phonon coupling into the metal lattice, the high surface‐to‐volume ratio of the small Ag nanoparticles favors the dissipation of the energy of the hot electrons from the metal nanoparticles (Ag0) towards the zeolitic micro‐environment. This finding is encouraging for further applications of Ag‐containing zeolites in photocatalysis and plasmonic chemistry.

One Way Traffic: Base‐to‐Backbone Hole Transfer in Nucleoside Phosphorodithioate

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by Renata Kaczmarek, Samuel Ward, Dipra Debnath,Taisiya Jacobs, Alexander D. Stark, Dariusz Korczyński, Anil Kumar, Michael D. Sevilla, Sergey A. Denisov, Viacheslav Shcherbakov, Pascal Pernot, Mehran Mostafavi, Roman Dembinski, Amitava Adhikary

Chem. Eur. J. 2020, 26, 9495-9505; https://doi.org/10.1002/chem.202000247

The directionality of the hole‐transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S−)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G.+‐P(S−)=S. The ionization potential of G‐P(S−)=S was calculated to be slightly lower than that of guanine in 5′‐dGMP. Subsequent thermally activated hole transfer from G.+ to P(S−)=S led to dithiyl radical (P‐2S.) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P‐2S. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G‐P(S−)=S concentrations showed a bimolecular conversion of P‐2S. to the σ2‐σ*1‐bonded dimer anion radical [‐P‐2S- 2S‐P‐]− [ΔG (150 K, DFT)=−7.2 kcal mol−1]. However, [‐P‐2S 2S‐P‐]− formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=−1.4 kcal mol−1]. Neither P‐2S. nor [‐P‐2S 2S‐P‐]− oxidized guanine base; only base‐to‐backbone hole transfer occurs in phosphorodithioate.

Mechanisms of metal nanoparticles nucleation and growth studied by radiolysis

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by J.Belloni, J.-L.Marignier, M.Mostafavi

Radiation Physics and Chemistry 2020, 169, 107952; doi.org/10.1016/j.radphyschem.2018.08.001

The ultradivided matter is used for long in various applications, for example in colloids, inks and paints, cosmetics, stained glasses, catalysts, photographic emulsions, … But the progressive need of nanoparticles for various miniaturized devices and the different approaches for the synthesis have suddenly increased.

All of the bottom-up synthesis methods from a diluted precursor to metal nanoparticles imply several steps: a reduction reaction of ionic precursors by electron transfer, inducing the nucleation of atoms then the growth of the seeds into particles, more or less inhibited by stabilizers. The final size, shape, structure and dispersity of the particles strongly depend on the thermodynamics and the kinetics of these steps. The interaction of high energy radiation with the solvent provides, quantitatively and homogeneously distributed in the bulk, strong electron donors (solvated electrons, reducing radicals) which reduce metal ions as precursors into atoms. The radiation chemistry, on one hand in the steady state regime with an accurate knowledge of the yields of all the radiolytic products, and on the other hand in the pulse regime giving access to time-resolved data, constitutes a unique tool to elucidate the detailed mechanisms and to provide the keys of really controlling these processes in view of various applications.