Semiquinonate

For most color photographic systems, development is the rate determining step, and within that step the formation of semiquinone is the slow process (37). The fate of the highly reactive QDI is deterrnined by the relative rates of a number of competing processes (38). The desired outcome is reaction with ionized coupler to produce dye (eq. 3). Typically, the second-order rate constant for this process with ionized coupler is about 10 to 10 ... 

The ionized developers are then capable of diffusing. Transfer of an electron reduces the silver and generates the semiquinone ion radical of the auxiUary developer (eq. 10). In turn, a dye developer molecule of the adjacent layer transfers an electron to the semiquinone, returning the auxiUary developer to its original state and leaving the dye developer in the semiquinone state (eq. 11). Further oxidation of the semiquinone leads to the quinone state of the dye developer. 

Methylphenazinium methyl sulfate [299-11-6] M 306.3, m 155-157 (198°dec by rapid heating). It forms yellow prisms from EtOH (charcoal). Solubility in H2O at 20° is 10%. In the presence of aqueous KI it forms a semiquinone which crystallises as blue leaflets from EtOH. [Wieland and Roseen Chem Ber 48 1117 1913, Voriskova Collect Czech Chem Commun 12 607 1947, Biilow Chem Ber 57 1431 1924.]... 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Oxidation of this UQHg occurs in two steps. First, an electron from UQHg is transferred to the Rieske protein and then to cytochrome C. This releases two to the cytosol and leaves UQ , a semiquinone anion form of UQ, at... 

The second half of the cycle (Figure 21.12b) is similar to the first half, with a second molecule of UQHg oxidized at the Q site, one electron being passed to cytochrome C and the other transferred to heme bj and then to heme bfj. In this latter half of the Q cycle, however, the bn electron is transferred to the semiquinone anion, UQ , at the Q site. With the addition of two from... 

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

Kurfuerst, M., Ghisla, S., and Hastings, J. W. (1986). Bacterial luciferase intermediates the neutral flavin semiquinone, its reaction with superoxide, and the flavin 4a-hydroxide. Method. Enzymol. 133 140-149. 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

Transition metal complexes of o-benzoquinone, o-semiquinone and catecholate ligands. C. G. Pier-pontand R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45-87 (72). 

A proof for the formation of alkyl radicals was found by their addition to the aci-nitromethane anion (CH2=N02 ) and by their reaction with p-benzoquinone to give the optically active nitroalkane radical-anion and the semiquinone radicals, respectively. In the case of di-r-butyl sulfoxide the f-butyl radical was observed directly by its absorption spectra. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

Peroxides oxidize N,N-DPDD to Wurster s red, a semiquinone diimine derivative [4]. Similarly Wurster s red is also produced from N,N-DPDD by reaction with halogen-containing substances in the presence of sodium ethylate and UV light and by reaction with the chlorinated triazines produced by reaction with chlorine [7]. 

Peroxides oxidize TPDD to Wurster s blue, a product with a semiquinone diimine structure [1]. Similarly Wurster s blue is also produced from TPDD by reaction with halogen-containing substances produced by the reaction of aromatic amines and triazines with chlorine gas. 

Hence, the HMO method is inherently incapable of giving any picture for disproportionation equilibria, the reason being the neglect of electronic repulsion. In the SCF treatment, it is convenient to assume that all three systems in the redox equilibrium are built up from the same MO s, the only difference being the number of n electrons. Adopting this convention and denoting the oxidized form, radical, and reduced form as Ox, Sem (semiquinone), and Red, we can write... 

Therefore, the Rieske protein has to switch between the positional states during turnover. The following reaction scheme combines the movement of the catalytic domain of the Rieske protein with the redox-dependent stabilization of the intermediate semiquinone (Fig. 19) (42) ... 

After the electron transfer (step 3), the resulting semiquinone is tightly hound to the reduced Rieske cluster in the b positional state (D) in this state, the semiquinone intermediate will he stabilized (116). 

After the second electron transfer from semiquinone to heme 6l (step 4), the interaction between the Rieske cluster and the resulting quinone is weakened so that the reduced Rieske protein can now occupy the preferred ci positional state (E), which allows rapid electron transfer from the Rieske cluster to heme Ci (step 5). 

How is the affinity of Rieske clusters for the binding of ligands (e.g., semiquinone) controlled, and what is the role of the exposed histidine residues ... 

The addition of sulfite to APS reductase results in changes of the flavin visible spectrum that are explained by the formation of an adduct between the sulfite and the FAD group (135). Addition of AMP to the as-isolated enzyme causes no change in the spectroscopic properties. Addition of AMP to the sulfite-reacted enzyme causes the reduction of center I. However, the formation of a semiquinone signal has never been observed either by EPR or visible spectroscopies. Also, Mossbauer and EPR data indicate that AMP closely interacts with center I (139). 

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