Electron redox chemistry

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

Similarities exist between the chemical characteristics of the actinides and those of the lanthanides. The metal ions are generally considered to be relatively hard Lewis acids, susceptible to complexation by hard (i.e., first row donor atom) ligands and to hydrolysis. Both actinide and lanthanide ions are affected by the lanthanide contraction, resulting in a contraction of ionic radius and an increasing reluctance to exhibit higher oxidation states later in the series. Most species are paramagnetic, although the electron spin-nuclear spin relaxation times often permit observation of NMR spectra, and disfavor observation of ESR spectra except at low temperatures. The elements display more than one accessible oxidation state, and one-electron redox chemistry is common. 

Inorganic Models for Two-Electron Redox Chemistry in Biological Systems... 

For two-electron redox chemistry the quantity of interest is the free energy change for the disproportionation of the one-electron intermediate (AGdisp, eq 3). This quantity equals the energy difference between the dianion and monoanion (ET2) minus the energy difference between the monoanion and neutral (ETl) AGdisp = AET = ET2 - ETl. AGdisp < 0 indicates that the disproportionation of A is spontaneous and that two-electron transfer will ensue via inversion of potentials Ei° and E2°. ... 

The arsole (34) has been studied by cyclic voltametry (Pt, MeCN, 0.1 M Bu4N" C104 ). It undergoes reversible one-electron redox chemistry with the radical anion (36) (Eq = —1.77 V vs the saturated calomel electrode (SCE)) <83JOM(249)335>. Cyclovoltammetry of the l,T-diarsaferrocene (41) gives redox potentials for the arsole/radical cation of 1/2 = 0.730+ 0.015 V (—38°C in... 

Cys-11, Asp-14, Cys-17, and Cys-56. One consequence of the partial noncysteinyl ligation to the 4Fe cluster is that the Fe atom that would be normally coordinated by a cysteinyl sulfur is more easily removed when coordinated by a carboxyl group of Asp. This means that the ferredoxin can be quantitatively converted to the 3Fe form by chemical oxidation that specifically removes the Asp-coordinated Fe atom. The P. furiosus protein also contains two other Cys residues, at positions 21 and 48 (Fig. 1), and these are redox active and can form a disulfide bond. The 4Fe center is also redox active, and like most other clusters of this type, it undergoes one-electron redox chemistry with a low midpoint potential (—370 mV, 23°). Hence, P. furiosus ferredoxin can exist in four formal redox states, where the cluster is either reduced (Fdred) or oxidized (Fdox), and the two Cys residues 21 and 48 are either in the form of a disulfide (form A) or exist as free thiols (form B). These four distinct redox states of the protein are stable and can be prepared at room temperature. The lack of rapid equilibration between the different states appears to be due to the high stability of the protein. For example, the wild-type ferredoxin exhibits limited degradation even when incubated at 95° for many days. 

The ability of flavins to engage in both 1-electron and 2-electron redox chemistry is key to their functions in electron transfer. In POR, they are an essential intermediate between NADPH, a two-electron donor, and the heme of P450, a one-electron acceptor. Furthermore, utihzation of two flavins, located in separate domains, provides a mechanism for control of the kinetics of electron transfer by regulating the distance between, and the relative orientation of, the two flavins. The flavin cofactors can exist as the oxidized (ox), one-electron reduced semiquinone (sq), and two-electron, fully reduced (red) forms (Fig. 2.3). 

The functional diversity of flavoproteins results from the broad range of redox potentials that are accessible to the flavin cofactors, as well as their ability to switch between one or two electron redox chemistry. In solution, flavins are found in equilibrium between the oxidized, reduced and the semi-quinone radical forms, and have a redox potential of about —210 mV (versus the normal hydrogen electrode) at neutral pH. However, in the protein-bound form, the redox equilibrium can be shifted and the redox potential may span up to 600 mV (Massey 2000). This arises from the fact that flavin-protein interactions may engage a number of non-covalent interactions such as 7i-stacking, hydrophobic effects, hydrogen bonding and electrostatic interactions, which will ultimately determine the flavin redox potential. 

RE one-electron redox chemistry is now emerging, driven by the highly reducing nature of the RE + — RE + + e couple (Table 1), and some examples of one-electron redox chemistry in organometallic RE chemistry are described below and illustrated in Figure 17 in order to give a flavor of the area and to demonstrate its breadth and potential capability. 

Ni-Ni) = 3.397(1) A. This compares with an Ni-Ni bond distance of < (Ni-Ni) = 2.506 A in the neutral complex, Ni2(/x-PPh2)2(GO)4. The cyclic voltammetry of the dianionic complex [Ni2(/x-PPh2)2(GO)4] shows a reversible oxidation at —1.5V (versus FeGp2 ) and a quasi-reversible oxidation at — I.IV. This established a chemically reversible two-electron redox chemistry connecting the dianionic and neutral complexes (Equation (18)). 

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