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Chemical Reviews Mar 2019Pulse radiolysis is a powerful method for generating highly reduced or oxidized species and free radicals. Combined with fast time-resolved spectroscopic measurement, we... (Review)
Review
Pulse radiolysis is a powerful method for generating highly reduced or oxidized species and free radicals. Combined with fast time-resolved spectroscopic measurement, we can monitor the reactions of intermediate species on time scales ranging from picoseconds to seconds. The application of pulse radiolysis to water generates hydrated electrons (e) and specific radicals, rendering this technique useful for investigating a number of biological redox processes. The first pulse radiolysis redox investigations explored in this review involved intramolecular electron transfer processes in protein with multiple electron-accepting sites. Pulse radiolysis enabled direct monitoring of the internal electron transfer rates and the distribution of electrons within proteins. Structural information from X-ray data has allowed analysis of the rate constants and their activation parameters in relation to the mechanisms with current theoretical treatments. The second set of pulse radiolysis redox investigations explored here concerned the intermediates of enzyme reactions after redox reactions. Pulse radiolysis allowed the extremely rapid donation of electrons to a redox center in a protein. It makes it possible to observe the unstable intermediates after the reduction and the following subsequent steps. For example, the intermediates generated through the one-electron reduction of oxygenated hemoproteins, such as cytochrome P450 and nitric oxide synthase, were characterized. Interestingly, ligand exchange can occur upon the reduction of heme iron, in which different amino acid residues bind to heme in the ferrous and ferric states, respectively. We directly observed the ligand-switching intermediates of bacterial CooA, a CO sensor, and bacterial iron response regulator protein. These ligand exchange processes are physiologically important for regulating the electrode potential and effective formation of superoxide anion or HO. The third set of pulse radiolysis redox investigations explored in this review concerns free-radical processes in biological systems. Free radicals are produced in cells and organisms in a variety of processes. The cell has developed special and very effective machinery for controlling and detoxifying reactive radicals. Radiation-generated radicals allow studies of the reactions between specific radicals and solutes, often revealing the mechanisms underlying the initial and subsequent reactions. The crucial contribution was made using pulse radiolysis techniques and knowledge of the identities, properties, and reactions of radicals. These radicals include superoxide (O), nitric monoxide (NO), ascorbate, urate, and protein radicals. This review focuses on the reactions of these radicals and their physiological functions.
Topics: Electron Transport; Electron-Transferring Flavoproteins; Free Radicals; Hydroxyl Radical; Models, Chemical; Models, Molecular; Oxidation-Reduction; Proteins; Pulse Radiolysis; Water
PubMed: 30741537
DOI: 10.1021/acs.chemrev.8b00405 -
Methods in Enzymology 1993
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Methods in Enzymology 1984
Topics: Electron Spin Resonance Spectroscopy; Free Radicals; Kinetics; Microwaves; Oxygen; Pulse Radiolysis; Superoxides
PubMed: 6328176
DOI: 10.1016/s0076-6879(84)05023-0 -
Journal of Photochemistry and... Nov 2001The contributions of pulse radiolysis towards characterisation of unstable ortho-quinones relevant to melanogenesis are reviewed. The quinones discussed include... (Review)
Review
The contributions of pulse radiolysis towards characterisation of unstable ortho-quinones relevant to melanogenesis are reviewed. The quinones discussed include dopaquinone, the precursor of both eumelanogenesis and phaeomelanogenesis, and 5-S-cysteinyldopaquinone, an early component of the phaeomelanogenic pathway. Redox exchange between dopaquinone and 5-S-cysteinyldopa is shown to be a determinant of the balance between eumelanogenesis and phaeomelanogenesis. Ortho-quinones resulting from the oxidation of tertiary N,N-dialkylcatecholamines cyclise to redox-inactive betaines which fail to autoactivate tyrosinase. This is consistent with the dopa detected during melanogenesis catalysed by tyrosinase being formed indirectly by a combination of dopaquinone intramolecular reductive addition to form leucodopachrome (cyclodopa), followed by redox exchange between remaining dopaquinone and leucodopachrome. Rapid tautomerism of the ortho-quinone of 4-cyanomethylcatechol to a redox-inactive quinomethane likewise inhibits tyrosinase autoactivation. The incorporation of trihydric phenol moieties in melanin is modelled by the reactions of several ortho-quinones with phloroglucinol, which itself is not directly oxidised by tyrosinase due to the meta-positioning of the hydroxyl groups. The importance of a susceptibility towards nucleophilic attack as well as a propensity to undergo redox-exchange, in the chemistry of melanogenic ortho-quinones, is emphasised.
Topics: Benzoquinones; Catechols; Cysteine; Cysteinyldopa; Dihydroxyphenylalanine; Enzyme Activation; Humans; Melanins; Molecular Structure; Monophenol Monooxygenase; Oxidation-Reduction; Phloroglucinol; Pulse Radiolysis; Quinones
PubMed: 11744399
DOI: 10.1016/s1011-1344(01)00220-2 -
ChemSusChem Nov 2017Taking inspiration from natural photosystems, the goal of artificial photosynthesis is to harness solar energy to convert abundant materials, such as CO and H O, into... (Review)
Review
Taking inspiration from natural photosystems, the goal of artificial photosynthesis is to harness solar energy to convert abundant materials, such as CO and H O, into solar fuels. Catalysts are required to ensure that the necessary redox half-reactions proceed in the most energy-efficient manner. It is therefore critical to gain a detailed mechanistic understanding of these catalytic reactions to develop new and improved catalysts. Many of the key catalytic intermediates are short-lived transient species, requiring time-resolved spectroscopic techniques for their observation. The two main methods for rapidly generating such species on the sub-microsecond timescale are laser flash photolysis and pulse radiolysis. These methods complement one another, and both provide important spectroscopic and kinetic information. However, pulse radiolysis proves to be superior in systems with significant spectroscopic overlap between the photosensitizer and other species present during the reaction. Herein, the pulse radiolysis technique and how it has been applied to mechanistic investigations of halfreactions relevant to artificial photosynthesis are reviewed.
Topics: Carbon Dioxide; Catalysis; Coordination Complexes; Hydrogen; Kinetics; Methane; Molecular Structure; Oxidation-Reduction; Photosynthesis; Pulse Radiolysis; Solar Energy; Sunlight; Water
PubMed: 28898568
DOI: 10.1002/cssc.201701559 -
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 -
BJR Supplement 1992
Review
Topics: Drug Design; Free Radicals; Models, Chemical; Oxidation-Reduction; Pulse Radiolysis; Superoxides; Water
PubMed: 1337838
DOI: No ID Found -
In Vivo (Athens, Greece) 2016Cancer therapy by means of high-energy ions is very efficient. As a consequence of the linear-energy-transfer effect only a negligible part of the produced free radicals...
Cancer therapy by means of high-energy ions is very efficient. As a consequence of the linear-energy-transfer effect only a negligible part of the produced free radicals can escape combination processes to form molecular products and to cause undesired side processes. Positrons (e⁺) and γ-rays, generated by the nuclear interaction of high-energy ions in the medium, serve in monitoring the radiation dose absorbed by the tumor. However, due to the dipole nature of water molecules a small proportion of thermalized positrons (e⁺th) can become solvated (e⁺aq). Hence, they are stabilized, live longer and can initiate side reactions. In addition, positronium (Ps), besides solvated electrons (e⁺aq), can be generated and involved in the reaction mechanisms. For a better understanding of the reaction mechanisms involved and to improve cancer therapy, a time-resolved pulse radiolysis instrument using high-energy particles is discussed here. The proposed method is examined and recommended by CERN experts. It is planned to be realized at the MedAustron Radiation Therapy and Research Centre in Wiener Neustadt, Austria.
Topics: Electrons; Gamma Rays; Humans; Ions; Neoplasms; Pulse Radiolysis
PubMed: 26912822
DOI: No ID Found -
Journal of Photochemistry and... Feb 1998The endogenous indole melatonin and the melatonin receptor agonist 6-chloromelatonin block the proliferation of both dermal and uveal melanoma cells by mechanisms that...
The endogenous indole melatonin and the melatonin receptor agonist 6-chloromelatonin block the proliferation of both dermal and uveal melanoma cells by mechanisms that may involve redox reactions. The interactions of hydrated electrons, the azide radical, hydroxyl radicals and superoxide with melatonin and its 6-chloro analogue have been studied using the technique of pulse radiolysis. The reaction rate constants of eaq- and N3 x with these compounds were found to be dependent on substitution at the sixth position. The rate constants for reaction of 6-chloromelatonin and melatonin with solvated electrons are 4.5 x 10(9) M-1 s-1 and 4.2 x 10(8) M-1 s-1, respectively. The reaction rate constants of N3 x with malatonin and chloromelatonin are 9.8 x 10(9) M-1 s-1 and 3.5 x 10(9) M-1 s-1 and 3.5 x 10(9) M-1 s-1, respectively. Melatonin and 6-chloromelatonin react with hydroxyl radicals at near diffusion controlled rates (1.3 x 10(10) M-1 s-1, 8.2 x 10(9) M-1 s-1). Melatonin and 6-chloromelatonin did not react with superoxide radicals and we calculate an upper limit of 1.0 x 10(4) M-1 s-1 for the rate constant for reaction of melatonin and 6-chloromelatonin with superoxide ion.
Topics: Electron Spin Resonance Spectroscopy; Hydroxyl Radical; Melatonin; Models, Chemical; Pulse Radiolysis; Superoxides
PubMed: 9540219
DOI: 10.1016/s1011-1344(97)00132-2 -
International Journal of Radiation... Nov 1998The aim of the study was to obtain evidence to support the hypothesis that the radioprotection by DNA-binding bibenzimidazoles is due to reduction by the DNA-bound...
PURPOSE
The aim of the study was to obtain evidence to support the hypothesis that the radioprotection by DNA-binding bibenzimidazoles is due to reduction by the DNA-bound ligand of transient radiation-induced oxidizing species on DNA, by following oxidation of the ligand after pulse radiolysis. A second aim was to compare the activities of methylproamine and Hoechst 33342 in the pulse radiolysis system, with the view to seeking a correlation with radioprotective activity.
METHODS
Solutions of deoxyguanosine or DNA, with or without Hoechst 33342 or methylproamine, and containing sodium selenate and tert-butanol were subjected to pulse radiolysis, and the oxidation of the ligand followed by time-resolved spectrophotometry.
RESULTS
The initial pulse radiolysis experiments using deoxyguanosine (dG) established that pulse radiolysis of sodium selenate produces a transient oxidant SeO3*-, which oxidizes dG to a species (presumably dG*+), with spectral characteristics indistinguishable from those described in previous pulse radiolysis studies using Br2*- as the oxidant. The estimate obtained for the bimolecular rate constant (k2) for the reaction of the selenite radical with dG, was 1.2 x 10(9) M(-1) s(-1). The corresponding reaction of SeO3*- with DNA is much slower (k2 3 x 10(7) M(-1) s(-1)). Although unbound Hoechst 33342 is oxidized directly by SeO3*- (k2 2.3 x 10(9) M(-1) s(-1)), experiments with mixtures of Hoechst 33342 with an excess of dG (or DNA) indicated that ligand oxidation was mediated by dG*+ (or DNAoxid). For example, successive dilution of a DNA-Hoechst solution had little impact on the rate of ligand oxidation, consistent with an intramolecular rate-determining step. When the concentration of DNA was maintained at 1.0 mM DNA bp, increasing the concentration of the ligand resulted in a linear increase in the rate of oxidation; the increase being steeper for methylproamine than for Hoechst 33342. Investigation of the dependence of yield of oxidized ligand on ligand occupancy also indicated that the methylproamine was more active than Hoechst 33342, with the estimates for the range of electron transfer from the ligand to DNAoxid being 14 and 31 bp for Hoechst 33342 and methylproamine, respectively.
CONCLUSIONS
At this stage we conclude that radioprotection by these DNA-binding ligands is mediated by electron transfer, and that the improved radioprotective activity of methylproamine may be attributable to the observed kinetic differences. However, further studies are required to confirm the correlation, and if it is sustained, pulse radiolysis could be useful in evaluating new analogues in an attempt to further improve the radioprotective properties of methylproamine, which already has considerable clinical potential.
Topics: Benzimidazoles; DNA; Electrons; Pulse Radiolysis; Radiation-Sensitizing Agents
PubMed: 9845105
DOI: 10.1016/s0360-3016(98)00316-2