Redox-active cobalt

Wu JH, Winn PJ, Ferenczy GG, Reynolds CA (1999) Solute polarization and the design of cobalt complexes as redox-active therapeutic agents. Int J Quant Chem 73(2) 229-236... 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

Two final concerns must be addressed surface oxidation state and temperature dependence. Whenever one deposits a redox-active species on a metal surface, the oxidation state of the adsorbate (and therefore the OMTS bands) may change. One example is the adsorption of a biaxially substituted dicyano cobalt phthalocy-anine salt, MCoPc(CN)2 (where M = K or Cs), on gold to form the reduced species CoPc [111]. A second example is provided by the adsorption of TCNE on gold, silver, and copper. In that order, the charge state of TCNE on the surface ranges from 0 to 3, and the OMTS reflects these changes. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Enzymes containing amino acid radicals are generally associated with transition metal ions—typically of iron, manganese, cobalt, or copper. In some instances, the metal is absent it is apparently replaced by redox-active organic cofactors such as S -adenosylmethionine or flavins. Functionally, their role is analogous to that of the metal ion in metalloproteins. 

Table 1 lists some of the binding constants and rate constants measured for the reaction of CO2 with redox-active molecules. Various techniques have been used to measure these constants including cyclic voltammetry, pulsed radiolysis, and bulk electrolysis followed by UV-visible spectral measurements. The binding constants span an enormous range from less than 1 to 10 M [13-17]. Co(I) and Ni(I) macrocyclic complexes have been studied in some detail [13-16]. For the cobalt complexes, the CO2 binding constants K) and second-order rate constants for CO2 binding (kf) are largely determined by the Co(II/I) reduction potentials... 

Walker, G.W., Geue, R.J., Sargeson, A.M. and Behm, C.A. (2003) Surface-active cobalt cage complexes synthesis, surface chemistry, biological activity, and redox properties, J. Chem. Soc., Dalton Trans. 2992-3001. 

The superoxide anion radical and hydrogen peroxide are not particularly harmful to cells. It is the product of hydrogen peroxide decomposition, the hydroxyl radical (HO ), that is responsible for most of the cytotoxicity of oxygen radicals. The reaction can be catalyzed by several transition metals, including copper, manganese, cobalt, and iron, of which iron is the most abundant in the human body (Reaction 2 also called the Fenton reaction). To avoid iron-catalyzed reactions, iron is transported and stored chiefly as Fe(III), although redox active iron can be formed in oxidative reactions, and Fe(III) can be reduced by semiquinone radicals (Reaction 3). 

Only transition-metal ions are redox-active and hence are possible sources of radicals. Ions such as Mg2+, Ca2+, Zn2+ etc. are not important in this context. By far the most important is iron, with copper, molybdenum, cobalt and nickel also participating on a more minor scale. Of course, any transition metal ion may be ingested, and hence may be a fortuitous source of radical formation and damage. We focus attention on iron for illustrative purposes. 

Redox-active metals are the initiators of perhaps greatest importance for lipid oxidation in oils, foods, and biological systems because they are ubiquitous and active in many forms, and trace quantities (electron transfers appear to be active catalysts these include cobalt, iron, copper, manganese, magnesium, and... 

Studies of redox-active metallointercalation agents in the presence of dsDNA have been done with solutions containing the redox complexes of cobalt, iron and osmium [64,68,72,95]. Osmium tetroxide complexes with tertiary amines (Os, L) have been used as a chemical probe of DNA structure. The simultaneous determination, based on a sufficient peak separation on the potential scale of (Os, L)-DNA adducts and free (Os, L), was obtained by Fojta et al. [96] using a p5Tolytic graphite electrode. 

In direct analogy with the behavior found in solution phase, redox active irreversibly adsorbed macrocycles involving iron and cobalt centers also display pH dependent voltammetric features, as shown in Figure 3.37 for FeTsPc (solid circles) and CoTsPc (open circles) [64]. 

A cobalt complex 8 containing a redox active tetradentate bis-iminopyridine framework has been reported to support a water reduction catalyst, with activities of observed rate constant, of 10 M s derived from voltammetry measurements (Scheme 3) [16]. Ligand-centered reduction of the coordinated imine function has been proposed as the first electrocatalytic step followed by protonation. Notably, this compound was shown to operate even under basic conditions at pH 8 (buffer) to give 10 liter of H (mol catalyst" h" ) albeit with a modest Faradaic efficiency of only 60%. 

Related approaches were recently employed in mediating (stoichiometric) oxidative addition and reductive elimination reactions at cobalt centers bearing redox active ligands [7b, 7c, 25,34]. These reactions also proceed without a change in the d-electron configuration of the metal. 

One merit of this polymer preparation method is that it allows creation of heterometal polymer chains with the intended sequence.93 The stepwise formation of a heterometal double-layer film [IColFe] was monitored by cyclic voltammetry during film construction (Figure 9.16a). When the bis(tpy)iron complex units were connected to the already prepared bis(tpy)cobalt complex layer, the redox activity of the Fenl/Fen couple appeared without... 

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