Reduced forms, electrons

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

This mechanism differs from that initially proposed, as the one-electron reduced active species [Re°(bpy)(CO)3] results here via loss of Cl- instead of CO. Since these first results, the arguments concerning the reaction mechanisms have still continued.74"78 However, some recent IR spectro-electrochemical studies79 have confirmed the occurrence of the C02 attack on both the one and two electron reduced forms [Re(bpy)(CO)3] and [Re(bpy)(CO)3]. ... 

It is well-established that electroreduced nickel(I) complexes of cyclam and a variety of substituted cyclams add oxidatively to alkyl halides to give alkylnickel(III) complexes in organic solvents,251,276 the lifetime of the carbon-nickel bond governing the overall behavior of the system. However, it was shown that [Ni (tmc)]+ (one-electron reduced form of complex (17) tmc= 1,4,8,11-teramethyl 1,4,8,11-tetraazacyclotetradecane) reacts with alkyl chlorides in aqueous alkaline solution in a one-electron process.277,278... 

However, ubihydroquinone, a two-electron reduced form of ubiquinone, can produce superoxide on reaction with molecular oxygen ... 

C-Nitroso compounds, oximes, N-hydroxyguanidines and N-hydroxyureas each contain an N-O bond and release nitric oxide (NO) or one of its redox forms under some conditions. The nitrogen atom of a C-nitroso compound formally exists in the +1 oxidation state, the same oxidation state as nitroxyl (HNO), the one-electron reduced form of N O. The nitrogen atoms of oximes, N-hydroxyguanidines, and N-hydroxyureas each formally exist in the -1 oxidation state, the same oxidation state as hydroxylamine. Consequently, the direct formation of NO (formal oxidation state = +2) from any of these species requires oxidation, one electron for a C-nitroso compound and three electrons for an oxime, N-hydroxyguanidine or N-hydroxyurea. This chapter summarizes the syntheses and properties, NO-releasing mechanisms and the known structure-activity relationships of these compounds. 

The disposition of the different metal centres of bovine heart CcOx is represented in Figure 14.9. The dimetallic CuA site receives electrons directly from cytochrome c, and is located in a globular domain of subunit II, which protrudes into the intermembrane space (the periplasmic space in bacteria). This centre, which was widely believed to be mononuclear is a dicopper site (Figure 14.10) in which the coppers are bridged by two cysteine sul-furs each copper in addition has two other protein ligands. In the one electron-reduced form,... 

A cation radical chain cycloaddition-polymerization catalysed by tris(4-bromophenyl)aminium hexachloroantimonate has been reported to afford polymers with an average molecular weight up to 150000. Both cyclobutanation and Diels-Alder polymers were obtained. " The reactivity of the phospine radical cation towards nucleophiles was studied. Tributylphosphine reacted with l,l-dimethyl-4,4-bipyridinium (methyl viologen, MV) in the presence of an alcohol or thiol (RXH X = O, S), which resulted in the gradual formation of the one-electron reduced form... 

To obtain an expression for the g-electron reduced form of L-appears in this equation, we first group the monomials in according... 

Superoxide (02 ) is the one electron reduced form of molecular oxygen. It reacts irreversibly and at close to the diffusion limit with nitric oxide (Huie and Padmaja, 1993) to form the powerful oxidant peroxynitrite anion (ONOO ). 

How the NOS isoforms compare to these related dual flavin enzymes is a matter of ongoing investigation. Characterization of the neuronal NOS revealed that it normally exists in its one-electron reduced form and maintains an air-stable, flavin semiquinone radical (Stuehr and Ikeda-Saito, 1992), as seen for NADPH-cytochrome P450 reductase. It is unknown which flavin in NOS contains the odd electron, although precedent argues that it probably resides on... 

For NP, not only the dissociation rate of the formally NO+ ligand is undetectable, but neither can the coordination of NO+ to [Fen(CN)5H20]3- be studied, because NO+ is very rapidly converted to nitrite in aqueous solutions (12,13). Similar considerations can be raised on the coordination ability of the one-electron reduced form of NO, namely NO- or HNO. These species are also highly reactive as precursors of N20 in aqueous media (13), and therefore the formation reactions, as in Eqs. (1) and (3), cannot be studied. 

Examples of this class of enzymes are glucose oxidase and D-amino acid oxidase The classification of the flavoproteins used here is that originally suggested which has been modified recently . In the author s own view the original classification has the advantage of being simple and yet quite useful whereas the new classification does not add to simplify and classify the rather complex picture of flavoprotein catalysis. Nevertheless, in flavoprotein oxidases, the 1,5-dihydroflavin is very reactive towards Oj. On the other hand, the two-electron reduced form of flavoprotein oxidases reacts slowly with pure one-electron acceptors, e.g. ferricyanide. That the two-electron transition is biologically favoured in these enzymes explains why they can react easily with sulfite... 

C(8) to be parallel with the nitrogen orbitals, thus providing evidence for the planarity of flavin semiquinones 2. More recent X-ray crystallographic data on the Cl. MP flavodoxin provide confirmatory data on the planarity of the iso-alloxazine ring in its one-electron reduced form. 

When O2 is the electron acceptor, the reduction can occur in either two-electron steps with FADH2 as reductant and H2O2 as the product or in a one-electron manner with 02 as the product. In the latter case, the reduced form of the flavin could be either FADH2 or FAD Recent studies on the reaction of O2 with reduced xanthine oxidase has shown that reoxidation of the six-electron reduced enzyme by O2 proceeds initially with two sequential two-electron steps to form two moles of H2O2 and the two-electron reduced form of the enzyme. Oxidation of the two-electron reduced form by O2 then proceeds via two-sequential one-electron steps to form two moles of O2 and oxidized enzyme. The differential rate of O2 release is suggestive of one mole arising from the one-electron... 

Two derivatives of nicotinamide (pyridine-3-carboxylic amide), one of the B2 vitamins, nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ), serve as redox coenzymes. Of the three heterocyclic ring systems found in these coenzymes, i.e. those of purine, ribose and pyridine, it is the pyridine portion that is reactive in redox reactions. Biologically, two oxidation states are important the oxidized form, NAD(P)+, and the 1,4-dihydro isomer of the two-electron reduced form, NAD(P)H (Scheme 1). Nicotinamide coenzymes interconvert between these two oxidation states in... 

The two oxidation states of (17) that are relevant in biopterin-dependent redox reactions are the four-electron and two-electron reduced forms, tetrahydrobiopterin (19) and p-quinonoid dihydrobiopterin (20), respectively. The oxidation state between these two, i.e. a radical, may also be relevant though it has not been detected as an intermediate in enzymatic reactions. Structurally, pteridines and flavins are rather similar and hence show similar chemical behavior in many respects. As a redox coenzyme, (19) is not encountered nearly as frequently as nicotinamides or flavins. It is, however, the cofactor of three very... 

Flavin coenzymes exist in three spectrally distinguishable oxidation states that account in part for their catalytic functions the yellow oxidized form, the red or blue one-electron reduced form, and the colorless two electron re-... 

Of course, the azoxybenzene anion radical can form a pair with the potassium cation during an electrode reaction too. This stabilizes the one-electron reduced form and hampers electron back-transfer. The electrode reaction is favored and proceeds further. Saveant (2001) gave complete thermodynamic reasons for this phenomenon. 

---