Nonredox reactions

Mechanistic Speculations. It is doubtful that the iodosylbenzene monomer, OlPh, is a stable entity, even when associated with a metal center. Solid iodosylbenzene is an insoluble polymer linked by -O-I-O- bonds (40). Typical nonredox reactions of iodosylbenzene give products with three bonds to iodine, e.g. (MeO)2lPh or the series of i-oxo dimer complexes which have recently been isolated and characterized (41). 

Biochemically, the niacin coenzymes function as cofactors for a number of dehydrogenases due to their oxidation-reduction capabilities (19,93,96). They are involved in the metabolism of carbohydrates, fatty acids, and amino acids. Nicotinamide can also participate in nonredox reactions, such as the ribosylation of ADP. 

The beef heart enzyme (M, = 80,000) (117) is a component of the citric acid cycle. Its active form contains one [4Fe-4S] cluster. Although such a cluster is normally associated with electron transfer, the enzyme catalyzes the nonredox reaction of citrate-isocitrate interconversion via a dehydration-hydration pathway. The current state of understanding of cluster structures and reactions of beef heart aconitase has been thoroughly reviewed by Emptage (130). When isolated aerobically, aconitase is inactive and contains one [3Fe-4S] cluster. Upon incubation of the reduced protein with Fe(ll), the fully active enzyme is generated. When a 3-Fe center is reduced to [3Fe-4S]°, Reaction 10 builds a 4-Fe cluster in a nonredox process. The Mossbauer spectra in Fig. 8 address the question of subsite specificity in this reaction of aconitase (124). If the externally supplied iron is Fe, the resultant spectrum reveals the intrinsic (original) Fe atoms... 

The use of a mixed-valent, dinuclear iron site, similar to those in hemerythrin and ribonucleotide reductase,to catalyze a nonredox reaction such as phosphate ester hydrolysis is novel and unexpected for a variant of the familiar oxo(hydroxo)-bridged diiron center. In contrast to the general agreement that exists regarding the spectroscopic and physical properties of the PAPs, their kinetics properties and especially their mechanism of action remain controversial. Much of the disagreement stems from the different pH dependences of the catalytic activity of BSPAP and Uf, which is due to the fact that the former is isolated in a proteolytically activated form while the latter is not. Proteolysis results in a substantial increase in optimal pH in addition to an increase in catalytic activity at the optimal pH. "" Current data suggest that many of the spectroscopic studies described in the literature were performed on a catalytically inactive form of the enzyme. As a result, the roles of the trivalent and divalent metal ions in catalysis and in particular the identity of the nucleophilic hydroxide that directly attacks the phosphate ester remain unresolved. 

Equation 5.14 represents the redox reaction that takes place when mercury(H) oxide (HgO) is heated. Mercury metal (Hg) and oxygen gas (O2) are the products. This reaction was used by Joseph Priestley in 1774 when he discovered oxygen. Equation 5.15 represents a nonredox reaction used commercially to produce lime (CaO) by heating limestone (CaCOs) to a high temperature. The decomposition of H2O2 represented by Figure 5.3 is shown in Figure 5.4. 

Single-replacement reactions, sometimes called substitution reactions, are always redox reactions. One element reacts with a compound and displaces another element from the compound. Double-replacement reactions, also called metathesis reactions, are always nonredox reactions. They can be recognized by their partner-swapping characteristics. 

Classify each of the reactions represented by the following equations, first as a redox or nomedox reaction. Then further classify each redox reaction as a decomposition, single-replacement, or combination reaction, and each nonredox reaction as a decomposition, double-replacement, or combination reaction ... 

The ultimate products are FeO(OH) and H2O, but they are formed by a nonredox reaction. The hydrated FeO(OH) is what we know as rust.)... 

The isomerization of 1-butene to cis- and trans- 2-butene onPd/C/Nafion and Pd-Ru/Nafion electrodes is one of the most remarkable and astonishing electrochemical promotion studies which has appeared in the literature.39,40 Smotkin and coworkers39,40 were investigating the electrocatalytic reduction of 1-butene to butane on high surface area Pd/C and Pd-Ru cathodes deposited on Nafion 117 when, to their great surprise, they observed at slightly negative overpotentials (Fig. 9.31) the massive production of 1-butene isomerization, rather than reduction, products, i.e. cis- and trans-2-butenes. This is extremely important as it shows that electrochemical promotion can be used also to enhance nonredox catalytic reactions such as isomerization processes. 

CODH/ACS is an extremely oxygen-sensitive protein that has been found in anaerobic microbes. It also is one of the three known nickel iron-sulfur proteins. Some authors would consider that there are only two, since the CODH and ACS activities are tightly linked in many organisms. However, there is strong evidence that the ACS and CODH activities are associated with different protein subunits and the reactions that the two enzymes catalyze are quite different. CODH catalyzes a redox reaction and ACS catalyzes the nonredox condensation of a methyl group, a carbonyl group, and an organic thiol (coenzyme A). 

However, a more discouraged fact is that benzoquinone accelerated SOD-inhibitable part of cytochrome c reduction, which is usually considered as a reliable proof of superoxide formation. Such a phenomenon has been first shown by Winterbourn [7], who suggested that SOD may shift the equilibrium of Reaction (4) to the right even for nonredox cycling quinones. The artificial enhancement of superoxide production by SOD in the presence of quinones was demonstrated in the experiments with lucigenin-amplified CL, in which benzoquinone was inhibitory [6],... 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

If a metal complex can be reduced by superoxide and if its reduced form can be oxidized by superoxide, both at rates competitive with superoxide disproportionation, the complex can probably act as an SOD by Mechanism I. Mechanism II has been proposed to account for the apparent catalysis of superoxide disproportionation by Lewis acidic nonredox-active metal ions under certain conditions. However, this mechanism should probably be considered possible for redox metal ions and the SOD enzymes as well. It is difficult to distinguish the two mechanisms for redox-active metal ions and the SOD enzymes unless the reduced form of the catalyst is observed directly as an intermediate in the reaction. So far it has not been possible to observe this intermediate in the SOD enzymes or the metal complexes. 

This is a hydrolytic nonredox process, and for some time it was thought that aconitase was a simple Fe + protein wherein the ferrous iron was involved in the Lewis-acid function of facilitating the hydrolytic reaction. Indeed, aconitase is inactive when isolated from mitochondria, and requires the addition of Fe to achieve activity. 

---