Two-electron redox reactions

The surface waves were simulated assuming the presence of two different functionalities, each undergoing a reversible two electron redox reaction. It was assumed that these surface functionalities were qulnones In Nernstlan equilibrium with the electrode potential before each DPV pulse. It was also assumed that the current during... 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

A number of binuclear iron complexes have also been isolated (with a neutral base attached to each metal in an axial position). The iron complexes undergo net two-electron redox reactions with dioxygen to yield products containing two identical low-spin Fe(n) metal sites superoxide or peroxide are simultaneously generated. Remarkably, the reaction can be partially reversed by removal of 02 from the system by, for example, flushing with N2 in a mixed aqueous solvent at 0°C. 

The prosthetic group associated with the molybdenum atom of the molybdenum cofactor found in most molybdenum-containing enzymes except nitrogenase (See Molybdenum Cofactor). Many of these enzymes catalyze two-electron redox reactions involving the net exchange of an oxygen atom between the substrate and water. In bacterial enzymes a nucleotide is linked to the phosphoryl group. 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Inasmuch as flavins can accommodate two electrons but possess a relatively stable one-electron intermediate, an obvious question which can be asked of any flavin-mediated two electron redox reaction is whether or not the mechanism includes the radical species on a direct line between reactants and products. The mere observation of semiquinones in a reaction mixture is not sufficient evidence for their intermediacy, due to the existence of side reactions such as comproportionation (F -I- FH2 2 FH-) which can generate radicals rapidly. Bruice has discussed this question from a physical-organic point of view and concluded that there must exist a competition between one-electron and two-electron processes and that the actual mechanism should be determined mainly by the free energy of formation of substrate radical and the nucleophilicity of the substrate. Bruice has analyzed a variety of systems which he feels should proceed via one-electron mechanisms among these are quinone and carbonyl group reduction by FH2... 

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

For the sake of completeness, a list of the general types of synthetically useful two-electron redox reactions is given in Table 2. It should be noted that combinations of the simple categories are feasible too, making possible combined carbon-skeleton construction and functionalization in a single step. 

These studies show that Cu(III)-peptide complexes have relatively low electrode potentials and suggest that Cu(III) may be a far more common oxidation state than had previously been thought possible. Furthermore, the decomposition reactions of Cu(III)-peptides indicate that two-electron transitions to give Cu(I) species are possible. Two-electron redox reactions in biological systems are intriguing because high energy, free radical intermediates are avoided. However, as yet we know very little about possible Cu(I) complexes. This oxidation state is poorly characterized in aqueous solution, and studies with various model complexes are needed. 

In conclusion, the [NiS] mediated formation of thioesters from alkyl, CO, and thiol groups lends support to an acetyl-CoA formation pathway that comprises CO insertion into a Ni Me and an intramolecular S -C bond formation between nickel-bound acyl groups and thiolate ligands. These reactions are favored at square-planar nickel complexes that enable two-electron redox reactions and readily add fifth ligands. 

As shown in Figure 7.4, flavins can undergo a one-electron reduction to the semiquinone radical or a two-electron reduction to dihydroflavin. This means that flavins can act as intermediates between obligatory two-electron redox reactions involving nicotinamide nucleotides (Section 8.4.1) and obligatory one-electron reactions involving cytochromes, iron-sulfur proteins, and ubiquinone (Section 14.6). 

The thiazole ring is synthesized biochemically by enzymatic post-translational modifications of cysteine-containing peptides. Heterocyclization between cysteine side-chains and neighboring carbonyl groups produces dihydrohetero-aromatic thiazolines as initial products followed by a two-electron redox reaction yielding either thiazole or thiazo-lidine rings (Scheme 104). All three oxidation states are seen in natural products. 

Molybdenum hydroxylases (i.e., AO and XO) are flavoproteins that contain in addition to a FAD, a pterine cofactor coordinated to a molybdenum atom, and an iron sulfur center for their catalytic activity. They catalyze the two-electron oxidation of substrates with transfer to molecular oxygen to produce H2O2, and insert an atom of oxygen from water into a wide range of N-hctcrocycies and aldehydes via two-electron redox reaction as shown in equation 1.4 ... 

The third recurring structural motif with non-amino-acid components found in metalloproteins is the metal-dithiolene unit found in molybdenum- and tungsten-containing oxidases or dehydrogenases (see Chapter 8.18). The dithiolene is typically a pterin derivative (often with phosphate and/or nucleotide appendages) and coordinated to Mo or W in a 1 1 or 2 1 stoichiometry (Figure 7)." These units usually function in two-electron redox reactions (cf 0x0 transfer ), shuttling between... 

In short, these examples of Mo(vi) reactions with tetrahydropterins were described by one research group as simple ligand substitution and by another research group as two-electron redox reactions (Scheme 2.8). 

Mechanism. Since its discovery in 1959 (2), galactose oxidase has attracted considerable interest in the scientific community because of its enigmatic catalytic mechanism. Various aspects of early research have been discussed in two excellent reviews (J, 4), The most puzzling property of this enzyme has been the ability of the monomeric protein to carry out two-electron redox reactions without any apparent involvement of a cofactor. Although a number of elegant explanations of this phenomena have been proposed (3, 5), the mechanism still remains elusive. On the... 

As mentioned previously, the vast majority of pyranopterin Mo enzymes catalyze two-electron redox reactions that are coupled to the formal transfer of an oxygen atom between the Mo center and the substrate. As such. 

We will now turn to XAS studies of the enzymes themselves. The molybdenum enzymes are divided into three distinct but related families, one of which can include the tungsten enzymes. With a small number of notable exceptions, these Mo and W enzymes catalyze two-electron redox reactions in which an oxygen atom, originating in water, is transferred between the metal and substrate. Nitrogenase catalyzes the six-electron reduction of N2 in a biochemically unique process, and will be considered separately. We will discuss selected examples of each family, but our focus will be on the contribution that XAS has made to our understanding of these systems. 

Platinum(II) catalysis in these systems is believed to proceed by a mechanism that involves an activated bridged complex and a two-electron redox reaction. Such a process is represented by the reaction schemes (30) to (33). In reaction... 

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