Dithionite redox potential

Cultivation of strictly anaerobic organisms requires not only that the medium be oxygen-free, but also that the redox potential of the medium be compatible with that required by the organisms. This may be accomplished by addition of reducing agents such as sulfide, dithionite, titanium(III) citrate, or titanium(IIl) nitrilotriacetate. Any of these may, however, be toxic so that only low concentrations should be employed. Attention has been drawn to the fact that titanium(III) citrate-reduced medium may be inhibitory to bacteria during initial isolation (Wachenheim and Hespell 1984). 

Mayhew, S.G. 1978. The redox potential of dithionite and SO,- from equilibrium reactions with flavodoxins, methyl viologen and hydrogen plus hydrogenase. European Journal of Biochemistry 85 535-547. 

Sodium dithionite is well established [ 1 ] as a powerful reducing agent under alkaline conditions. Its redox potential is close to that of sodium borohydride [2] and, in several respects, there are advantages in the use of sodium dithionite as an alternative to the metal hydrides under phase-transfer catalytic conditions, particularly in the reduction of carbonyl compounds [3],... 

The one-electron electrochemical reduction of NP (57) is a reversible process in aqueous solution, provided the measurements are performed at pH > 8 (—0.123 V vs. NHE) (57a,57b). Different chemical reductants such as sodium in liquid ammonia, tetrahydroborate, ascorbic acid, quinol, dithionite, superoxide or thiolates are also known to generate the [Fen(CN)5NO]3 ion (48,57). However, care must be taken in the products analysis, because the negative redox potentials of some of these reductants allow for further nitrosyl reduction (57a). Also, the reduced product is unstable toward cyanide... 

According to recent data, the property of dithionite as an electron donor for nitrogenase is different from that of the natural donor flavodoxin (Burgess and Lowe, 1996). Flavodoxin from Azotobacter vinelandii has the redox potential equal to -0.515 V for the reversible transition between the semiquinone and hydroquinone forms of flavodoxin. Unlike dithionite, flavodoxin can reversibly reduce the [Fe4S4]+l cluster Av2 by one electron to the [Fe4S4]° state in which all iron ions exist in the ferrous form. It is assumed that, under natural conditions, two electrons can transfer from Av2 to Avl. Flavodoxin reduces both Av2 bound to Avl and free Av2 in a solution. The apparent rate constants of these reactions are 400 s 1 and > 1000 s"1, respectively (Duyvis et al. 1998). 

First dark redox potential, determined with sodium dithionite. 

The potentials of the redox centers of xanthine oxidase have been investigated by titrations in the presence of redox mediator dyes. An early study (245) used dithionite to generate reducing equivalents and quantified the reduced species by EPR measurements at low temperature. Subsequent studies as a function of pH showed that the potential of the molybdenum center was sensitive to pH (246). Concern over the effect of temperature on the observed potentials led to redox titrations monitored by room temperature CD and EPR spectroscopy (247). These experiments indicated that the redox potentials of all of the prosthetic... 

It is now accepted that the reduced iron protein binds two moles MgATP, followed by a change of conformation and a decrease in redox potential. The (MgATP)2(Fe) adduct complexes the MoFe protein (one of the two independently functioning halves of the tetrameric a2 2 structure). This is followed by hydrolysis of two moles MgATP and the transfer of one electron to the MoFe protein (and ultimately to the bound substrate). The oxidized Fe protein is reduced under physiological conditions by transfer of electrons from pyruvate, via pyruvate flavodoxin oxido-reductase and fiavodoxin. In vitro sodium dithionite is usually used and involves S02 as the... 

The presence of reduced [FeS-A/FeS-B] apparently also has an influence on the thermodynamic properties of FeS-X, as illustrated by its redox potential in the two different environments. The titration curve for FeS-X in the core complex shown in the upper panel of Fig. 7 (C) yielded a curve for a one-electron change with a midpoint potential of -610 mV, which is only slightly lower than that of the FeS-B cluster. By comparison, redox titration of the PS-1 native complex in which FeS-A/FeS-B was reduced chemically by dithionite and methyl viologen yielded, as shown in the bottom panel in Fig. 7 (C), a curve for a one-electron change but with a midpoint potential about 60 mV more negative than that for FeS-X in the core complex, namely -670 mV. It has been noted above that the midpoint potential for FeS-X in the PS-1 native complex reported here is still 60-70 mV more positive than those determined previously but the reason for this discrepancy is not yet clear. 

Svensson et al. [100] compared dithionite bleaching of spruce TMP at pH 10 and pH 5, using principal components analysis to reduce reflectance spectra to contributing subspectra. The authors concluded that at pH 5 (the common pH for commercial dithionite bleaching) quinones were the major chromophores bleached. At pH 10, quinones were formed due to alkali darkening. In spite of the higher redox potential of dithionite at pH 10, bleaching was less efficient than at pH 5, and there was no evidence that any chromophores other than quinones were removed. 

When PDR is titrated with NADH or dithionite, two stages of reduction are observed. First, the iron-sulfur center and FMN react simultaneously to form a reduced iron—sulfur center and a neutral flavin semiquinone. Further titration reduces the semiquinone to the hydroquinone. During the reduction, a maximum semiquinone concentration of 80% of the total enzyme concentration is reached. The redox potential of the [2Fe-2S] center and that of the oxidized flavin—semiquinone couple are the same, —174 mV. The semiquinone— hydroquinone couple is well resolved from this at —287 mV." These midpoint potentials favor spontaneous electron transfer from NADH to FMN to [2Fe-2S]. 

The FeMoco has been observed in three redox states. When the enzyme is isolated in the presence of dithionite, the FeMoco is in the M ( native ) form. Most c stallographic studies have taken place on enzyme in which the FeMoco is in the M state. M° can be generated by one-electron oxidation of M with dyes, and can be reactivated by reduction. The M° /M redox potential is dependent on the organism from which the nitrogenase is derived, lying in the range OmV to — 180mV. " The X-ray crystal structure of MoFe protein with the FeMoco in the M° state shows no major differences from M is the turnover state of the enzyme,... 

Evidence for the [4Fe S] cluster as the active form of lysine aminomutase was obtained by Frey and co-workers, who showed by a combination of EPR spectroscopy and enzyme assays that the [4Fe-4S] -LAM generated in the presence of AdoMet was catalytically active. Unlike aRNR-AE, however, LAM catalyzes a reversible reductive cleavage of AdoMet, and thus methionine production and cluster oxidation could not be monitored as evidence of turnover. It is of interest to note that in the case of LAM, the presence of AdoMet facilitates reduction to the [4Fe-4S] state very little [4Fe-4S]" cluster is produced by the reduction of LAM with dithionite in the absence of AdoMet, while the presence of AdoMet or its analogue S-adenosylhomocysteine dramatically increases the quantity of [dFe-dS] " produced. It is not clear whether the presence of AdoMet affects the redox potential of the cluster or whether some other effect, such as accessibility of the cluster by the reductant, is at work. 

A recent study has shown that the same reaction may take place smoothly at room temperature in 7—IOM-H2SO4. Redox potentials of aqueous solutions containing dithionite, sulphite, and thiosulphate and for the irreversible system sulphite-dithionite have been measured. 

After each addition of dithionite or foricyanide, the sample was stirred magnetically until a stable redox potential was obtained. A fluorescence emission spectrum, using 460 nm excitation, was then recorded, and the fluorescence intensities at the emission maxima of BChl d (764 nm) and BChl a (812 nm) were noted. The emission intensities at these wavelengths in the initial non-reduced sample were then subtracted to obtain the oxidant-sensitive fluorescence. 

Further observations on this effect by Wang et al. (16) have shown that cells taken from the culture medium anaerobically and diluted in anaerobic buffer exhibit a 100% efficiency of excitation transfer in the absence of dithionite. They also observed that the intensity of fluorescence from BChl d (measured at 760 nm) decreased under aerobic conditions. This suggests that aerobic conditions create quenchers within the chlorosome that compete with excitation transfer for BChl d excited states. Wang et al. (16) also found that oxidants other than O2, such as benzc uinone and Fe(CN)6 + TMPD, caused a similar decrease in the efficiency of excitation transfer, indicating that the critical factor is redox potential and not the presence of O2 per se. 

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