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PubMed2001

Enzymatic and electrochemical oxidation of N-hydroxy compounds. Redox potential, electron-transfer kinetics, and radical stability.

A series of N-hydroxyacetanilide and 1-hydroxybenzotriazole analogues derivatized by various functional substituents were studied with regard to redox potential, oxidation by laccase, oxidative stability, and correlation to the electronic and steric properties of the substituents. It was found that substituents carrying conjugative/pi-electron function influenced the redox potential more than substituents carrying inductive/sigma-electron function, and that the electron-transfer from an N-hydroxy moiety to laccase was significantly affected by the redox potential. Electron-withdrawing substituents tended to reduce the electron density on the N-hydroxy group, leading to higher redox potential and lower oxidation rate. Bulky substitution or absence of N-phenyl tended to increase the Km of the N-hydroxy group, leading to lower oxidation rate. Oxidized N-hydroxy compounds were stabilized by N-phenyl or N-carbonyl group, but not by N-azo or highly strained structure. Potential implication of these effects on laccase-based, N-hydroxy compound-mediated biocatalysis is discussed.

Catalysis↗

PubMed2005

Electric-field-induced redox potential shifts of tetraheme cytochromes c3 immobilized on self-assembled monolayers: surface-enhanced resonance Raman spectroscopy and simulation studies.

The tetraheme protein cytochrome c(3) (Cyt-c(3)) from Desulfovibrio gigas, immobilized on a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid, is studied by theoretical and spectroscopic methods. Molecular dynamics simulations indicate that the protein docks to the negatively charged SAM via its lysine-rich domain around the exposed heme IV. Complex formation is associated with only little protein structural perturbations. This finding is in line with the resonance Raman and surface-enhanced resonance Raman (SERR) spectroscopic results that indicate essentially the same heme pocket structures for the protein in solution and adsorbed on SAM-coated Ag electrodes. Electron- and proton-binding equilibrium calculations reveal substantial negative shifts of the redox potentials compared to the protein in solution. The magnitude of these shifts decreases in the order heme IV (-161 mV) > heme III (-73 mV) > heme II (-57 mV) > heme I (-26 mV), resulting in a change of the order of reduction. These shifts originate from the distance-dependent electrostatic interactions between the SAM headgroups and the individual hemes, leading to a stabilization of the oxidized forms. The results of the potential-dependent SERR spectroscopic analyses are consistent with the theoretical predictions and afford redox potential shifts of -160 mV (heme IV), -90 mV (heme III), -70 mV (heme II), and +20 mV (heme I) relative to the experimental redox potentials for Cyt-c(3) in solution. SERR spectroscopic experiments reveal electric-field-induced changes of the redox potentials also for the structurally very similar Cyt-c(3) from Desulfovibrio vulgaris, although the shifts are somewhat smaller compared to Cyt-c(3) from D. gigas. This study suggests that electric-field-induced redox potential shifts may also occur upon binding to biomembranes or partner proteins and thus may affect biological electron transfer processes.

Biophysical Phenomena↗

PubMed1997

Redox potential measurements of plasma in patients undergoing coronary artery bypass graft and its clinical significance.

The apparent redox potentials (Em) of plasma as a marker of oxidant injury during coronary artery bypass graft (CABG) is determined, and their clinical significance is discussed. We measured plasma Em of normal volunteers (n = 20) and samples drawn at different time points from patients undergoing elective CABG (n = 60) directly and by adding 5 microl (20 mM) oxidants or reductants with known redox potential to plasma (95 microl), using a micro Pt/AgCl combination redox electrode. The Em value stays elevated up to 30 min during the surgery, after the administration of protamine it came down toward a more reduced state. Similar changes are seen with the lactate pyruvate ratio. Smaller changes of Em than normal are observed in plasma samples from patients treated with Aprotinin (antiprotease), Carmeda (heparin-coated) circuit and aspirin reflecting their protective effect. Redox potential (Em) measurements appear to be effective and useful in monitoring redox shifts wherever oxidative stress needs to be monitored.

Anticoagulants↗

PubMed2002

Ferrocene encapsulated within symmetric dendrimers: a deeper understanding of dendritic effects on redox potential.

Ferrocene has been encapsulated within a symmetric ether-amide dendritic shell and its redox potential monitored in a variety of solvents. The dendritic effect generated by the branched shell is different in different solvents. In less polar, non hydrogen bond donor solvents, attachment of the branched shell to ferrocene increases its E(1/2), indicating that oxidation to ferrocenium (charge buildup) becomes thermodynamically hindered by the dendrimer, a result explained by the dendrimer providing a less polar medium than that of the surrounding electrolyte solution. The effect of electrolyte concentration on redox potential was also investigated, and it was shown that the concentration of "innocent" electrolyte has a significant effect on the redox potential by increasing the overall polarity of the surrounding medium. Dendritic destabilization of charge buildup is in agreement with the majority of reported dendritic effects. A notable exception to this is provided by the asymmetric ferrocene dendrimers previously reported by Kaifer and co-workers, in which the branching facilitated oxidation, and it is proposed that in this case the dendritic effect is generated by a different mechanism. Interestingly, in methanol, the new symmetric ferrocene dendrimer exhibited almost no dendritic effect, a result explained by the ability of methanol to interact extensively with the branched shell, generating a more open superstructure. By comparison of all the new data with other reports, this study provides a key insight into the structure-activity relationships which control redox processes in dendrimers and also an insight into the electrochemical process itself.

Journal Article↗

PubMed1976

The size pH, and redox potential of the cecum in mice associated with various microbial floras.

Cecal size and in situ redox potential and pH of cecal contents were determined in conventionally reared mice and mice reared under a variety of gnotobiotic conditions: germfree, monoassociated with a cecal Clostridium sp., hexaflora-associated and thermoduric polyflora-associated. The mean Eh was approximately +200 mV in germfree and -200 mV in conventional mice. The Eh was close to zero in the monoassociated mice, thus occupying a position intermediate between the germfree and conventional mice. The potentials observed in the hexaflora and the thermoduric flora groups were indistinguishable from those of conventional animals. The degree of normalization was more advanced with respect to the redox potential than to the cecal size in the various gnotobiotic groups. In the thermoduric polyflora-associated group, normalization was observed in both cecal size and redox potential. This demonstrates that normalization can be accomplished with a relatively simplified microflora, at least with regard to the parameters studied.

Animals↗

PubMed1997

Differential stabilization of the three FMN redox forms by tyrosine 94 and tryptophan 57 in flavodoxin from Anabaena and its influence on the redox potentials.

Flavodoxins are electron transfer proteins that carry a noncovalently bound flavin mononucleotide molecule as the redox-active center. The redox potentials of the flavin nucleotide are profoundly altered upon interaction with the protein. In Anabaena flavodoxin, as in many flavodoxins, the flavin is sandwiched between two aromatic residues (Trp57 and Tyr94) thought to be implicated in the alteration of the redox potentials. We have individually replaced these two residues by each of the other aromatic residues, by alanine and by leucine. For each mutant, we have determined the redox potentials and the binding energies of the oxidized FMN--apoflavodoxin complexes. From these data, the binding energies of the semireduced and reduced complexes have been calculated. Comparison of the binding energies of wild-type and mutant flavodoxins at the three redox states suggests that the interaction between Tyr94 and FMN stabilizes the apoflavodoxin--FMN complex in all redox states. The oxidized and semireduced complexes are, however, more strongly stabilized than the reduced complex, making the semiquinone/hydroquinone midpoint potential more negative in flavodoxin than in unbound FMN. Trp57 also stabilizes all redox forms of FMN, thus cooperating with Tyr94 in strong FMN binding. On the other hand, Trp57 seems to slightly destabilize the semireduced complex relative to the oxidized one. Finally, we have observed that reduction of mutants lacking Trp57 is slow relative to that of wild-type or mutants lacking Tyr94, which suggests that Trp57 could play a role in the kinetics of flavodoxin redox reactions.

Anabaena↗

PubMed2006

Arsenic release from iron rich mineral processing waste: Influence of pH and redox potential.

This paper presents the effect of pH and redox potential on the potential mobility of arsenic (As) from a contaminated mineral processing waste. The selected waste contained about 0.47 g kg(-1) of As and 66.2 g kg(-1) of iron (Fe). The characteristic of the waste was identified by acid digestion, X-ray diffraction and sequential extraction procedures. Less than 2% of the total As was acid extractable with the remaining 98% associated with Fe-oxyhydroxides and oxides. Batch leaching tests at different pH conditions showed a strong pH dependence on arsenic and iron leaching. Arsenic leaching followed a "V" shaped profiles with significant leaching in the acidic and alkaline pH region. Acid extractable phases dissolved at acidic pH, while desorption of arsenic due to increase in pH resulted in high arsenic concentration at alkaline pH. Under aerobic conditions and pH 7, As solubility was low, probably due to its precipitation on Fe-oxyhydroxides. Maximum As solubilization occurred at pH 11 (3.59 mg l(-1)). Similarity in the As and Fe leaching profiles suggested that the release of As was related to the dissolution of Fe in the low pH region. In general, redox potential did not play a significant role in arsenic or iron solubilization. It was thus concluded that for this solid waste, desorption was the predominant mechanism in arsenic leaching. A simple thermodynamic model based on arsenic and iron redox reactions was developed to identify the more sensitive redox couple.

Arsenicals↗

PubMed1974

Redox potentials of certain vitamins K: implications for a role in sulfite reduction by obligately anaerobic bacteria.

Redox potentials of a menaquinone (MK-6), isolated in earlier researches from two species of the obligately anaerobic genus, Desulfovibrio, as well as two other vitamins K(2)-menaquinones (MK-5) and (MK-9)- have been determined polarographically. The measurements have been validated by determination of redox potentials of 1,4-naphthoquinone and vitamin K(1) which agree with published potentiometric values. E(m7) for menaquinone (MK-6) is -0.067 +/- 0.010 V. Redox potentials calculated for terminal acceptor couples currently proposed in the mechanisms of sulfate reduction by Desulfovibrio are consistent with the involvement of menaquinone (MK-6) in at least one of the steps postulated during electron transfer with ultimate production of sulfide.

Anaerobiosis↗

PubMed1999

Scale-up of citric acid fermentation by redox potential control

To obtain high citric acid productivity in Aspergillus niger fermentation on beet molasses substrate, a certain redox potential profile with two maxima (260 and 280 mV) and two minima (180 and 80 mV) must be maintained. The most effective regulation of redox potential is by regulation of aeration and agitation. It has been shown that control of redox potential by aeration and agitation is a most successful method for scale-up from 10-L laboratory scale to the 100- and 1000-L pilot-plant scale, even in geometrically dissimilar stirred-tank reactors. Copyright 1999 John Wiley & Sons, Inc.

Journal Article↗

PubMed1999

Unusually high standard redox potential of acrylyl-CoA/propionyl-CoA couple among enoyl-CoA/acyl-CoA couples: a reason for the distinct metabolic pathway of propionyl-CoA from longer acyl-CoAs.

The standard redox potential of acrylyl-CoA/propionyl-CoA couple (C(3)) was determined to be 69 mV (vs. standard hydrogen electrode) at pH 7 and 25 degrees C. This value implies that the 2, 3-dehydrogenation of propionyl-CoA is thermodynamically much more unfavorable than that of longer acyl-CoAs because the standard redox potentials of crotonyl-CoA/butyryl-CoA (C(4)), octenoyl-CoA/octanoyl-CoA (C(8)), and hexadecenoyl-CoA/palmitoyl-CoA (C(16)) are all about -10 mV. The unusually high standard redox potential of the acrylyl-CoA/propionyl-CoA couple is thought to be one of the reasons that in mammals propionyl-CoA is not metabolized by beta-oxidation as in the case of longer acyl-CoAs, but by a methylmalonyl-CoA pathway. The obvious structural difference between C(3) and C(4) (and longer) is whether an H or the C(4) atom is connected to -C(3)H=C(2)H-C(1)O-S-CoA. The molecular orbital calculations (MOPAC) for the enoyl and acyl forms of C(3) and C(4) revealed that this structural feature is the main cause for the higher standard redox potential of the C(3) couple. That is, the C(4)-C(3) bond is stabilized by the dehydrogenation to a greater degree than the H-C(3) bond.

Acyl Coenzyme A↗

PubMed1989

Effect of redox potential on the catalytic properties of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1.

The effect of redox potential on the catalytic activities of the soluble hydrogenase from the hydrogen bacterium Alcaligenes eutrophus Z1 was studied. Several transitions were observed on the enzyme catalytic activity vs potential profiles. The coenzyme-dependent activities of the hydrogenase, its diaphorase activity and activity toward NAD, are controlled by the Em -300 mV, while the process of hydrogen evolution from reduced methyl viologen is governed by the midpoint redox potential of -435 mV. This value of Em was independent of pH in the range 5 to 8. The redox potential of the medium appears to be one of the major factors determining the hydrogenase activation, inactivation, and catalytic properties. It is suggested that a change in the redox state of the enzyme electron transport chain is followed by structural rearrangements within the protein affecting both the hydrogenase catalytic activity and stability. The probable mechanism of enzyme activity regulation is discussed.

Alcaligenes↗

PubMed2001

Cessation of cell proliferation by adjustment of cell redox potential.

A variety of chemical substances are known to stop cell proliferation, although the mechanisms are obscure. We suggest that many of these chemicals employ the braking power of the retinoblastoma (RB) protein to stop proliferation by raising the intracellular redox potential. The elevation of the redox potential above a threshold prevents the phosphorylation of RB protein, which in turn impedes the release of transcription factors necessary for the progression of the cell cycle. The redox potential of normal proliferating fibroblasts has been found to be below that of fibrosarcoma cells. The possibility thus exists that, for cells with this property, the dosage of a redox-raising agent can be adjusted to stop cancer-cell proliferation without affecting the proliferation of normal cells.

Animals↗

PubMed1988

Computed redox potentials and the design of bioreductive agents.

Anti-cancer agents that have been made selective for tumour cells by exploiting the known differences in the availability of oxygen between normal and transformed cells are a promising development in cancer chemotherapy. We have recently suggested a new type of bioreductive activity which would depend on a two-electron reduction. For rational design of such compounds, it is essential to be able to predict the redox potentials and the chemical modifications needed to produce the optimum redox value. Calculating redox potentials is a daunting task for the theoretician, however, as the effect of water solvation is clearly of major significance. Recent successful calculations of differences in the free energies of biologically important molecules in aqueous solution using the free-energy perturbation method prompted us to apply the technique to the computation of two-electron redox potentials. The results are accurate to within 20 mV, suggesting that we should be able to manipulate redox potentials by successfully predicting structures with the appropriate value.

Antineoplastic Agents↗

PubMed2005

Dications of fluorenylidenes. The relationship between redox potentials and antiaromaticity for meta- and para-substituted diphenylmethylidenefluorenes.

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.

Journal Article↗

PubMed1985

Cytosolic redox potential and phosphate transport in the proximal tubule of the rabbit. A study in the isolated perfused tubules.

Recently it has been proposed that cytosolic NAD+ may play a regulatory role in phosphate transport across the renal proximal tubules. To test this thesis, we have examined the effect of altering the cytosolic redox potential (NAD+/NADH) on phosphate flux across the isolated perfused rabbit proximal tubules. The cytosolic redox potential was shifted to a more oxidized state either by changing the substrate in the bathing medium from lactate to pyruvate or by adding methylene blue to the bath medium. On the other hand, cytosolic redox potential was shifted to a more reduced state by changing the substrate in the bath medium from pyruvate to lactate. In either case the phosphate flux across the proximal tubule was unaffected. It was concluded that cytosolic redox potential may not play a significant role in regulating the phosphate transport in renal proximal tubules.

Animals↗

PubMed2000

Skeletal muscle ryanodine receptor is a redox sensor with a well defined redox potential that is sensitive to channel modulators.

Hyperreactive sulfhydryl groups associated with the Ca(2+) release protein from sarcoplasmic reticulum are shown to have a well defined reduction potential that is sensitive to the cellular environment. Ca(2+) channel activators lower the redox potential of the ryanodine receptor, which favors the oxidation of thiols and the opening of the Ca(2+) release protein. In contrast, channel inhibitors increase the redox potential, which favors the reduction of disulfides and the closure of the release protein. Modulation of redox potential of reactive thiols may be a general control mechanism by which sarcoplasmic/endoplasmic reticulum, ryanodine receptors/IP(3) receptors, control cytoplasmic Ca(2+) concentrations.

Animals↗

PubMed2006

Modelling the impact of geometric parameters on the redox potential of blue copper proteins.

The synthesis and structure of a homologous series of cationic N(2)S(2) copper(I) Schiff base complexes constructed using o-tert-butylthiobenzaldehyde and a series of terminal diamines (ethane, propane, butane) are reported. The complexes differ only in the length of the methylene chain between the imine groups. This simple modification forces the copper centre to shift geometry from a planar (1,2-diaminoethane) to a more distorted tetrahedral motif (1,4-diaminobutane). The redox potentials of the three cations were measured using cyclic voltammetry in donor (acetonitrile) and non-donor solvents (dichloromethane). The S-Cu-N angles for each complex are correlated against the respective redox potential allowing an analysis of the geometric impact on the redox potential in soft copper centres. The redox potential is observed to increase as the metal centre moves from a planar towards a tetrahedral motif. Comparing this data with the reported structures of the blue copper proteins (rusticyanin and plastocyanin) allows an assessment of the contribution of the geometry of the metal binding site to the operating potential of these proteins to be made.

Azurin↗

PubMed1980

Modulation of the flavin redox potential as mode of regulation of succinate dehydrogenase activity.

The redox properties of flavin in active and non-active (oxaloacetate reacted) soluble succinate dehydrogenase were studied. Quantitative analysis of reductive activation titrations of redox titrations of active and non-active enzyme reveal that the redox potential of the histidyl-flavin in the active enzyme (-3 +/- 15 mV) is high enough to allow reduction by succinate, whereas in the non active enzyme it is -196 +/- 19 mV, far to low to be reduced by substrate. The flavin radical in the active enzyme attains 60% of total flavin at a poised redox potential of about +60 mV, upon addition of oxaloacetate the magnitude of the signal is diminished and the potential where it reaches maximal concentration is shifted by about -200 mV. A mechanism is proposed which ascribes the fundamental difference between active and non-active enzyme to the inability of the latter to be reduced by substrate.

Animals↗