Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Redox potential”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
PubMedDate not supplied

Construction of platinum-tipped redox probes for determining soil redox potential.

Redox probes are typically constructed by soldering Pt wire to a metal wire or rod, such as copper or brass. The junction between the Pt and wire or rod is often sealed with an epoxy resin and hardener or with heat-shrink tubing. Microcracks (small cracks invisible to the unaided eye) can form in the hardened resin and result in incorrect readings. The hardened resin is not easily removed, making repairs difficult. Heat-shrink tubing is thin, lacks rigidity, and can be damaged in the soil. The method described in this paper used a thick-walled, adhesive-lined terminal insulator to seal the junction. The terminal insulators were easily applied and removed, which made faulty probes easy to repair. Two-hundred forty probes were made with this method and eight were made with a marine epoxy resin. The probes were tested with a redox buffer solution (Light Solution) and were usable if they read +476 +/- 10 mV. The probes were installed 0.76 and 1.5 m deep in the soil. The ability of the probes to provide reliable redox readings was examined by testing selected probes after 10 mo of use and testing all of the probes after completion of the study (19 mo). Ten of the twelve probes tested after 10 mo worked satisfactorily, while the other two clearly malfunctioned before testing. After the study was completed, 236 of the 240 of the probes worked satisfactorily. These results indicate that the construction method presented produces reliable, long-lasting probes.

Environmental Monitoring↗
PubMed2004

Investigating mitochondrial redox potential with redox-sensitive green fluorescent protein indicators.

Current methods for determining ambient redox potential in cells are labor-intensive and generally require destruction of tissue. This precludes single cell or real time studies of changes in redox poise that result from metabolic processes or environmental influences. By substitution of surface-exposed residues on the Aequorea victoria green fluorescent protein (GFP) with cysteines in appropriate positions to form disulfide bonds, reduction-oxidation-sensitive GFPs (roGFPs) have been created. roGFPs have two fluorescence excitation maxima at about 400 and 490 nm and display rapid and reversible ratiometric changes in fluorescence in response to changes in ambient redox potential in vitro and in vivo. Crystal structure analyses of reduced and oxidized crystals of roGFP2 at 2.0- and 1.9-A resolution, respectively, reveal in the oxidized state a highly strained disulfide and localized main chain structural changes that presumably account for the state-dependent spectral changes. roGFP1 has been targeted to the mitochondria in HeLa cells. Fluorometric measurements on these cells using a fluorescence microscope or in cell suspension using a fluorometer reveal that the roGFP1 probe is in dynamic equilibrium with the mitochondrial redox status and responds to membrane-permeable reductants and oxidants. The roGFP1 probe reports that the matrix space in HeLa cell mitochondria is highly reducing, with a midpoint potential near -360 mV (assuming mitochondrial pH approximately 8.0 at 37 degrees C). In other work (C. T. Dooley, T. M. Dore, G. Hanson, W. C. Jackson, S. J. Remington, and R. Y. Tsien, submitted for publication), it is shown that the cytosol of HeLa cells is also unusually reducing but somewhat less so than the mitochondrial matrix.

Cell Line↗
PubMed1997

Site-directed mutants of pseudoazurin: explanation of increased redox potentials from X-ray structures and from calculation of redox potential differences.

In order to understand the origins of differences in redox potentials among cupredoxins (small blue type I copper-containing proteins that reversibly change oxidation state and interact with redox partners), we have determined the structures of the native and two mutants (P80A and P80I) of pseudoazurin from Alcaligenes faecalis S-6 in oxidized and reduced forms at resolutions of 2.2 A in the worst case and 1.6 A in the best case. The P80A mutation creates a surface pocket filled by a new water molecule, whereas the P80I mutant excludes this water. Distinct patterns of change occur in response to reduction for all three molecules: the copper position shifts, Met 7 and Pro 35 move, and the relative orientations of residues 81 to 16, 18 to the amide planes of 77 and 86, all change. Systematic changes in the weak electrostatic interactions seen in the structures of different oxidation states can explain the Met 7/Pro 35 structural differences as well as some fluctuating solvent positions. Overall displacement parameters increase reversibly upon reduction. The reduced forms are slightly expanded over the oxidized forms. The geometries of the mutants become more trigonal in their reduced forms, consistent with higher redox potentials (+409 mV for P80A and +450 mV for P80I). Calculations of the differences in redox potentials, using POLARIS, reveal that a water unique to the P80A mutant is required (with correctly oriented hydrogens) to approximate the observed difference in redox potential. The POLARIS calculations suggest that the reduced forms are additionally stabilized through changes in the solvation of the copper center, specifically via the amides of residues 16, 39, 41, 79, and 80 which interact with either Phe 18, Met 86, or Cys 78. The redox potential of P80A is increased largely due to solvation effects, whereas the redox potential of P80I is increased largely due to geometrical effects.

Alcaligenes↗
PubMed1997

Inhibitory effects on Epstein-Barr virus activation of anthraquinones: correlation with redox potentials.

The redox potentials have been determined for nine anthraquinones in phosphate buffer at pH 7.2 by means of cyclic voltammetry. A definite correlation has been found between the redox potentials and the inhibitory effects of the anthraquinones on the EBV-EA activation. It has further been shown that the correlation can be made better by introducing an electronic property, i.e. the atomic charge at O12 as an additional parameter.

Anthraquinones↗
PubMed1993

Amino acid sequence of a high redox potential ferredoxin (HiPIP) from the purple phototrophic bacterium Rhodopila globiformis, which has the highest known redox potential of its class.

Rhodopila globiformis HiPIP has a redox potential (ca. 450 mV) that is 100 mV higher than any other known iron-sulfur protein. The amino acid sequence contains 57 residues and can be aligned with that of Thiobacillus ferrooxidans without any insertions or deletions and is 51% identical. Rp. globiformis HiPIP is also similar to that of Rhodocyclus tenuis, but six- and two-residue gaps must be postulated and there is only 37% identity. Most of the amino acid residues near the iron-sulfur cluster are similar in these two species based on inspection of the three-dimensional structure of Rc. tenuis HiPIP. The reason for the higher redox potential may be a more hydrophilic environment of the Rp. globiformis HiPIP iron-sulfur cluster due to the above two deletions and to substitution of Ser 32 for Gly. Rp. globiformis is unusual in that it has a cytochrome c2 in addition to the HiPIP, and it too has a very high redox potential. These results suggest that the cytochrome c2 and HiPIP may function interchangeably and that the species normally resides in a very high potential environment, although it is not known to grow aerobically in the dark.

Amino Acid Sequence↗
PubMed1978

A high-potential redox component located within cyanobacterial photosystem II.

The calcium-dependent oxygen evolution activity of preparations of Phormidium luridum shows a marked selectivity in favor of ferricyanide over benzoquinone as Hill oxidant. In addition, the rate of oxygen evolution increases with increasing solution redox potential over the range +350 to +550 mV vs. the standard hydrogen electrode. These properties pertain to both 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive and -insensitive fractions of the total oxygen evolution activity. Neither changes in solution potential nor use of oxidants other than ferricyanide obviate the need for added Ca(2+). To explain these observations, two models are proposed, each of which invokes the existence of a redox component located within Photosystem II and having a midpoint potential greater than +450 mV. In one model, the postulated species is a donor which competes with water for oxidizing equivalents generated by System II. In the other model, the 450 mV species is a high-potential primary acceptor of System II electrons.

Calcium↗
PubMed1985

Kinetics of reduction of high redox potential ferredoxins by the semiquinones of Clostridium pasteurianum flavodoxin and exogenous flavin mononucleotide. Electrostatic and redox potential effects.

We have measured the ionic strength dependence of the rate constants for the electron-transfer reactions of flavin mononucleotide (FMN) and flavodoxin semiquinones with 10 high redox potential ferredoxins (HiPIP's). The rate constants were extrapolated to infinite ionic strength by using a theoretical model of electrostatic interactions developed in our laboratory. In all cases, the sign of the electrostatic interaction was the same as the protein net charge, but the magnitudes were much smaller. The results are consistent with a model in which the electrical charges are approximately uniformly distributed over the HiPIP surface and in which there are both short- and long-range electrostatic interactions. An electrostatic field calculation for Chromatium vinosum HiPIP is consistent with this. The presumed site of electron transfer includes that region of the protein surface to which the iron-sulfur cluster is nearest and appears to be relatively hydrophobic. The principal short-range electrostatic interaction would involve the negative charge on the iron-sulfur cluster. For some net negatively charged proteins, this effect is magnified, and for net positively charged HiPIP's, it is counterbalanced. The rate constants extrapolated to infinite ionic strength can be correlated with redox potential differences between the reactants, as has previously been shown for cytochrome-flavin semiquinone reactions. Both electrostatic and redox potential effects are magnified for the flavodoxin semiquinone as compared to the FMN semiquinone-HiPIP reactions. This was also observed previously for the flavin semiquinone-cytochrome reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzoquinones↗
PubMed1996

Anion binding to mitochondrial cytochromes c studied through electrochemistry. Effects of the neutralization of surface charges on the redox potential.

The redox potential of horse and bovine heart cytochromes c determined through cyclic voltammetry is exploited to probe for anion-protein interactions, using a Debye-Hückel-based model. In parallel, protein charge neutralization resulting from specific anion binding allows monitoring for surface-charge/E(o) relationships. This approach shows that a number of anions, most of which are of biological relevance, namely CI-, HPO(2-)4, HCO3-, NO3, SO(2-)4, CIO4-, citrate3- and oxalate2-, bind specifically to the protein surface, often in a sequential manner as a result of the presence of multiple sites with different affinities. The binding stoichiometries of the various anions toward a given cytochrome are in general different. Chloride and phosphate appear to bind to a greater extent to both proteins as compared to the other anions. Differences in binding specificity toward the two cytochromes, although highly sequence-related, are observed for a few anions. The data are discussed comparatively in terms of electrostatic and geometric properties of the anions and by reference to the proposed location and amino acid composition of the anion binding sites, when available. Specific binding of this large set of anions bearing different charges allows the electrostatic effect on Eo due to neutralization of net positive protein surface charge(s) to be monitored. (J)H NMR indeed indicates the absence of significant salt-induced structural perturbations, hence the above change in Eo is predominantly electrostatic in origin. A systematic study of protein surface-charge/Eo relationships using this approach is unprecedented. Values of 15-25 mV (extrapolated at zero ionic strength) are obtained for the decrease in Eo due to neutralization of one positive surface charge, which are of the same order of magnitude as previous estimates obtained with either mutation or chemical modification of surface lysines. The effects of the anion-induced decrease of net positive charge on Eo persist also at a relatively high ionic strength and add to the general effects related to the charge shielding of the protein as a whole due to the surrounding ionic atmosphere: hence the ionic strength dependence of the rate of electron transfer between cytochromes c and redox partners could also involve salt-induced changes in the driving force.

Animals↗
PubMedDate not supplied

[Rumen bacterial metabolism as affected by extracellular redox potential].

The redox potential (Eh) in the digestive tract of ruminants varies mostly within the ranges from -300 to +200 mV, in the rumen medium: from -130 to -200 mV. Eh and pH changes are of a linear character. Enhanced fermentation moves Eh towards negative values and improves growth of microorganisms. Eh values become even more negative as affected by Na2S, NaOH, cysteine and NaHSO3. Addition of picrate, sodium nitrate, hydrochloric acid, copper sulphate, gaseous oxygen and particularly heavy metals (silver and mercury) promotes positive Eh values and inhibits metabolism. Optimum Eh values for Megasphaera elsdenii are more negative, than those for Streptococcus bovis. In the process of S. bovis growth, the production of lactate, VFA, CH2O2 and NH3 increases, Eh and pH values decrease. More positive Eh values inhibit metabolism and growth rates of Selenomonas ruminantium, Bacteroides amylophilus, Bacteroides succinogenes and Streptococcus bovis to different degrees.

Animals↗
PubMed1972

Effects of nonpolar environments on the redox potentials of heme complexes.

A method has been described for measurement of the oxidation-reduction potentials of redox couples in nonaqueous solutions. This method has been applied to a study of the effect of a nonpolar environment on the redox potential of a heme complex. A positive potential difference of some 300 mV is observed for the measured redox potentials by comparison with reported values for the corresponding heme complex in aqueous solution. The proposal is made that the redox potentials of many high-potential cytochromes may be accounted for by a local heme environment of low-dielectric constant, characteristic of nonpolar amino-acid side chains, and that this factor may play a dominant role in the determination of the oxidation-reduction properties of these proteins.

Amino Acids↗
PubMed1977

[Aqueous chloride and bromine solutions as disinfectants: composition, redox potential, differences of reactivity (author's transl)].

A method is given to calculate the equilibrium concentrations of the molecules and ions emerging from the hydrolysis and dissociation of chlorine and bromine in aqueous soltuion. The differing influence the pH value and the initial (total) halogene concentration exerts to the HOX- and X2-equilibrium concentration is discussed. From the calculated equilibrium concentrations and tabulated standard reduction potentials the redox potentials of chlorine and bromine solutions at various pH values are calculated. No correlation between the reactivity and the redox potential of the two halogens was observed. Two possible explanations for the differing reactivity of chlorine and bromine against proteins are presented.

Bromine↗
PubMed1996

Structural origins of redox potentials in Fe-S proteins: electrostatic potentials of crystal structures.

Redox potentials often differ dramatically for homologous proteins that have identical redox centers. For two types of iron-sulfur proteins, the rubredoxins and the high-potential iron-sulfur proteins (HiPIPs), no structural explanations for these differences have been found. We calculated the classical electrostatic potential at the redox site using static crystal structures of four rubredoxins and four HiPIPs to identify important structural determinants of their redox potentials. The contributions from just the backbone and polar side chains are shown to explain major features of the experimental redox potentials. For instance, in the rubredoxins, the presence of Val 44 versus Ala 44 causes a backbone shift that explains a approximately 50 mV lower redox potential in one of the four rubredoxins. This result is consistent with experimental redox potentials of five additional rubredoxins with known sequence. Also, we attribute the unusually lower redox potentials of two of the HiPIPs studied to a less positive electrostatic environment around their redox sites. Finally, molecular dynamics simulations of solvent around static rubredoxin crystal structures indicate that water alone is a major factor in dampening the contribution of charged side chains, in accord with experiments showing that mutations of surface charges produce relatively little effect on redox potentials.

Alanine↗
PubMedDate not supplied

Automated and continuous redox potential measurements in soil.

Redox potential (Eh) describes the electrical state of a matrix. In soils, Eh is an important parameter controlling the persistence of many organic and inorganic compounds. A popular, but also criticized, manual measuring method makes use of a small tip of Pt placed on a copper wire that is placed in the soil; a reference electrode is placed in the same soil at a fixed distance. Fluctuations in redox potential values measured in the soil can be very large and depth-dependent. This will be overlooked when making single-point measurements. We developed the datalogger Hypnos 2.0 for continuous redox potential and temperature measurements at various depths in the soil and without disturbance of the site. Hypnos is field-deployable, relatively cheap, and runs on batteries. The datalogger can use a "sleep mode" between sampling events. In sleep mode, there is no constant voltage on the Pt wire or the reference electrode, but there is only a short pulse during sampling. We did not measure an effect of this short pulse on the measured redox potential. In sandy soils in mesocosms and in a salt marsh soil we measured changes in the Eh as large as from -400 to +100 mV within 4 d, and daily cycles of 200 mV. Both absolute redox potential values and their diurnal variations were depth-dependent. Because single redox measurements are insufficient in describing redox conditions in some soil systems, Hypnos can be a powerful tool when studying the effects of fluctuating redox conditions on metal availability and pollutant degradation.

Automation↗
PubMed1986

Factors influencing redox potentials of electron transfer proteins.

The redox potentials of electron transfer proteins vary over a wide range, even when the type of redox center is the same. Rees [Proc. Natl. Acad. Sci. USA (1985) 82, 3082-3085] proposed that this variation of redox potential partly reflects the different net charges of the proteins, and he presented a linear correlation between these two properties for 36 proteins. A review of the factors that influence protein redox potentials makes it clear that this linear correlation is fortuitous. The key factors influencing redox potentials are the contributions to the Gibbs energy difference between the two redox states, resulting from bonding interactions at the redox center, electrostatic interactions between the redox-center charge and polar groups within the protein and solvent, and redox-state conformational changes. The relative importance of these terms is likely to vary from protein to protein.

Chemical Phenomena↗
PubMed1998

Increase of xylitol production rate by controlling redox potential in Candida parapsilosis.

The effect of redox potential on xylitol production by Candida parapsilosis was investigated. The redox potential was found to be useful for monitoring the dissolved oxygen (DO) level in culture media, especially when the DO level was low. An increase in the agitation speed in a 5 L fermentor resulted in an increased culture redox potential as well as enhanced cell growth. Production of xylitol was maximized at a redox potential of 100 mV. As the initial cell concentration increased from 8 g/L to 30 g/L, the volumetric productivity of xylitol increased from 1.38 g/L. h to 4.62 g/L. h. A two-stage xylitol production strategy was devised, with stage 1 involving rapid production of cells under well-aerated conditions, and stage 2 involving cultivation with reduced aeration such that the culture redox potential was 100 mV. Using this technique, a final xylitol concentration of 180 g/L was obtained from a culture medium totally containing 254.5 g/L xylose in a 3,000 L pilot scale fermentor after 77 h fermentation. The volumetric productivity of xylitol during the fermentation was 2.34 g/L. h.

Biotechnology↗