Free radicals generated by radiolysis of aqueous solutions

Schwarz, Harold A.

doi:10.1021/ed058p101

Cited by 233 publications

(162 citation statements)

References 22 publications

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“…Solutions contained enzyme, 30 mM formate, and 0.5 M ethanol (as hydroxyl radical scavengers), 50 M EDTA, and 2 mM Taps at pH 8.2. Superoxide radicals formed in an aqueous air saturated solution via the mechanisms described by Schwarz (27). Decay of superoxide was observed spectrophotometrically at 260 nm.…”

Section: Methodsmentioning

confidence: 99%

Activation of the Proton Transfer Pathway in Catalysis by Iron Superoxide Dismutase

Greenleaf

Silverman

2002

Journal of Biological Chemistry

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Catalysis by Escherichia coli and Porphyromonas gingivalis iron superoxide dismutase was activated by addition of primary amines, as measured by pulse radiolysis and stopped-flow spectrophotometry. This activation was saturable for most amines investigated, and a free energy plot of the apparent second-order rate constant of activation was linear as a function of the pK a of the amine, indicating activation by proton transfer. Amines provide an alternate rather than the only pathway for proton transfer, and catalysis was appreciable in the absence of amines. Solvent hydrogen isotope effects were near unity for amine activation, which is consistent with rate-contributing proton transfer if the pK a of the proton acceptor on the enzyme is not in the region of the pK a values of the amines studied, from 7.8 to 10.6. The activation of catalysis by these amines was uncompetitive with respect to superoxide, interpreted as proton transfer in a ternary complex of amine with the enzyme-bound peroxide dianion.The superoxide dismutases (SODs) 1 are cyclic redox enzymes that provide protection against oxidative damage caused by the reactions of the superoxide radical anion O 2 . . SODs have been characterized with manganese, iron, copper/zinc, or nickel as the redox active metal (1-3). The catalysis of the dismutation of superoxide occurs in two stages in which the metal ion, designated M in Reactions 1 and 2, alternates between the M 3ϩ and M 2ϩ forms (2-4),Iron superoxide dismutase (Fe-SOD), found in prokaryotes, and Mn-SOD, found in mitochondria as well as in prokaryotes, are highly similar in sequence (5) and in crystal structure (6, 7). Despite this high degree of structural homology, substitution of one metal for the other generally results in an inactive enzyme (8,9). A subclass of SOD exists that is able to utilize either manganese or iron as catalytic metal ion; these are termed cambialistic SODs and are of significant interest from the perspective of metal specificity as well as catalytic mechanism. A well studied cambialistic SOD is that from Porphyromonas gingivalis, the crystal structure of which is reported for the manganese-and iron-containing forms (10). Although they all share common features of their catalytic pathways, the copper/ zinc-and nickel-containing SODs have structures unrelated to the manganese-and iron-containing SODs (2, 11).The crystal structures of the iron-and manganese-containing superoxide dismutases generally show a trigonal bipyramidal geometry about the oxidized metal with three histidines, one aspartic acid, and one solvent molecule as ligands (6, 7). Major features of the active site appear very similar in each enzyme, including the highly conserved hydrogen-bonded network connecting the metal-coordinated solvent molecule and residues Gln-69 2 (in Fe-SOD) or Gln-143 (in Mn-SOD), Tyr-34, His-30, and Tyr-166 from the adjacent subunit. Although this active site Gln emanates from a different backbone position in Fe-SOD and Mn-SOD, the side chain orientation is very similar in both. Thi...

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Section: Methodsmentioning

confidence: 99%

Activation of the Proton Transfer Pathway in Catalysis by Iron Superoxide Dismutase

Greenleaf

Silverman

2002

Journal of Biological Chemistry

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“…While not as aggressive as OH, hydrogen radical is very reactive having a standard reduction potential of 2.3 V vs. SHE (46). Unlike OH, the hydrogen atom could be very aggressive toward the C-F bonds of PFSAs.…”

Section: The Hydrogen Radical Mechanismmentioning

confidence: 99%

The Chemistry of Fuel Cell Membrane Chemical Degradation

2008

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A detailed thermochemical and kinetic analysis of PFSA ionomers and the reactive oxygen species formed in an operating fuel cell is reported. The analysis reveals that hydroxyl radical is the only oxygen species capable of abstracting a hydrogen atom from carboxylic acid intermediates, thereby propagating the PFSA main chain unzipping process. Two chemically specific, main chain scission mechanisms are proposed. The first involves formation of sulfonyl radicals under dry conditions and the second invokes the action of hydrogen atoms formed from hydrogen gas and hydroxyl radical. The thermochemical analysis provides a strong basis from which to rationalize the results from a broad range of in-situ and ex-situ fuel cell degradation studies.

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“…Dosimetry was measured using the (SCN) 2 Ϫ dosimeter (16). The radiolysis of water, either by electrons or ␥-rays, yields the species described in Reaction 1, where the numbers in parentheses are G values, that is the number of molecules/100 eV of energy absorbed by the medium (17).…”

Section: Neelaredoxin Reactivity Toward Superoxidementioning

confidence: 99%

The Mechanism of Superoxide Scavenging byArchaeoglobus fulgidus Neelaredoxin

Abreu

Saraiva

Soares

et al. 2001

Journal of Biological Chemistry

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Neelaredoxin is a mononuclear iron protein widespread among prokaryotic anaerobes and facultative aerobes, including human pathogens. It has superoxide scavenging activity, but the exact mechanism by which this process occurs has been controversial. In this report, we present the study of the reaction of superoxide with the reduced form of neelaredoxin from the hyperthermophilic archaeon Archaeoglobus fulgidus by pulse radiolysis. This protein reduces superoxide very efficiently (k ‫؍‬ 1.5 ؋ 10 9 M ؊1 s ؊1 ), and the dismutation activity is rate-limited, in steady-state conditions, by the much slower superoxide oxidation step. These data show unambiguously that the superfamily of neelaredoxin-like proteins (including desulfoferrodoxin) presents a novel type of reactivity toward superoxide, a result of particular relevance for the understanding of both oxygen stress response mechanisms and, in particular, how pathogens may respond to the oxidative burst produced by the defense cells in eukaryotes. The actual in vivo functioning of these enzymes will depend strongly on the cell redox status. Further insight on the catalytic mechanism was obtained by the detection of a transient intermediate ferric species upon oxidation of neelaredoxin by superoxide, detectable by visible spectroscopy with an absorption maximum at 610 nm, blueshifted ϳ50 nm from the absorption of the resting ferric state. The role of the iron sixth ligand, glutamate-12, in the reactivity of neelaredoxin toward superoxide was assessed by studying two site-directed mutants: E12Q and E12V.

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Free radicals generated by radiolysis of aqueous solutions

Abstract: This article will discuss the identity and nature of free radicals produced in radiolysis.

Cited by 233 publications

References 22 publications

Activation of the Proton Transfer Pathway in Catalysis by Iron Superoxide Dismutase

Activation of the Proton Transfer Pathway in Catalysis by Iron Superoxide Dismutase

The Chemistry of Fuel Cell Membrane Chemical Degradation

The Mechanism of Superoxide Scavenging byArchaeoglobus fulgidus Neelaredoxin

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