Cyclic Voltammetric and E.P.R. Spectroscopic Studies on Apical Substituent Effects in Radical Anions of 9-Substituted and 9,10-Disubstituted Nitroethanoanthracenes

1996 ◽  
Vol 49 (12) ◽  
pp. 1279 ◽  
Author(s):  
PA Lay ◽  
RK Norris ◽  
PK Witting
Keyword(s):  
Electron Paramagnetic Resonance Spectroscopy ◽  
Substituent Effects ◽  
Spectroscopic Studies ◽  
Resonance Spectroscopy ◽  
Radical Anions ◽  
Redox Potentials ◽  
Clear Trend ◽  
Paramagnetic Resonance ◽  
Molecular Electron ◽  
Proton Transfer Reactions

Cyclic voltammetry and electron paramagnetic resonance spectroscopy were used to examine apical substituent effects on the properties of Me2SO solutions of the radical anions from 9-substituted and 9,10-disubstituted 2- and 3-nitro-9,10-dihydro-9,10-ethanoanthracenes (1)-(24). The reductions of the nitro group are, in general, reversible at 100 mV s-1 and at 20°, except where there are coupled intra-or inter-molecular electron or proton transfer reactions with aliphatic bridgehead substituents, such as a carboxylic acid or iodine. The substituent effects for the meta- and para-nitroethanoanthracene systems are both similar and additive. This similarity in the meta and para polar substituent effects is attributed to the orthogonality of the π*(ArNO2/ArNO2-) orbital with the σ* orbital of the carbon substituent (C-X) groups at the bridgehead positions. Overall, the meta substituent effect was slightly lower than that for corresponding simple meta-nitrobenzyl systems, but the para-nitro systems showed substituent effects that are a factor of 2 smaller than that for corresponding simple para-nitrobenzyl systems. These linear correlations between the substituent effects and redox potentials have been used to estimate the redox potentials of irreversible systems, which are required for digital simulations of reactions involving nitroaryl radical anions. Only small substituent effects are present in hyperfine coupling with the aliphatic and aromatic protons of the nitroaromatic radical anions of ethanoanthracenes (1)-(13), but a clear trend to lower nitrogen hyperfine values was observed with increasing electron-withdrawing ability of the apical substituent. In addition, no spin density was transferred to the benzylic bridgehead substituent in any of the nitro radical anions studied, clearly demonstrating that the bond between the bridgehead substituent and the carbon at a benzylic position is orthogonal to the π-system of the nitroaromatic ring bearing the odd electron.

scholarly journals Oxidation--reduction potentials of turkey liver xanthine dehydrogenase and the origins of oxidase and dehydrogenase behaviour in molybdenum-containing hydroxylases

1977 ◽  
Vol 163 (2) ◽  
pp. 279-289 ◽  
Author(s):  
M J Barber ◽  
R C Bray ◽  
R Cammack ◽  
M P Coughlan
Keyword(s):  
Uric Acid ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Xanthine Dehydrogenase ◽  
Resonance Spectroscopy ◽  
Redox Potentials ◽  
Paramagnetic Resonance ◽  
Reduction Potentials ◽  
Oxidation Reduction ◽  
Temperature Electron ◽  
Full Reduction

Redox potentials for the various centres in the enzyme xanthine dehydrogenase (EC 1.2.1.37) from turkey liver determined by potentiometric titration in the presence of mediator dyes, with low-temperature electron-paramagnetic-resonance spectroscopy. Values at 25 degrees C in pyrophosphate buffer, pH 8.2, are: Mo(VI)/Mo(V)(Rapid),-350 +/- 20mV; Mo(V) (Rapid)/Mo(IV), -362 +/- 20mV; Fe-S Iox./Fe-S Ired., -295 +/- 15mV; Fe-S IIox./Fe-S IIred., -292 +/- 15mV; FAD/FADH,-359+-20mV; FADH/FADH2, -366 +/- 20mV. This value of the FADH/FADH2 potential, which is 130mV lower than the corresponding one for milk xanthine oxidase [Cammack, Barber & Bray (1976) Biochem. J. 157, 469-478], accounts for many of the differences between the two enzymes. When allowance is made for some interference by desulpho enzyme, then differences in the enzymes' behaviour in titration with xanthine [Barber, Bray, Lowe & Coughlan (1976) Biochem. J. 153, 297-307] are accounted for by the potentials. Increases in the molybdenum potentials of the enzymes caused by the binding of uric acid are discussed. Though the potential of uric acid/xanthine (-440mV) is favourable for full reduction of the dehydrogenase, nevertheless, during turnover, for kinetic reasons, only FADH and very little FADH2 is produced from it. Since only FADH2 is expected to react with O2, lack of oxidase activity by the dehydrogenase is explained. Reactivity of the two enzymes with NAD+ as electron acceptor is discussed in relation to the potentials.

scholarly journals Midpoint redox potentials of plant and algal ferredoxins

1977 ◽  
Vol 168 (2) ◽  
pp. 205-209 ◽  
Author(s):  
R Cammack ◽  
K K Rao ◽  
C P Bargeron ◽  
K G Hutson ◽  
P W Andrew ◽  
...  
Keyword(s):  
Green Algae ◽  
Higher Plants ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Photosynthetic Electron Transport ◽  
Resonance Spectroscopy ◽  
Redox Potentials ◽  
Higher Plant ◽  
Paramagnetic Resonance ◽  
Blue Green Algae ◽  
Nostoc Strain

Midpoint potentials of plant-type ferredoxins from a range of sources were measured by redox titrations combined with electron-paramagnetic-resonance spectroscopy. For ferredoxins from higher plants, green algae and most red algae, the midpoint potentials (at pH 8.0) were between —390 and —425 mV. Values for the major ferredoxin fractions from blue-green algae were less negative (between —325 and —390 mV). In addition, Spirulina maxima and Nostoc strain MAC contain second minor ferredoxin components with a different potential, —305 mV (the highest so far measured for a plant-algal ferrodoxin) for Spirulina ferrodoxin II, and —455 mV (the lowest so far measured for a plant-algal ferredoxin) for Nostoc strain MAC ferredoxin II. However, two ferredoxins extracted from a variety of the higher plant Pisum sativum (pea) had midpoint potentials that were only slightly different from each other. These values are discussed in terms of possible roles for the ferredoxins in addition to their involvement in photosynthetic electron transport.

scholarly journals Engineered holocytochrome c synthases that biosynthesize new cytochromes c

2017 ◽  
Vol 114 (9) ◽  
pp. 2235-2240 ◽  
Author(s):  
Deanna L. Mendez ◽  
Shalon E. Babbitt ◽  
Jeremy D. King ◽  
John D’Alessandro ◽  
Michael B. Watson ◽  
...  
Keyword(s):  
Circular Dichroism ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Axial Ligand ◽  
Heme Iron ◽  
Resonance Spectroscopy ◽  
Redox Potentials ◽  
Hydrogen Electrode ◽  
Paramagnetic Resonance ◽  
Axial Ligands ◽  
Circular Dichroïsm

Cytochrome c (cyt c), required for electron transport in mitochondria, possesses a covalently attached heme cofactor. Attachment is catalyzed by holocytochrome c synthase (HCCS), leading to two thioether bonds between heme and a conserved CXXCH motif of cyt c. In cyt c, histidine (His19) of CXXCH acts as an axial ligand to heme iron and upon release of holocytochrome c from HCCS, folding leads to formation of a second axial interaction with methionine (Met81). We previously discovered mutations in human HCCS that facilitate increased biosynthesis of cyt c in recombinant Escherichia coli. Focusing on HCCS E159A, novel cyt c variants in quantities that are sufficient for biophysical analysis are biosynthesized. Cyt c H19M, the first bis-Met liganded cyt c, is compared with other axial ligand variants (M81A, M81H) and single thioether cyt c variants. For variants with axial ligand substitutions, electronic absorption, near-UV circular dichroism, and electron paramagnetic resonance spectroscopy provide evidence that axial ligands are changed and the heme environment is altered. Circular dichroism spectra in far UV and thermal denaturation analyses demonstrate that axial ligand changes do not affect secondary structures and stability. Redox potentials span a 400-mV range (+349 mV vs. standard hydrogen electrode, H19M; +252 mV, WT; −19 mV, M81A; −69 mV, M81H). We discuss the results in the context of a four-step mechanism for HCCS, whereby HCCS mutants such as E159A are enhanced in release (step 4) of cyt c from the HCCS active site; thus, we term these “release mutants.”

Spin-coupling in dimers of 2,3-dicyano-5,6-dichlorosemiquinone radical anions in the crystalline state

2013 ◽  
Vol 70 (1) ◽  
pp. 181-190 ◽  
Author(s):  
Krešimir Molčanov ◽  
Darko Babić ◽  
Biserka Kojić-Prodić ◽  
Jernej Stare ◽  
Nadica Maltar-Strmečki ◽  
...  
Keyword(s):  
Crystal Engineering ◽  
Radical Anion ◽  
Crystalline State ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Separation Distance ◽  
Spin Coupling ◽  
Resonance Spectroscopy ◽  
Radical Anions ◽  
Paramagnetic Resonance ◽  
Periodic Models

A crystal engineering approach is used to stabilize a radical anion in the crystalline state and to modulate the separation distance within π-stacks of anion radicals. Alkali metal salts of 2,3-dicyano-5,6-dichlorosemiquinone (C8Cl2N2O2, DDQ^{\bullet -}) radical anions were prepared and their crystal structures determined: LiDDQ·2H2O·(CH3)2CO, RbDDQ·2H2O and CsDDQ·2H2O. In these structures, stacked dimers of radical anions are formed within π-stacked columns. Within the stacked dimers, interplanar separation distances are significantly shorter than the sum of the van der Waals radii for two C atoms; the shortest is 2.812 Å for the Li salt and the longest is 2.925 Å for the Cs salt. Diamagnetic character, observed by electron paramagnetic resonance spectroscopy, indicates spin-coupling of the unpaired electrons within the radical anion dimer. The electron-rich cyano substituents on DDQ^{\bullet -} influence the electron redistribution within the ring skeleton. The crystalline compounds are also characterized by IR spectroscopy, complemented by quantum-chemical calculations based on both isolated and periodic models.

scholarly journals Tauroursodeoxycholic Acid (TUDCA)—Lipid Interactions and Antioxidant Properties of TUDCA Studied in Model of Photoreceptor Membranes

Membranes ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 327
Author(s):  
Michał J. Sabat ◽  
Anna M. Wiśniewska-Becker ◽  
Michał Markiewicz ◽  
Katarzyna M. Marzec ◽  
Jakub Dybas ◽  
...  
Keyword(s):  
Antioxidant Activity ◽  
Plasma Membranes ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Protective Action ◽  
Antioxidant Properties ◽  
Model Membranes ◽  
Dynamics Simulation ◽  
Resonance Spectroscopy ◽  
Paramagnetic Resonance ◽  
Tauroursodeoxycholic Acid

Tauroursodeoxycholic acid (TUDCA), a hydrophilic bile acid containing taurine conjugated with the ursodeoxycholic acid (UDCA), has been known and used from ancient times as a therapeutic compound in traditional Chinese medicine. TUDCA has recently been gaining significant interest as a neuroprotective agent, also exploited in the visual disorders. Among several mechanisms of TUDCA’s protective action, its antioxidant activity and stabilizing effect on mitochondrial and plasma membranes are considered. In this work we investigated antioxidant activity of TUDCA and its impact on structural properties of model membranes of different composition using electron paramagnetic resonance spectroscopy and the spin labeling technique. Localization of TUDCA molecules in a pure POPC bilayer has been studied using a molecular dynamics simulation (MD). The obtained results indicate that TUDCA is not an efficient singlet oxygen (1O2 (1Δg)) quencher, and the determined rate constant of its interaction with 1O2 (1Δg) is only 1.9 × 105 M−1s−1. However, in lipid oxidation process induced by a Fenton reaction, TUDCA reveals substantial antioxidant activity significantly decreasing the rate of oxygen consumption in the system studied. In addition, TUDCA induces slight, but noticeable changes in the polarity and fluidity of the investigated model membranes. The results of performed MD simulation correspond very well with the experimental results.

scholarly journals Ambient base-free glycerol oxidation over bimetallic PdFe/SiO2 by in situ generated active oxygen species

2021 ◽  
Author(s):  
Ricci Underhill ◽  
Mark Douthwaite ◽  
Richard J. Lewis ◽  
Peter J. Miedziak ◽  
Robert D. Armstrong ◽  
...  
Keyword(s):  
Heterogeneous Catalysts ◽  
Bimetallic Catalyst ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Active Oxygen Species ◽  
Catalytic Performance ◽  
Resonance Spectroscopy ◽  
Oxygen Species ◽  
Paramagnetic Resonance ◽  
Temperature Oxidation

AbstractLow temperature oxidation of alcohols over heterogeneous catalysts is exceptionally challenging, particularly under neutral conditions. Herein, we report on an efficient, base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO2 catalyst at ambient temperature in the presence of gaseous H2 and O2. The exceptional catalytic performance was attributed to the in situ formation of highly reactive surface-bound oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe bimetallic catalyst was determined to be significantly more active than corresponding monometallic analogues, highlighting the important role both metals have in this oxidative transformation. Fe leaching was confirmed to occur over the course of the reaction but sequestering experiments, involving the addition of bare carbon to the reactions, confirmed that the reaction was predominantly heterogeneous in nature. Investigations with electron paramagnetic resonance spectroscopy suggested that the reactivity in the early stages was mediated by surface-bound reactive oxygen species; no homogeneous radical species were observed in solution. This theory was further evidenced by a direct H2O2 synthesis study, which confirmed that the presence of Fe in the bimetallic catalyst neither improved the synthesis of H2O2 nor promoted its decomposition over the PdFe/SiO2 catalyst.

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