SEARCH · PubMed Health

Results for “Pulse Radiolysis”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources

PubMed2006

Product and pulse radiolysis studies on radical-ion splitting of N(1)-C(5')-linked dimer hydrates of 5-substituted uracils by one-electron reduction in anoxic aqueous solution.

Steady-state gamma-radiolysis, pulse radiolysis, and cyclic voltammetry have been performed to identify the mechanism by which N(1)-C(5')-linked homodimer hydrates [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)thymine (2a) and [1-(5'-fluoro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-fluorouracil (2b)], N(1)-C(6')-linked dimer hydrate [1-(5'-hydroxy-5',6'-dihydrothymin-6'-yl)thymine (3a)], and N(1)-C(5')-linked heterodimer hydrate [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)-5-fluorouracil (2ba)] undergo radiolytic reductive splitting to regenerate the parent monomers in anoxic aqueous solution. Radiolytic reductions of the thymine homodimer hydrates 2a and 3a by hydrated electrons (e(aq)-) regenerated the parent thymine (1a) almost quantitatively, while the 5-fluorouracil homodimer hydrates cis-2b and trans-2b afforded 1-(uracil-5'-yl)-5-fluorouracil efficiently along with a small amount of the parent 5-fluorouracil (1b). In contrast to 2b, the heterodimer hydrate analogue 2ba with noneliminating 5'-methyl substituent releases 5-fluorouracil 1b almost quantitatively in the radiolytic reduction. The pulse radiolysis studies suggested that the electron adducts are produced primarily at the thymine and 5-fluorouracil structural unit in the dimer hydrates 2a,b, respectively, in which the resulting dimer hydrate radical anion of 2b (2b*-) was more stable than that of 2a (2a*-). As characterized by pulse radiolysis and cyclic voltammetry, the 5-fluorouracil homodimer hydrate 2b bearing F-substituent at C(5') undergoes one-electron reduction to eliminate exclusively fluoride ion along with the formation of dimer hydrate C(5') radical (2b(-F)*) with oxidizing property. The formation of a possible dimer hydrate radical intermediate 2b(-F)* was also supported by the effect of amines as the reducing additives on the yields of 1b and 4b in the radiolytic reduction of 2b.

Journal Article↗

PubMed2006

S-S bond mesolysis in alpha,alpha'-dinaphthyl disulfide radical anion generated during gamma-radiolysis and pulse radiolysis in organic solution.

A dissociation mechanism of the S-S bond in the alpha,alpha'-dinaphthyl disulfide radical anion (NpSSNp*-) in organic solution was investigated on the basis of transient absorption measurements and DFT calculations. NpSSNp*- generated during gamma-radiolysis of NpSSNp in MTHF at 77 K showed the absorption band at 430 nm, which shifted to 560 nm with an increase of the ambient temperature up to room temperature. With the aid of DFT calculations at the B3LYP/6-31G(d) level, the shift of the absorption band was interpreted in terms of molecular conformational changes of NpSSNp*- due to the elongation of the S-S bond. It was observed that NpSSNp*- dissociates into naphthylthiyl radical and thionaphtholate anion in organic solution with a first-order rate constant in the magnitude of 10(6) s-1. From Arrhenius plots of the decay rate constants of NpSSNp*- in a temperature range of 160-293 K, an activation energy for the S-S bond cleavage in NpSSNp*- in solution was determined along with a frequency factor. Based on the state energies of NpSSNp*- calculated at the B3LYP/6-31G(d) level, a Morse-like energy potential for the S-S bond cleavage of NpSSNp*- is depicted as a function of the S-S bond distance.

Anions↗

PubMed1978

Interaction of aminoacridines with nucleic acids. A pulse-radiolysis study.

Pulse radiolysis has been used to study the interaction of aminoacridines with nucleic acids. The data confirm that there are two modes of binding. These are: a weak interaction which has a maximum binding ratio of one site per dye; and a strong binding process effected by both electrostatic and Van der Waals interactions. The limit of this latter, strong binding mode is approximately six sites per dye. The radiation-induced transient absorption spectrum of benzoflavine is characterized by a pronounced bleaching at 440 nm, which is quenched by the addition of nucleic acids. Mechanisms have been proposed for the reactions of both eaq-and .OH with benzoflavine which account both for the observed bleaching of benzoflavine solutions and for the protective effect of nucleic acids. It is proposed that eaq-reacts with benzoflavine to form a stable benzoflavine semiquinone radical and that .OH reacts with subsequent formation of a very stable benzoflavine hydroxycyclohexadienyl radical.

Acridines↗

PubMed1986

Interactions of thiyl free radicals with oxygen: a pulse radiolysis study.

A pulse radiolysis study of glutathione in aqueous solution at pH 5.5 containing N2O/O2 mixtures at various ratios indicates that oxygen rapidly adds to the thiyl glutathione radical yielding a transient absorption, with a maximum at 540 nm, whose characteristics appear to be compatible with assignment to the GSOO. radical. The reaction (Formula: see text) appears to be an equilibrium whose kinetic constants have been estimated (kf = 2.0 X 10(9) dm3 mol-1, kb = 6.2 X 10(5) s-1). Evidence for electron transfer from ascorbate to the GSOO. radical has been obtained and the respective rate constant has been determined to be 1.75 +/- 0.15 X 10(8) dm3 mol-1 s-1.

Free Radicals↗

PubMed1996

Reaction of hydroxyl radical with phenylpropanoid glycosides from Pedicularis species: a pulse radiolysis study.

Using pulse radiolysis technique, the reaction between hydroxyl radical and 7 phenylpropanoid glycosides: echinacoside, verbascoside, leucosceptoside A, martynoside, pediculariosides A, M and N which were isolated from Pedicularis were examined. The rate constants of these reactions were determined by transient absorption spectra. All 7 phenylpropanoid glycosides react with hydroxyl radical at high rate constants within (0.97-1.91) x 10(10)L.mol-1.s-1, suggesting that they are effective hydroxyl radical scavengers. The results demonstrate that the numbers of phenolic hydroxyl groups of phenylpropanoid glycosides are directly related to their scavenging activities. The scavenging activities are likely related to o-dihydroxy group of phenylpropanoid glycosides as well.

Drugs, Chinese Herbal↗

PubMed1988

Radicals produced by the reactions of SO4- with uridine and its derivatives. Studies by pulse radiolysis and gamma-radiolysis.

Using a pulse radiolysis technique, the sulphate radical, SO4-, has been demonstrated to react with uridine, 3'-UMP and 5'-UMP to produce intermediates of a new type, decaying according to a first-order rate law with k = 2.1, 4.1, 3.0 x 10(5) s-1 respectively at pH 7. The rate for uridine increases slightly with increasing pH. With deoxyuridine and 2', 3'-isopropylidene uridine, on the other hand, OH adduct radicals with longer lives (second-order decay) were found in accordance with the known fact that similar radicals are produced from N(1)-methylated uracils with SO4-. On gamma-irradiation of similar solutions containing the substrates that produce such short-lived species, chain reactions and high yields of unaltered uracil were found. Some optical properties are detailed and a probable structure of the short-lived species is discussed in relation to the substituent effect at the 2' position of the sugar part and to its strong reducing ability revealed by the rapid reaction with TNM.

Free Radicals↗

PubMed2005

Effect of pressure on pulse radiolysis reduction of proteins.

Pulse radiolysis experiments were performed on proteins under pressure. Whereas many spectroscopic techniques have shown protein modifications at different pressure ranges, the present measurements performed using the water radiolysis allowed to generate radical species and to study the mechanisms implied in their reactions with proteins. This work gives the first results obtained on the effects of pressure on the rate constants of the proteins reduction by the hydrated electron at pressures up to 100 MPa. The reaction with the hydrated electron was investigated on two classes of protein: the horse myoglobin and the mussel metallothioneins. We have successively studied the influence of the pH value of metmyoglobin solutions (pH 6, 7 and 8) and the influence of the metals nature (Zn,Cu,Cd) bound to metallothioneins. For both protein, whatever the experimental conditions, the pressure does not influence the value of the reduction rate constant in the investigated range (0.1-100 MPa).

Animals↗

PubMed1977

Pulse radiolysis of lactate dehydrogenase.

Pulse radiolysis has been used to investigate the rates and transient spectra for the reactions of free radicals with beef heart lactate dehydrogenase at pH 7. Analysis of the results leads to second-order rate-constants for eaq-, .OH, .I, .Br2-, .I2- and .(CNS)2- which are, respectively, 24, 21, 10, 0.55, 0.43 and 0.15 in units of 10(10) M-1 s-1 with uncertainties of +/- 20 per cent. Those for .I and .I2- are similar to the corresponding rate-constants for the related enzyme alcohol dehydrogenase. The spectra of the transient species produced by .OH, .Br2- and .(CNS)2- all showed evidence for reactions with tyrosine and tryptophan residues, and in general terms the magnitudes of the rate-constants appeared to increase with the oxidizing abilities of the radicals. The implication of the results for understanding the mechanism of deactivation by free radicals is discussed.

Animals↗

PubMed1978

Quaternary states of methemoglobin and its valence-hybrid. A pulse radiolysis study.

Using the pulse radiolysis technique on solutions of stripped adult human methemoglobin, we found that the heme-iron within a single subunit in the tetramer was reduced to iron(II). The valence-hybrid thus formed was reacted with oxygen and with carbon monoxide. Kinetics of the reactions were studied. The effects of pH, inositol hexaphosphate, and temperature on these reactions were examined. The kinetics of the ligation of O2 and CO were used to characterize the affinity states of the valence-hybrid and its parent methemoglobin. Our results support the description of stripped methemoglobin A as residing in an R state. In the presence of inositol hexaphosphate methemoglobin is stabilized in a T state, but it switches into a high affinity state when the pH is raised a0ove 8.0. This structural transition was not found to coincide with the switch of spin state of the heme-iron that accompanies the ionization of water in aquomethemoglobin A.

Adult↗

PubMed2006

Reaction of hydroxyl radicals with azacytosines: a pulse radiolysis and theoretical study.

Pulse radiolysis and density functional theory (DFT) calculations at B3LYP/6-31+G(d,p) level have been carried out to probe the reaction of the water-derived hydroxyl radicals (*OH) with 5-azacytosine (5Ac) and 5-azacytidine (5Acyd) at near neutral and basic pH. A low percentage of nitrogen-centered oxidizing radicals, and a high percentage of non-oxidizing carbon-centered radicals were identified based on the reaction of transient intermediates with 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), ABTS2-. Theoretical calculations suggests that the N3 atom in 5Ac is the most reactive center as it is the main contributor of HOMO, whereas C5 atom is the prime donor for the HOMO of cytosine (Cyt) where the major addition site is C5. The order of stability of the adduct species were found to be C6-OH_5Ac*>C4-OH_5Ac*>N3-OH_5Ac*>N5-OH_5Ac* both in the gaseous and solution phase (using the PCM model) respectively due to the additions of *OH at C6, C4, N3, and N5 atoms. These additions occur in direct manner, without the intervention of any precursor complex formation. The possibility of a 1,2-hydrogen shift from the C6 to N5 in the nitrogen-centered C6-OH_5Ac* radical is considered in order to account for the experimental observation of the high yield of non-oxidizing radicals, and found that such a conversion requires activation energy of about 32 kcal/mol, and hence this possibility is ruled out. The hydrogen abstraction reactions were assumed to occur from precursor complexes (hydrogen bonded complexes represented as S1, S2, S3, and S4) resulted from the electrostatic interactions of the lone pairs on the N3, N5, and O8 atoms with the incoming *OH radical. It was found that the conversion of these precursor complexes to their respective transition states has ample barrier heights, and it persists even when the effect of solvent is considered. It was also found that the formation of precursor complexes itself is highly endergonic in solution phase. Hence, the abstraction reactions will not occur in the present case. Finally, the time dependent density functional theory (TDDFT) calculations predicted an absorption maximum of 292 nm for the N3-OH_5Ac* adduct, which is close to the experimentally observed spectral maxima at 290 nm. Hence, it is assumed that the addition to the most reactive center N3, which results the N3-OH_5Ac* radical, occurs via a kinetically driven process.

Aza Compounds↗

PubMedDate not supplied

Reactions of OH-radicals with procarbazine. A pulse radiolysis and computer simulation study.

Pulse radiolysis was applied to study the reactivity of *OH radicals with procarbazine (PC), a cytostatic agent widely used in radiation- and chemotherapy. An overall rate constant of k(*OH+PC) = 3.7 x 10(9) l.mol(-1).s(-1) was determined. The thereby formed transients had a strong absorption at 350 nm, epsilon350 = 4.46 x 10(3) l.mol(-1).cm(-1), and a weak absorption band around 530 nm. Computer simulation studies to elucidate the most probable sites of *OH attack on the PC molecule showed that *OH radical addition to the aromatic ring had the highest probability. These transients decayed by a first order reaction, k = 1.75 x 10(3) s(-1), whereby species having a maximum absorption at 300 nm and broad shoulder at 340-380 nm were formed. Similar absorptions were observed after gamma radiolysis of PC. A reaction mechanism is suggested. For the reaction of H-atoms with PC, a rate constant k(*H+PC) = 6.4 x 10(8) l.mol(-1).s(-1) was determined.

Antineoplastic Agents↗

PubMed1998

Intramolecular electron transfer in the dipeptide, histidyltyrosine: a pulse radiolysis study.

The technique of pulse radiolysis has been used to investigate the possibility of intramolecular charge transfer in the dipeptide histidyltyrosine, following one-electron oxidation of one of its amino acid residues. The radical anion, Br2.- was found to react with the dipeptide at pH 6.0 with a bimolecular rate constant of 2.3+/-0.2 x 10(7) dm3 mol(-1)s(-1) suggesting that it reacts very selectively with the histidine moiety. Spectral observations at, or close to the end of this reaction show only the presence of a tyrosinyl free radical (TyrO.), however, indicating that fast (>10(6) s(-1) intramolecular charge transfer has taken place between histidine radicals (His+.) and tyrosine (TyrOH). This finding was supported by the direct observation of the rate of formation of TyrO. in experiments with the free amino acids, histidine and tyrosine, under conditions where Br2.- reacted selectively with histidine. The bimolecular rate constant for the reaction between His+. and TyrOH was found to be 2.4+/-0.5 x 10(6) dm3 mol(-1)s(-1). Taken together, the results of the study indicate that His+. is a relatively strong oxidising agent where (E (His+./His) > 770 mV at pH 6.0.

Bromides↗

PubMed1982

The reaction of cytochrome aa3 with (porphyrin) cytochrome c as studied by pulse radiolysis.

(1) Using the pulse-radiolysis and stopped-flow techniques, the reactions of iron-free (porphyrin) cytochrome c and native cytochrome c with cytochrome aa3 were investigated. The porphyrin cytochrome c anion radical (generated by reduction of porphyrin cytochrome c by the hydrated electron) can transfer its electron to cytochrome aa3. The bimolecular rate constant for this reaction is 2 x 10(7) M-1 . s-1 (5 mM potassium phosphate, 0.5% Tween 20, pH 7.0, 20 degrees C). (2) The ionic strength dependence of the cytochrome c-cytochrome aa3 interaction was measured in the ionic strength range between 40 and 120 mM. At ionic strengths below 30 mM, a cytochrome c-cytochrome aa3 complex is formed in which cytochrome c is no longer reducible by the hydrated electron. A method is described by which the contributions of electrostatic forces to the reaction rate can be determined. (3) Using the stopped-flow technique, the effect of the dielectric constant (epsilon) of the reaction medium on the reaction of cytochrome C with cytochrome aa3 was investigated. With increasing epsilon the second-order rate constant decreased.

Animals↗

PubMed1998

Pulse radiolysis studies indicate that electron transfer is involved in radioprotection by Hoechst 33342 and methylproamine.

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.

Benzimidazoles↗

PubMed1996

Repair of dGMP hydroxyl radical adducts by verbascoside via electron transfer: a pulse radiolysis study.

The repair activity of verbascoside (VER), isolated from Pedicularis spicata, towards the oxidizing hydroxyl radical adduct of dGMP and its reaction mechanism were studied using pulse radiolysis. Upon pulse radiolysis of nitrous oxide saturated aqueous solution of 2'-deoxyguanosine-5'-monophosphate (dGMP) and VER, it was found that the transient absorption spectrum of the hydroxyl adduct of dGMP decays with the formation of that of the phenoxyl radical of VER, several tens of microseconds after the electron pulse. From the formation kinetics of the phenoxyl radical of VER, the rate constant of the repair reaction was determined to be 1.12 x 10(9) dm(3) mol(-1) s(-1).

Antioxidants↗

PubMed1991

Kinetic behavior of the monodehydroascorbate radical studied by pulse radiolysis.

The reactions of the monodehydroascorbate radical (As.-) with various biological molecules were investigated by pulse radiolysis. As.- reacted with both fully reduced and semiquinone forms of hepatic NADH-cytochrome b5 reductase with second-order rate constants of 4.3 x 10(6) and 3.7 x 10(5) M-1 s-1, respectively, at pH 7.0. In contrast, no reaction of As.- with ferrous cytochrome b5 could be detected by pulse radiolysis, whereas the oxidation of cytochrome b5 by As.- was observed by ascorbate-ascorbate oxidase method. This suggests that the rate constant of As.- with the ferrous cytochrome b5 must be several orders in magnitude smaller than that of the disproportionation of As.-. On the other hand, As.- reduced Fe3+EDTA with a second-order rate constant of 4.0 x 10(6) M-1 s-1 but did not reduce ferric hemoproteins such as metmyoglobin, methemoglobin, and cytochrome b5 by either the pulse radiolysis or the ascorbate-ascorbate oxidase method.

Animals↗

PubMed2000

Reaction of reducing hydroxyl radical adducts of pyrimidine nucleotides with riboflavin and flavin adenine dinucleotide (FAD) via electron transfer: a pulse radiolysis study.

Using the techniques of pulse radiolysis with time-resolved spectrophotometric detection, it has been demonstrated that the interaction of reducing OH radical adducts of dCMP, TMP and UMP with riboflavin (RF) and flavin adenine dinucleotide (FAD) does proceed via an electron transfer reaction. From buildup kinetics of radical species, the rate constants of electron transfer from reducing OH adducts of pyrimidines to RF and FAD have been determined, respectively. It could be deduced that RF and FAD would reduce the probability of repair of the damaged DNA in the presence of enzymes and antioxidants, accordingly RF and FAD might have a radiosensitization effect on DNA damage.

DNA↗

PubMed2005

Pulse radiolysis of aqueous thiocyanate solution.

The pulse radiolysis of N(2)O saturated aqueous solutions of KSCN was studied under neutral pH conditions. The observed optical absorption spectrum of the SCN(*) radical in solution is more complex than previously reported, but it is in good agreement with that measured in the gas phase. Kinetic traces at 330 and 472 nm corresponding to SCN(*) and (SCN)(2)(*-), respectively, were fit using a Monte Carlo simulation kinetic model. The rate coefficient for the oxidation of SCN(-) ions by OH radicals, an important reaction used in competition kinetics measurements, was found to be (1.4 +/- 0.1) x 10(10) M(-1) s(-1), about 30% higher than the normally accepted value. A detailed discussion of the reaction mechanism is presented.

Pulse Radiolysis↗