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At least 19 recordsLinked to original sources
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↗
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↗
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↗
PubMed1982

Evidence for catalytic dismutation of superoxide by cobalt(II) derivatives of bovine superoxide dismutase in aqueous solution as studied by pulse radiolysis.

By using the technique of pulse radiolysis to generate O2-., it is demonstrated that Co(II) derivatives of bovine superoxide dismutase in which the copper alone and both the copper and zinc of the enzyme have been substituted by Co(II), resulting in (Co,Zn)- and (Co,Co)-proteins, are capable of catalytically dismutating O2-. with 'turnover' rate constants of 4.8 X 10(6) dm3.s-1.mol-1 and 3.1 X 10(6) dm3.s-1.mol-1 respectively. The activities of the proteins are independent of the pH (7.4-9.4) and are about three orders of magnitude less than that of the native (Cu,Zn)-protein. The rate constants for the initial interaction of O2-. with the Co-proteins were determined to be (1.5-1.6) X 10(9) dm3.s-1.mol-1; however, in the presence of phosphate, partial inhibition is apparent [k approximately (1.9-2.3) X 10(8) dm3.s-1.mol-1]. To account for the experimental observations, two reaction schemes are presented, involving initially either complex-formation or redox reactions between O2-. and Co(II). This is the first demonstration that substitution of a metal into the vacant copper site of (Cu,Zn)-protein results in proteins that retain superoxide dismutase activity.

Hydrogen-Ion Concentration↗
PubMed1996

Oxidation reactions of a bovine serum albumin-bilirubin complex. A pulse radiolysis study.

Using the technique of pulse radiolysis, oxidation studies of the bovine serum albumin-bilirubin (BSA-BR) system with radicals like CCl3OO., N3., (SCN)2.-, Br2.- and OH. generated in neutral and alkaline medium are reported. In a neutral solution, BSA protects the bound BR very efficiently from the attack of these radicals. The experimental k/k' values for the reaction of CCl3OO., N3. and Br2.- radicals are 2.46, 1.78 and 2.55 respectively, where k and k' are the bimolecular rate constants for the formation of the semi-oxidized BSA and BR radicals respectively. The calculated ratios from our measurements of rate constants k and k' are 0.16, 0.28 and 1.38 for CCl3OO., N3. and Br2.- respectively. These values indicate protection of BR by BSA from free radical attack. For Br2.- radical-induced oxidation of the BSA-BR system, a radical transfer from protein to BR was observed. OH. shows very fast adduct formation with both BSA and BR. The bimolecular rate constant for the formation of BR-OH adducts at PH 8+/- 0.2 is 9.5 x 10(9) dm3 mol-1 s-1 (540 nm). OH. adds to BSA at neutral pH with a rate constant of 3.0 +/- 1.0 x 10(10) dm3 mol-1 s-1 (305 nm). In the BSA-BR complex, BSA fully protects BR from OH. attack and the (BSA-BR)-OH adduct further reacts with free BR molecule if present in solution.

Animals↗
PubMed1999

Characterization of the transient species generated by the photoexcitation of C-phycocyanin from Spirulina platensis: a laser photolysis and pulse radiolysis study.

Nanosecond laser flash photolysis and pulse radiolysis were used to generate and characterize the triplet state and cation radical of C-phycocyanin (C-PC) from Spirulina platensis. The transient absorption spectra of C-PC were measured from direct excitation and acetone sensitization in aqueous solution at room temperature by KrF (248 nm) laser flash photolysis. Laser-induced transient species have been characterized by the method of acetone sensitization and one-electron oxidation. In nitrous oxide-saturated phosphate buffer saline (pH = 7.0) of C-PC, the produced intermediates are assigned to the excited triplet state and the radical cation. Using acetone as photosensitizer, the C-PC excited triplet states produced via triplet-triplet energy transfer and the C-PC radical cation from electron transfer reaction were further confirmed. Furthermore, the corresponding kinetic parameters were determined. To our knowledge, the transient absorption spectra of C-PC have been reported for the first time.

Bacterial Proteins↗
PubMed1988

Properties of the radicals formed by one-electron oxidation of acetaminophen--a pulse radiolysis study.

The semi-iminoquinone radical of acetaminophen, which has previously been proposed as a possible hepatotoxic intermediate in the cytochrome P-450 catalysed oxidation of acetaminophen, has been generated and studied by pulse radiolysis. In the absence of other reactive solutes, the radical decays rapidly by second order kinetics with a rate constant (2k2) of (2.2 +/- 0.4) x 10(9) M-1 sec-1. In alkaline solutions the radical deprotonates with a pK of 11.1 +/- 0.1 to form a radical-anion, as confirmed by the effect of ionic strength on the rate of radical decay. The acetaminophen radical-anion reacts with resorcinol at high pH values, leading to the formation of a transient equilibrium from which the one-electron reduction potential of the semi-iminoquinone radical of acetaminophen is estimated to be +0.707 +/- 0.01 V at pH 7. This value predicts that acetaminophen should be oxidised by thiyl radicals. This was confirmed by pulse radiolysis experiments for reaction of the cysteinyl radical, for which rate constants of 7 x 10(6) M-1 sec-1 at pH 7 and 2.7 x 10(8) M-1 sec-1 at pH 11.3 were obtained. The reaction of O2 with the acetaminophen semi-iminoquinone radical could not be detected by pulse radiolysis, and alternative mechanisms for superoxide radical formation are discussed.

Acetaminophen↗
PubMed1997

Hydrated Electrons in a Quaternary Microemulsion System: A Pulse Radiolysis Study

Using the technique of pulse radiolysis, hydrated electrons have been produced and studied in quaternary microemulsion (sodium lauryl sulfate (NaLS)/water/cyclohexane/1-pentanol) system. Remarkably high lifetimes ( approximately 20 &mgr;s) for hydrated electrons have been obtained. In general, these are two orders of magnitude higher than those reported earlier in reverse micelles. The water droplet sizes and location of the probes have been determined from the decay kinetics of hydrated electrons. The yields and half-lives (t 1/2 ) of the hydrated electrons vary smoothly as the water droplet sizes are changed. We believe that these studies will be useful not only in interpreting electron transfer reactions occurring in small water pockets in biological systems but also in the production of metal nanoclusters by radiation chemical method.

Journal Article↗