Fatai A. Taiwo

Mechanism of tiron as scavenger of superoxide ions and free electrons

Sodium 4,5-dihydroxybenzene-1,3-disulfonate (tiron) has been reported to be an efficient chelator of certain metal ions, and a substrate in several enzyme reactions. Its small size facilitates cell entry and therefore modulates intracellular elec- tron transfer reactions as an antioxidant by scavenging free radicals. Its reduction by electrochemical and enzymatic methods gives identical products; a semiquinone detectable by EPR spectroscopy. In a test of its use as a spin trap, in comparison with DMPO, tiron does not form a molecular spin-adduct but proves more functional as an electron trap. Electron addition to tiron is more facile than reduction of dioxygen as observed by the non-formation of DMPO-OOH spin-adduct in the system XO/HPX/O2/DMPO/tiron. Rather, it is the tiron semiquinone radical which is formed quantitatively with increasing concen- tration of hypoxanthine independent of oxygen concentration. These results offer explanation for the action of tiron and its suitability for measuring electron release in hypoxic conditions, and also for mitigating redox-induced toxicity in drug regimes by acting as an electron scavenger.

Application of radiation and electron spin resonance spectroscopy to the study of ferryl myoglobin

Addition of an excess of hydrogen peroxide to aqueous metmyoglobin (Fe3+—OH2) followed by rapid freezing gave a species having no detectable e.s.r. features assignable to iron. However, on exposure to 60Co γ-rays at 77 K, e.s.r. features characteristic of a low-spin FeIII centre grew-in. On annealing above 77 K a modified centre with a larger spread of g values was identified as hydroxy-myoglobin. On further annealing this finally changed to normal high-spin Hb+ characterised by an intense feature at g= 6. These results strongly support the ‘ferryl’ structure, (FeO2+) previously assigned to the major product formed from hydrogen peroxide.We were unable to detect e.s.r. features assignable to the expected (Fe3+—OOtext-decoration:overlineH) intermediate even at relatively low H2O2 concentrations. This species has been previously identified by e.s.r. spectroscopy as an intermediate in the formation of (Fe3+H2O) following electron addition to oxymyoglobin (Fe2+O2). This result shows that, although the hydroperoxide derivative, (Fe3+—OOtext-decoration:overlineH), is probably an essential intermediate in the route to the ferryl derivative, it is never present in concentrations high enough to be detected by e.s.r. spectroscopy.

Electron paramagnetic resonance spectroscopic studies of iron and copper proteins

Transition metal (d-group) ions are widespread in nature, essential for structural characteristics and mechanistic specificity of many proteins. Iron and copper are the two most prevalent metals in proteins responsible for the storage and transport of molecules, ions, and electrons. Electron paramagnetic resonance (EPR) spectroscopy has been extensively used for the determination of these metal ions without extensive disruption of the native protein moiety. It also detects variations in coordination geometry due to ligand substitutions as well as multiple valencies of the same metal. This review highlights the unique application of EPR spectroscopy to the study of iron and copper in biological systems. Mention is made of a select number of other metalloproteins.

Diaquabis(3,6-dioxaheptanoato)copper(II): crystal structure and EPR characteristics

Abstract The structure of the title compound has been determined by X-ray diffraction at 190 K. The complex has an all trans configuration with an elongated tetragonally distorted octahedral CuO 6 chromophore. The elongated axis corresponding to the trans -Cu–O(ether) bonds. The ligand molecules are bidentate via the carboxyl and the 3-ether O atoms; the 6-ether O atoms are not coordinated and are remote from the Cu centres. The bond lengths to the Cu centres are Cu–O(ether) 2.355 A, Cu–O(Carboxyl) 1.933 A and Cu–OH 2 1.995 A. The EPR spectrum of both the powder and frozen solution forms is typical of a rhombic system with a d x 2 − y 2 1 electronic configuration. There were no significant differences in spectra recorded over the temperature range 77 K to room temperature. These results are discussed in relation to earlier published results on closely related oxa-carboxyl complexes.

FREE RADICALS IN IRRADIATED DRUGS : AN EPR STUDY

Exposure of dry powder forms of the drugs nitrendipine, nifedipine, felodipine, and nimodipine to gamma-radiation results in the formation of free radicals detected by electron paramagnetic resonance (EPR) spectroscopy. The four structurally related drugs show qualitatively identical EPR spectral features in terms of g-values, the qualitative descriptive parameter. These radicals are very stable, surviving long periods of time in excess of 9 months and possibly beyond conventional shelf-life of the drugs. The residual radical population is high enough to be detectable after long storage. Administration of such radiation-treated drugs may present patients with quantities of free radicals and possibilities of secondary cell damage.

Polymeric ternary metal thiols I. Products from reaction of Cu(II) with MoS42

Abstract The black, polymeric, product isolated from the reaction of Cu(II) salts with MoS42− has been shown to have some unexpected and unusual properties, i.e. a stoichiometry of Cu1.6MOS4Xy (X = Cl−, Br−, y ≈ 1; X = SO42−, y ≈ 0.5), reduction of the metal centres to Cu(I) and Mo(V), the presence of S radical centres, and the absorption of ca. 2 mol O2. EXAFS spectra have also been recorded. The W anion WS42− behaves identically.

Electron addition to xanthine oxidase. An electron spin resonance study of the effects of ionizing radiation

Exposure of aqueous xanthine oxidase to 60Co γ-rays at 77 K resulted in electron addition to an iron–sulphur centre at low doses. At higher doses, addition to the MoVI unit was also observed, using ESR detection. The results are interpreted in terms of rapid electron transfer from MoV to the nearest Fe/S cluster, with permanent trapping at molybdenum only when a second electron is generated in a given molecule. The ESR spectrum for the primary MoV centre closely resembles that for a centre previously only detected in the presence of a substrate molecule, such as xanthine, and known as the ‘very rapid’ centre. Hence we conclude that there is no major covalent bonding between MoV and the substrate in this complex.On annealing, the first change involved conversion of the primary MoV centre to a secondary MoV centre, exhibiting a 13 G proton hyperfine coupling. The ESR parameters closely resemble those for a centre previously described as the ‘rapid’ centre. This is the first detectable species in rapid-freeze experiments in the absence of specific substrates. When D2O was used, the proton coupling was lost. The ESR parameters are compatible with the postulate that the initial species has a sulphide ligand (Mo—S)– and the second species is protonated on sulphur (Mo—SH).Further annealing results in irreversible loss of the MoV signal and concomitant growth of a second Fe/S cluster signal. These results are discussed in terms of the remarkable selectivity of electrons ejected within the protein by γ-rays, and the very different rates of electron transfers between the different centres.There was also evidence for electron capture at an RS—SR unit giving, initially, an RS—SR– radical anion, followed by an irreversible conversion into a characteristic centre, probably formed from the anion by protonation. This centre is of interest since it is also formed from solvated electrons in pulse-radiolysis studies.

Studies of equid hoof horn material by EPR spectroscopy

EPR spectroscopy has been used to show that stable free radicals are formed when hoof horn material from equids is cut. The form of the EPR signal suggests that these are stable sulfur-centred radicals. The detection of melanin and thus the distinction between pigmentation levels of hoof horn is noted. Possible implications of these results are discussed.

Radiation damage to proteins: an electron paramagnetic resonance study

Exposure of a wide range of proteins to ionizing radiation at low temperatures has led to the following generalizations: (i) Electrons are ejected indiscriminately from any part of the protein. The residual radical cations largely become localized on the backbone amide units, and are trapped thereon by the N–H proton loss. (ii) These centres are characterized by their 14N hyperfine coupling constants and by coupling to the unique C–H proton. This varies in a manner that is characteristic of α-helical regions and β-sheet regions of the protein. (iii) The ejected electrons, in contrast, are very mobile through the protein until they reach good electron traps, such as transition metal ions or disulfide linkages.

Effects of AQ4N and its reduction product on radiation-mediated DNA strand breakage

Supercoiled plasmid pBR322 DNA was irradiated in phosphate buffer by 60Co gamma-rays at a dose rate 19.26 Gy/min and total dose of 10 Gy in the presence of a bioreductive antitumour prodrug namely 1,4-bis [¿2-(dimethylamino-N-oxide)ethyl¿ amino] 5, 8-dihydroxyanthracene-9,10-dione (AQ4N) and its DNA affinic reduction product 1,4-bis[¿2(dimethylamino)ethyl¿ amino] 5,8-dihydroxyanthracene-9,10-dione (AQ4) under air and nitrogen. AQ4N and AQ4 were found to protect against radiation-induced plasmid single and double strand breakage as assessed by agarose gel electrophoresis. The differences between the two agents, and between atmospheres of air or nitrogen were negligible. It was also found that the protection efficiencies of the compounds were pH dependent and showed maximum protection at pH 6. These results indicate that protection of DNA by AQ4 and AQ4N against radiation damage is an indirect effect since both agents are equally effective despite major differences in their DNA affinity. It is likely that radiation-induced phosphate buffer radicals are intercepted by AQ4 and AQ4N in a pH-dependent process.