Oxidation, one electron

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

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. 

Nitric oxide formation from hydroxyurea requires a three-electron oxidation (Scheme 7.15) [114]. Treatment of hydroxyurea with a variety of chemical oxidants produces NO or NO-related species , including nitroxyl (HNO), and these reactions have recently been extensively reviewed [114]. Many of these reactions proceed either through the nitroxide radical (25) or a C-nitroso intermediate (26, Scheme 7.15) [114]. The remainder of the hydroxyurea molecule may decompose into formamide or carbon dioxide and ammonia, depending on the conditions and type of oxidant (one-electron vs. two electron) employed. 

Let ns direct onr attention to the difference between the anion-radicals 804 and COj". While the latter is a one-electron rednctant (see section 1.7.4), the former is a one-electron oxidant. One-electron transfer from a snbstrate to the snlfate radical mostly follows diffusion rates. For instance, rate constants of one-electron oxidation of benzene and anisole with SO4 are equal to 3 X 10 and 5 X 10 L mol s respectively (Goldstein and McNelis 1984). 

To understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation- and anion-radicals. One-electron addition to an organic molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to... 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

A typical voltammogram associated with an irreversible oxidative one-electron-transfer process is shown in Fig. 15. A number of differences from the reversible case may be noted. 

Figure 19.15. A Plausible Scheme for Oxygen Evolution from the Manganese Center. A possible partial structure for the manganese center is shown. The center is oxidized, one electron at a time, until two bound H2O molecules are linked to form a molecule of O2, which is then released from the center. A tyrosine residue (not shown) also participates in the coupled proton-electron transfer steps. The structures are designated Sq through S4 to indicate the number of electrons that have been removed.

Flavins (El) catalyze many different bioreactions of physiological importance [7-9]. Riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) have the 7,8-dimethyl isoalloxazine ring in common but differ in the side chain attached to NIO. With their five redox states, fully oxidized, one-electron reduced semiquinoid (F1H and F1 ), and fully reduced hydroquinone (FIH2 and F1H ), flavins are involved in one-electron and two-electron transfer reactions [10]. 

At V > 16,000 V s (Fig. lb), the voltammogram of 2CH3O presents a single reversible oxidation one-electron wave (E° = 1.20 V). Thus, it could be concluded that the initially produced radical reacts according to the first order chemical reaction through a stepwise EC mechanism. 

Fig. 12. Mole fractions of cytochrome C3 D. vulgaris, Miyazaki F, in the five macroscopic oxidation states as a function of the equilibrium potential (E). The open circles, crosses, triangles and squares represent the measured mole fractions of cytochrome C3 in the fully oxidized, one-electron, two-electron, three-electron and fully reduced states, respectively. The solid curves were calculated with the best-fitting parameters obtained on non-linear least squares fitting. (Reproduced from [126], courtesy of the publisher)...

Two dinickel(ll) paddlewheel compounds with amidinate (77) or guanidinate (78) ligands were synthesized and their one-electron oxidation was studied (Scheme 10.36) [45]. In the Ni2 compounds 79 and 80, the Ni-Ni distance was found to be 0.lA shorter than the corresponding dinickel(ll) precursor (2.476(1) A [77] vs 2.3703(4) A [79] 2.4280(5) A [78] vs 2.3298(6)A [80]) (Entries 4 and 5, Table 10.5). Based on this observation, it was proposed that upon oxidation one electron was removed from a metal-based a orbital to give an overall Ni-Ni bond order of 1/2 in the Nij species. Furthermore, a single-point calculation with no simplified solid-state structure of 79 suggested that the unpaired electron in the Ni2 species is in a metal-based a orbital, and this was also verified by a solution EPR spectrum. 

Scheme 10.1 Comparison of A/ for (a) noncatalytic and (b) catalytic oxidation (one-electron redox)...

Sequential oxidation of trialkylboranes takes place with trimethylamine N-oxide and oxidation of a chiral boronic acid by flavoenzyme cyclohexanone oxygenase proceeds with retention of configuration at the migrating centre in an analogous manner to peroxide oxidation. One electron oxidation of alkyltriphenyl borate anions leads to carbon-boron bond cleavage and the formation of free alkyl radicals. ... 

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