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S S Eaton

Publications and source records attributed to S S Eaton.

31 records · Page 2Linked to original sources
PubMed1995

Electron-electron spin-spin interaction in spin-labeled low-spin methemoglobin.

Nitroxyl free radical electron spin relaxation times for spin-labeled low-spin methemoglobins were measured between 6 and 120 K by two-pulse electron spin echo spectroscopy and by saturation recovery electron paramagnetic resonance (EPR). Spin-lattice relaxation times for cyano-methemoglobin and imidazole-methemoglobin were measured between 8 and 25 K by saturation recovery and between 4.2 and 20 K by electron spin echo. At low temperature the iron electron spin relaxation rates are slow relative to the iron-nitroxyl electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl electron spin echo decay rate. Throughout the temperature range examined, interaction with the Fe(III) increases the spin lattice relaxation rate (1/T1) for the nitroxyl. The measured relaxation times for the Fe(III) were used to analyze the temperature-dependent changes in the spin echo decays and in the saturation recovery (T1) data for the interacting nitroxyl and to determine the interspin distance, r. The values of r for three spin-labeled methemoglobins were between 15 and 15.5 A, with good agreement between values obtained by electron spin echo and saturation recovery. Analysis of the nitroxyl spin echo and saturation recovery data also provides values of the iron relaxation rates at temperatures where the iron relaxation rates are too fast to measure directly by saturation recovery or electron spin echo spectroscopy. These results demonstrate the power of using time-domain EPR measurements to probe the distance between a slowly relaxing spin and a relatively rapidly relaxing metal in a protein.

Cyclic N-Oxides↗
PubMed1994

Spin trapping of azidyl and hydroxyl radicals in azide-inhibited rat brain submitochondrial particles.

Succinate-driven respiration in azide-inhibited rat brain submitochondrial particles (smps) produces azidyl and hydroxyl radicals that were detected by spin trapping with 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO). Production of radicals required succinate and oxygen and was eliminated by heat denaturation, which indicates that radical production is a result of respiration. The concentrations of both DMPO/.OH and DMPO/.N3 were decreased by addition of catalase to the smps, which indicates that H2O2 is involved in radical production. In the absence of azide anion, DMPO/.OH was not detected in the same system, even after five additions of succinate over a period of 24 h. It is proposed that azide inhibition of cytochrome c oxidase results in increased production of superoxide, which is efficiently converted to hydrogen peroxide by membrane-bound superoxide dismutase. Hydrogen peroxide activates endogenous peroxidase to react with azide anion forming azidyl radical, which damages the peroxidase, resulting in decreased production of azidyl radical with successive additions of succinate. Hydroxyl radical is produced from the hydrogen peroxide that is not removed by peroxidase. The increased production of superoxide in the azide-inhibited system suggests that loss of cytochrome c oxidase activity can lead to increased radical production if other proteins in the respiratory chain remain active. In the azide-inhibited system, reaction of azide anion with H2O2-activated endogenous peroxidase and spin-trapping of the resulting azidyl radical is a convenient monitor of H2O2 production.

Animals↗
PubMed1994

A neutral water-soluble broadening agent for spin-label studies.

Cr(maltolate)3 is proposed as a neutral water-soluble reagent for the broadening of accessible nitroxyl spin labels or spin probes in biological experiments. For situations in which the molecular charge is important, it supplements Cr(oxalate)3(3-), which is somewhat more effective on a molar basis. The interaction of the two reagents with spin-labeled creatine kinase is an example of a case in which the charge of the broadening agent is important.

Animals↗
PubMed1991

Effect of the synergistic anion on electron paramagnetic resonance spectra of iron-transferrin anion complexes is consistent with bidentate binding of the anion.

Continuous wave (cw) X-band EPR spectra at approximately 90 K were obtained for iron-transferrin-anion complexes with 18 anions. Each anion had a carboxylate group and at least one other polar moiety. As the second polar group was varied from hydroxyl to carbonyl to amine to carboxylate, the EPR spectra changed from a dominant signal at g' approximately 4.3 with a second smaller peak at g' approximately 9 to a broad signal with intensity between g' approximately 5 and 7. Computer simulation indicated that the changes in the EPR spectra were due to changes in the zero field splitting parameter ratio, E/D, from approximately 1/3 for carbonate anion to approximately 0.04 for malonate anion. Observation of iron-13C coupling in the electron spin echo envelope modulation (ESEEM) for iron transferrin [1-13C]pyruvate indicated that the carboxylate group was bound to the iron. It is proposed that all of the anions behave as bidentate ligands, with coordination to the iron through both the carboxylate and proximal groups, and the carboxyl group serves as a bridge between the iron and a positively charged group on the protein.

Anions↗
PubMed1990

Electron spin echo envelope modulation evidence for carbonate binding to iron(III) and copper(II) transferrin and lactoferrin.

Iron binding to transferrin and lactoferrin requires a synergistic anion, which is carbonate in vivo. The anion is thought to play a key role in iron binding and release. To understand better the iron-carbonate interaction, experiments were performed with iron(III) and copper(II) complexes of human milk lactoferrin and serum transferrin with carbon-13-labeled carbonate. Modulation frequencies were present in the Fourier transforms of two-pulse and three-pulse electron spin echo envelope modulation data for the Fe(III) and Cu(II) complexes, consistent with binding of carbonate to both metals. The metal-13C interaction was similar for the lactoferrin and transferrin complexes. Spin coupling to the nitrogen of a coordinated histidine imidazole was observed for both metals. Both the metal-nitrogen and the metal-carbon spin coupling constants were about a factor of 5 smaller for the iron complexes than for the copper complexes, which indicated substantial similarity in the metal-carbonate and metal-imidazole binding for the two metals.

Carbonates↗
PubMed1989

Comparison of the electron spin echo envelope modulation (ESEEM) for human lactoferrin and transferrin complexes of copper(II) and vanadyl ion.

Copper(II) and vanadyl ions were bound to human milk lactoferrin or serum transferrin with carbonate or oxalate as the synergistic anion. Electron spin echo envelope modulation (ESEEM) due to nitrogen of a coordinated histidine imidazole was observed for both the copper and vanadyl complexes. For both metals, the modulation frequencies in the Fourier transforms of the data were similar for the two proteins and were weakly dependent on anion. When data in D2O/glycerol-d3 were compared with data in H2O/glycerol, the deep deuterium modulation indicated multiple exchangeable protons in the vicinity of the metals with at most one proton within about 2.9 A of the metal. The distribution of exchangeable protons around the metals as probed by ESEEM was the same, within experimental uncertainty, for the copper or vanadyl complexes with either carbonate or oxalate as the anion. When 13C-labeled oxalate was used as the synergistic anion, 13C-ESEEM was observed for both the copper and vanadyl complexes of lactoferrin and transferrin. The deeper 13C modulation for copper and vanadyl transferrin [13C]oxalate than for vanadyl transferrin [13C]carbonate suggests that both ends of the oxalate are bound to the metal in the transferrin and lactoferrin complexes.

Copper↗
PubMed1983

A spin-label study of the disposition of the Fe-S cluster with respect to the active center of aconitase.

It has been reported by Johnson et al. ((1977) Biochem. Biophys. Res. Commun. 74, 384-389) that phenacyl bromide reacts with a single reactive sulfhydryl group of aconitase, abolishing enzyme activity. Substrate or analogs have a protective effect. This group is therefore at the catalytic site of the enzyme. Aconitase is also known to be an Fe-S protein, paramagnetic as obtained on purification (Ruzicka and Beinert (1978) J. Biol. Chem. 253, 2514-2517). We have attempted to obtain information on the location of the Fe-S cluster of aconitase with respect to the catalytically active site by attaching nitroxide-labelled sulfhydryl reagents of the bromoacyl and maleimide type to the sensitive sulfhydryl group. The EPR signals of those spin-labelled sulfhydryl reagents that abolish enzyme activity disappear during reaction with aconitase. EPR spectra at 13 K of the product obtained by reaction of three spin labels (two maleimides and one bromoacyl) with aconitase included a half-field transition at g approximately equal to 4.0 which is characteristic of spin-spin interaction. On the basis of calculations of the dependence of the intensity of the half-field transition on the distance between two interacting unpaired electrons (Eaton and Eaton, (1982) J. Am. Chem. Soc. 104, 5002-5003) the distances between the nitroxide N-O bond and the center of the Fe-S cluster for the three spin labels were calculated to be 10.5, 11 and 13 A. Combined distance and orientation data for the three spin labels indicate that the reactive sulfhydryl group is about 12 A from the center of the Fe-S cluster.

Aconitate Hydratase↗
PubMedDate not supplied

Multifrequency electron paramagnetic resonance of irradiated L-alanine.

The radical generated by gamma-irradiation of crystalline L-alanine was examined by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) at 1.8, 3.2, 4.9, 9.1 and 19.4 GHz. The spin-flip satellite lines that make a prominent contribution to the saturated spectra at 9.1 GHz are less conspicuous at lower frequencies because of overlap with the allowed transitions. The spin-lattice relaxation times measured by long-pulse saturation recovery and phase memory times measured by electron spin echo increase with increasing microwave frequency.

Alanine↗