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The Journal of Chemical Physics Dec 2023Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted...
Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M-1 s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1 s-1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.
PubMed: 38146832
DOI: 10.1063/5.0180065 -
FEBS Letters Apr 2022Flavohaemoglobins (FlavoHbs) function as nitric oxide dioxygenases, oxidizing nitric oxide with nitrite and shuttling electrons from NAD(P)H via FAD and O . Here, using...
Flavohaemoglobins (FlavoHbs) function as nitric oxide dioxygenases, oxidizing nitric oxide with nitrite and shuttling electrons from NAD(P)H via FAD and O . Here, using pulse radiolysis, we investigate intramolecular electron transfer between FAD and haem b in FlavoHbs. We found that reduction of FlavoHb with hydrated electrons proceeded via two phases: an initial fast phase and a second slower process. Absorbance measured at 600 nm revealed fast flavin reduction followed by a slower decrease corresponding to reoxidation of FAD. The slower process was partially lost in FlavoHbs lacking FAD. These results suggest that the slower phase is attributable to intramolecular electron transfer from FAD to the haem iron. The rate constant in the absence of azole compound (3.3 × 10 s ) was accelerated ~ 10-fold (2.7 × 10 s ) by the binding of econazole, reflecting a conformational change in the open-to-closed transition.
Topics: Anti-Bacterial Agents; Azoles; Candida; Electron Transport; Electrons; Flavin-Adenine Dinucleotide; Heme; Kinetics; NAD; Nitric Oxide; Oxidation-Reduction; Pichia
PubMed: 35253217
DOI: 10.1002/1873-3468.14327 -
Physical Chemistry Chemical Physics :... Sep 2020Radiation chemical studies of esculetin (E), a dihydroxycoumarin derivative, were performed using a pulse radiolysis technique employing kinetic spectrometer and quantum...
Radiation chemical studies of esculetin (E), a dihydroxycoumarin derivative, were performed using a pulse radiolysis technique employing kinetic spectrometer and quantum chemical calculations. Both the oxidizing radicals, hydroxyl (˙OH) and azide (N˙) radicals, and the reducing radical hydrated electron (e) and hydrogen atom (H˙) reactions of E were used for the present study. The reaction of ˙OH and N˙ radicals with E produced transients that absorbed at 410 nm; additionally, another broad band at 510 nm was observed for the ˙OH radical reaction. The reaction of ˙OH radicals with E formed the phenoxyl radical and ˙OH-adducts. It was revealed that 32% of the ˙OH radical reaction products of E were oxidizing in nature and 47% were reducing in nature. The carbonyl group of E was reduced by e and subsequently converted to a neutral radical adduct upon protonation. Similarly, the H˙ atom reaction with E yielded a neutral adduct along with H˙ atom addition products. The transient product absorbed at 380 nm when E was reduced by e and the H˙ atom; additionally, the H˙ atom addition product absorbed at 500 nm. In the case of E, the oxidizing radicals were reactive towards the aromatic ring and the phenolic OH group, whereas the reducing radicals were reactive towards the carbonyl group of E. Quantum chemical calculations using DFT and TD-DFT methods have supported the experimental observation. There was good agreement between the experimental and theoretical data on a number of occasions. Based on the energetics of the transients, it was suggested that the addition products were exothermic in nature. In the addition reaction with the ˙OH radical, there was a slight increase in the C-C bond length adjacent to the addition site compared to the remaining bonds. During the reduction process through the carbonyl group, the [double bond splayed left]C[double bond, length as m-dash]O bond length was increased from 1.221 Å to 1.358 Å. There was an excellent correlation between the calculated and experimentally observed absorption maximum for the oxidized product of E. Overall, these redox studies may find application in developing hydroxycoumarin derivatives as an antioxidant or as an electron transporting agent in biochemical processes. In addition, this information will be helpful for understanding the mechanism of removing pollutant dyes by advanced oxidation processes.
PubMed: 32785355
DOI: 10.1039/d0cp03130e -
Annual Review of Physical Chemistry Apr 2021Ionizing rays cause damage to genomes, proteins, and signaling pathways that normally regulate cell activity, with harmful consequences such as accelerated aging,... (Review)
Review
Ionizing rays cause damage to genomes, proteins, and signaling pathways that normally regulate cell activity, with harmful consequences such as accelerated aging, tumors, and cancers but also with beneficial effects in the context of radiotherapies. While the great pace of research in the twentieth century led to the identification of the molecular mechanisms for chemical lesions on the building blocks of biomacromolecules, the last two decades have brought renewed questions, for example, regarding the formation of clustered damage or the rich chemistry involving the secondary electrons produced by radiolysis. Radiation chemistry is now meeting attosecond science, providing extraordinary opportunities to unravel the very first stages of biological matter radiolysis. This review provides an overview of the recent progress made in this direction, focusing mainly on the atto- to femto- to picosecond timescales. We review promising applications of time-dependent density functional theory in this context.
Topics: Computer Simulation; DNA; Humans; Lipids; Models, Theoretical; Proteins; Pulse Radiolysis; Radiation, Ionizing; Radiochemistry
PubMed: 33878897
DOI: 10.1146/annurev-physchem-101419-013639 -
Free Radical Research Oct 2019Pulse radiolysis was conducted to investigate: several fundamental reactions of a natural flavonoid, rutin, and its glycosylated form (αG-rutin) as a basis for their...
Pulse radiolysis was conducted to investigate: several fundamental reactions of a natural flavonoid, rutin, and its glycosylated form (αG-rutin) as a basis for their radiation protection properties; the reactions with OH () and dGMP radical, dGMP (), which was used as a model of initial and not yet stabilised damage on DNA. Three absorption peaks were commonly seen in the reactions of the flavonoids with OH, showing that their reactive site is the common structure, i.e. . One among the three peaks was attributed to the flavonoid radical produced as a result of the removal of a hydrogen atom. The same peak was found in their reactions with dGMP, showing that dGMP is chemically repaired by obtaining a hydrogen atom supplied from the flavonoids. Such a spectral change due to the chemical repair was as clear as never reported. The rate constants of the chemical repair reaction were estimated as (9 ± 2)×10 M s and (6 ± 1)×10 M s for rutin and αG-rutin, respectively. The rate constants of the radical scavenging reactions towards OH were estimated as (1.3 ± 0.3)×10 M s and (1.0 ± 0.1)×10 M s for rutin and αG-rutin, respectively. In addition, there was no obvious difference between rutin and αG-rutin, indicating that the glycosylation does not change early chemical reactions of rutin.
Topics: Flavonoids; Pulse Radiolysis; Radiation Protection; Rutin
PubMed: 31514547
DOI: 10.1080/10715762.2019.1667991 -
Angewandte Chemie (International Ed. in... Sep 2020Herein, the synthesis and characterization of a hypervalent-iodine-based reagent that enables a direct and selective nitrooxylation of enolizable C-H bonds to access a...
Herein, the synthesis and characterization of a hypervalent-iodine-based reagent that enables a direct and selective nitrooxylation of enolizable C-H bonds to access a broad array of organic nitrate esters is reported. This compound is bench stable, easy-to-handle, and delivers the nitrooxy (-ONO ) group under mild reaction conditions. Activation of the reagent by Brønsted and Lewis acids was demonstrated in the synthesis of nitrooxylated β-keto esters, 1,3-diketones, and malonates, while its activity under photoredox catalysis was shown in the synthesis of nitrooxylated oxindoles. Detailed mechanistic studies including pulse radiolysis, Stern-Volmer quenching studies, and UV/Vis spectroelectrochemistry reveal a unique single-electron-transfer (SET)-induced concerted mechanistic pathway not reliant upon generation of the nitrate radical.
PubMed: 32530081
DOI: 10.1002/anie.202005720 -
Applied Spectroscopy Sep 2022We describe the first implementation of broadband, nanosecond time-resolved step-scan Fourier transform infrared (S-FT-IR) spectroscopy at a pulse radiolysis facility....
Coupling Pulse Radiolysis with Nanosecond Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy: Broadband Mid-Infrared Detection of Radiolytically Generated Transients.
We describe the first implementation of broadband, nanosecond time-resolved step-scan Fourier transform infrared (S-FT-IR) spectroscopy at a pulse radiolysis facility. This new technique allows the rapid acquisition of nano- to microsecond time-resolved infrared (TRIR) spectra of transient species generated by pulse radiolysis of liquid samples at a pulsed electron accelerator. Wide regions of the mid-infrared can be probed in a single experiment, which often takes < 20-30 min to complete. It is therefore a powerful method for rapidly locating the IR absorptions of short-lived, radiation-induced species in solution, and for directly monitoring their subsequent reactions. Time-resolved step-scan FT-IR detection for pulse radiolysis thus complements our existing narrowband quantum cascade laser-based pulse radiolysis-TRIR detection system, which is more suitable for acquiring single-shot kinetics and narrowband TRIR spectra on small-volume samples and in strongly absorbing solvents, such as water. We have demonstrated the application of time-resolved step-scan FT-IR spectroscopy to pulse radiolysis by probing the metal carbonyl and organic carbonyl vibrations of the one-electron-reduced forms of two Re-based CO reduction catalysts in acetonitrile solution. Transient IR absorption bands with amplitudes on the order of 1 × 10 are easily detected on the sub-microsecond timescale using electron pulses as short as 250 ns.
PubMed: 35414202
DOI: 10.1177/00037028221097429 -
Chemical Society Reviews Jul 2021The concept behind the research described in this article was that of marrying the 'soft' methods of radical generation with the effectiveness and flexibility of... (Review)
Review
The concept behind the research described in this article was that of marrying the 'soft' methods of radical generation with the effectiveness and flexibility of nucleophile/electrophile synthetic procedures. Classic studies with pulse radiolysis and laser flash photolysis had shown that free radicals could be more acidic than their closed shell counterparts. QM computations harmonised with this and helped to define which radical centres and which structural types were most effective. Radicals based on the sulfonic acid moiety and on the Meldrum's acid moiety (2,2-dimethyl-1,3-dioxane-4,6-dione) were found to be extreme examples in the superacid class. The ethyne unit could be used as a very effective spacer between the radical centre and the site of proton donation. The key factor in promoting acidity was understood to be the thermodynamic stabilisation of the conjugate anion-radicals released on deprotonation. Solvation played a key part in promoting this and theoretical microhydration studies provided notable support. A corollary was that heterolytic dissociations of free radicals to yield either electrophiles or nucleophiles were also enhanced relative to non-radical models. The most effective radical types for spontaneous releases of both these types of reagents were identified. Ethyne units were again effective as spacers. The enhancement of release of phosphate anions by adjacent radical centres was an important special case. Reactive oxygen species and also diradicals from endiyne antibiotics generate C4'-deoxyribose radicals from nucleotides. Radicals of these types spontaneously release phosphate and triphosphate and this is a contributor to DNA and RNA strand breaks.
Topics: DNA; DNA Damage; Free Radicals; Protons; Pulse Radiolysis; RNA
PubMed: 34019058
DOI: 10.1039/d1cs00193k -
Polymers Nov 2020Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review... (Review)
Review
Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons interaction with polymers display various mechanisms. While the interactions of gamma ray and X-ray photons are mainly through the photoelectric effect, Compton scattering, and pair-production, the interactions of the high-energy electrons take place through coulombic interactions. Despite the type of radiation used on materials, photons or high energy electrons, in both cases ions and electrons are produced. The interactions between electrons and monomers takes place within less than a nanosecond. Depending on the dose rate (dose is defined as the absorbed radiation energy per unit mass), the kinetic chain length of the propagation can be controlled, hence allowing for some control over the degree of polymerization. When polymers are submitted to high-energy radiation in the bulk, contrasting behaviors are observed with a dominant effect of cross-linking or chain scission, depending on the chemical nature and physical characteristics of the material. Polymers in solution are subject to indirect effects resulting from the radiolysis of the medium. Likewise, for radiation-induced polymerization, depending on the dose rate, the free radicals generated on polymer chains can undergo various reactions, such as inter/intramolecular combination or inter/intramolecular disproportionation, b-scission. These reactions lead to structural or functional polymer modifications. In the presence of oxygen, playing on irradiation dose-rates, one can favor crosslinking reactions or promotes degradations through oxidations. The competition between the crosslinking reactions of C-centered free radicals and their reactions with oxygen is described through fundamental mechanism formalisms. The fundamentals of polymerization reactions are herein presented to meet industrial needs for various polymer materials produced or degraded by irradiation. Notably, the medical and industrial applications of polymers are endless and thus it is vital to investigate the effects of sterilization dose and dose rate on various polymers and copolymers with different molecular structures and morphologies. The presence or absence of various functional groups, degree of crystallinity, irradiation temperature, etc. all greatly affect the radiation chemistry of the irradiated polymers. Over the past decade, grafting new chemical functionalities on solid polymers by radiation-induced polymerization (also called RIG for Radiation-Induced Grafting) has been widely exploited to develop innovative materials in coherence with actual societal expectations. These novel materials respond not only to health emergencies but also to carbon-free energy needs (e.g., hydrogen fuel cells, piezoelectricity, etc.) and environmental concerns with the development of numerous specific adsorbents of chemical hazards and pollutants. The modification of polymers through RIG is durable as it covalently bonds the functional monomers. As radiation penetration depths can be varied, this technique can be used to modify polymer surface or bulk. The many parameters influencing RIG that control the yield of the grafting process are discussed in this review. These include monomer reactivity, irradiation dose, solvent, presence of inhibitor of homopolymerization, grafting temperature, etc. Today, the general knowledge of RIG can be applied to any solid polymer and may predict, to some extent, the grafting location. A special focus is on how ionizing radiation sources (ion and electron beams, UVs) may be chosen or mixed to combine both solid polymer nanostructuration and RIG. LLET ionizing radiation has also been extensively used to synthesize hydrogel and nanogel for drug delivery systems and other advanced applications. In particular, nanogels can either be produced by radiation-induced polymerization and simultaneous crosslinking of hydrophilic monomers in "nanocompartments", i.e., within the aqueous phase of inverse micelles, or by intramolecular crosslinking of suitable water-soluble polymers. The radiolytically produced oxidizing species from water, •OH radicals, can easily abstract H-atoms from the backbone of the dissolved polymers (or can add to the unsaturated bonds) leading to the formation of C-centered radicals. These C-centered free radicals can undergo two main competitive reactions; intramolecular and intermolecular crosslinking. When produced by electron beam irradiation, higher temperatures, dose rates within the pulse, and pulse repetition rates favour intramolecular crosslinking over intermolecular crosslinking, thus enabling a better control of particle size and size distribution. For other water-soluble biopolymers such as polysaccharides, proteins, DNA and RNA, the abstraction of H atoms or the addition to the unsaturation by •OH can lead to the direct scission of the backbone, double, or single strand breaks of these polymers.
PubMed: 33266261
DOI: 10.3390/polym12122877 -
Biochemistry Nov 2022The functioning of cytochrome oxidases involves orchestration of long-range electron transfer (ET) events among the four redox active metal centers. We report the...
The functioning of cytochrome oxidases involves orchestration of long-range electron transfer (ET) events among the four redox active metal centers. We report the temperature dependence of electron transfer from the Cu site to the low-spin heme-() site, i.e., Cu + heme-() → Cu + heme-() in three structurally characterized enzymes: A-type from (PDB code 3HB3) and bovine heart tissue (PDB code 2ZXW), and the B-type from (PDB codes 1EHK and 1XME). , data sets were obtained with the use of pulse radiolysis as described previously. Semiclassical Marcus theory revealed that λ varies from 0.74 to 1.1 eV, , varies from ∼2 × 10 eV (0.16 cm) to ∼24 × 10 eV (1.9 cm), and β varies from 9.3 to 13.9. These parameters are consistent with diabatic electron tunneling. The II-Asp111Asn Cu mutation in cytochrome had no effect on the rate of this reaction whereas the II-Met160Leu Cu-mutation was slower by an amount corresponding to a decreased driving force of ∼0.06 eV. The structures support the presence of a common, electron-conducting "wire" between Cu and heme-(). The transfer of an electron from the low-spin heme to the high-spin heme, i.e., heme-() + heme- → heme-() + heme-, was not observed with the A-type enzymes in our experiments but was observed with the ; its Marcus parameters are λ = 1.5 eV, = 26.6 × 10 eV (2.14 cm), and β = 9.35, consistent also with diabatic electron tunneling between the two hemes. The II-Glu15Ala mutation of the K-channel structure, ∼ 24 Å between its CA and Fe-, was found to completely block heme- to heme- electron transfer. A structural mechanism is suggested to explain these observations.
Topics: Cattle; Animals; Thermus thermophilus; Electron Transport Complex IV; Cytochrome b Group; Electrons; Pulse Radiolysis; Temperature; Oxidation-Reduction; Heme
PubMed: 21028883
DOI: 10.1021/bi100548n