Redox potentials flavins

Although the reduction potentials of DNA bases and UV induced DNA lesions inside a DNA double strand or inside the active site of a DNA photolyase, together with the reduction potential of the photoexcited FADH- in the photolyases, are not known, currently available redox potentials indicate that the single electron reduction of a nucleobase or a UV induced dimer lesion by a reduced and deprotonated flavin coenzyme is a weakly exothermic process. The reduced and deprotonated FADH- in its photoexcited state is... 

Redox mediators, such as flavins or quinones, are usually involved in the azo bond reduction. Therefore, the azo bond cleavage is a chemical, unspecific reaction that can occur inside or outside the cell, relying on the redox potential of the redox mediators and of the azo compounds. Also the reduction of the redox mediators can be both a chemical and an enzymatic process. As a consequence, it is an evidence that environmental conditions can affect the azo dyes degradation process extent both directly, depending on the reductive or oxidative status of the environment, and indirectly, influencing the microbial metabolism. 

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

New supramolecular compounds mimicking biological cofactors have been proposed which have high redox potentials and, further, can be Hnked covalently onto supports in biotransformation systems [52]. Bioelectrochemical methods available for regeneration of nicotinamide- and flavin-dependent systems are comprehensively reviewed by Kohhnan et al. [53]. 

Table 7. Some Redox Potentials (E in mV) of Free and Protein-Bound Flavins... 

In spite of this progress, the gaps in our knowledge of the molecular mechanisms of the participation of flavins in one-electron transfer reactions are enormous. Whether the reduction of flavins by obligatory two-electron donors occurs by a concerted two-electron process or by sequential one-electron transfers remains a matter of controversy and is a subject under current active investigation. It is hoped that this review will convince the reader of the usefulness and necessity of redox potential measurements in the understanding of electron transfer reactions in flavoenzymes. These type of measurements have become more numerous in recent years however, more information of this type is needed. We have seen that the apoprotein environment can alter the one-electron potentials of their respective bound flavin coenzymes by several hundred millivolts, yet virtually nothing is known, on a molecular basis, of how this is achieved. 

Flavin coenzymes are usually bound tightly to proteins and cycle between reduced and oxidized states while attached to the same protein molecule. In a free unbound coenzyme the redox potential is determined by the structures of the oxidized and reduced forms of the couple. Both riboflavin and the pyridine nucleotides contain aromatic ring systems that are stabilized by resonance. Part of this resonance stabilization is lost upon reduction. The value of E° depends in part upon the varying amounts of resonance in the oxidized and reduced forms. The structures of the coenzymes have apparently evolved to provide values of E° appropriate for their biological functions. 

This unique redox catalyst links the oxidation of H2 or of formate to the reduction of NADP+229 and also serves as the reductant in the final step of methane biosynthesis (see Section E) 228 It resembles NAD+ in having a redox potential of about -0.345 volts and the tendency to be only a two-electron donor. More recently free 8-hydroxy-7,8-didemethyl-5-deazaribo-flavin has been identified as an essential light-absorbing chromophore in DNA photolyase of Methanobacterium, other bacteria, and eukaryotic algae.230 Roseoflavin is not a coenzyme but an antibiotic from Streptomyces davawensisP1 Many synthetic flavins have been used in studies of mechanisms and for NMR232 and other forms of spectroscopy. 

The equilibria governing semi-quinone formation from quinones are similar to those for the flavin semiquinones which were discussed in Section B,6. Two consecutive one-electron redox steps can be defined. Their redox potentials will vary with pH because of a pfCa for the semiquinone in the pH 4.5 -6.5 region. For ubiquinone this pKa is about 4.9 in water and 6.45 in methanol. A pKa of over 13 in the... 

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

Electron-transfer in biological systems takes place through the mediation of a number of proteins, which contain a variety of active sites such as heme, Fe—S, Cu, and flavin. These active sites are protected from the solvent by a hydrophobic environment created by the peptide chain 48). The redox potential of a biological redox couple in vivo lies, for the most part, between —0.5 and +0.85 V. The former and latter potentials correspond to the redox potentials of H20/H2 and H20/02 respectively 49). 

A high content of linolenate in the thylakoid membranes would, most probably, make them more fluid and also provide a medium of low dielectric constant. In this medium, the electron-transport chains that are inhibited by water can function well.384-386 It was found that photoreduction of cytochrome C is increased by the addition of MGDG and DGDG.387 A complex that contained 12% of manganese, DGDG, and a flavine was isolated from a variety of leaves388 this was found to have a high redox potential, and thus, it may participate as an oxidizer. 

Konig, B., Pelka, M., Reichenbach-Klinke, R., Schelter, J., Daub, J. A model system for flavoenzyme activity - binding of flavin and modulation of its redox potentials through coordination to a Lewis-acidic azamacrocyclic zinc(II) complex, Eur. J. Org. Chem. (2001), 2297-2303. 

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