Rate constants-semiquinones

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 ... 

Enthalpies, Activation Energies, and Rate Constants of the Reduction of Peroxides by Ketyl, Semiquinone, and Hydroperoxyl Radicals Calculated by the I PM Model [68]... 

Disproportionation of sterically nonhindered semiquinone radicals occurs with diffusion rate constants. On the other hand, sterically hindered semiquinone radicals react by several orders of magnitude more slowly (see Table 15.11). 

Rate Constants of Disproportionation of Semiquinone Radicals at 298 K—continued... 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

Peroxyl radicals are not only ones, which are able to react with ubihydroquinones. Poderoso et al. [245] showed that the short-chain ubihydroquinones Q0 and Q2 are oxidized by nitric oxide with the rate constants of 0.49 x 104 and 1.6x 1041 mol-1 s 1, respectively. The reaction apparently proceeded by one-electron transfer mechanism because the formation of intermediate semiquinone radicals has been registered. 

A flash photolysis method has been developed that prepares the MoVI-Fe11 state and thus allows the rate constants k3 and k 3 to be measured. Solutions containing 5-deazariboflavin, semicarbazide, and sulfite oxidase are subjected to 555 nm flash photolysis. The deazariboflavin is excited to a triplet state, which is then reduced by semicarbazide to form the 5-deazariboflavin semiquinone radical. This radical is then rapidly oxidized back to its parent species through the one-electron reduction of sulfite oxidase. 

They were found not to react with BESOD, the rate constant was estimated to be < 10 M s , if there was a reaction at all The reaction of BESOD was also investigated with several other radicals generated by pulse radiolysis. With the semiquinone of riboflavin 5 -phosphate no reaction was detected. The semiquinone of 9,10-anthraquinone-2-sulfonate and the radical anion of 4-nitroacetophenone converted the enzyme into an unreactive form... 

In Table 3 is a compilation of some of the rate constants which have been determined for various flavin semiquinone reactions. Note that whereas many of these reactions are quite rapid (at or near diffusion control), others are relatively slow. Of particular interest (see below) is the reaction of Oj with FH-, which is too slow to measure (due to the competing disproportionation). Note also that side chain and electrostatic repulsion effects can be seen in some of these reactions. 

A recent study has shown that solvent dielectric constant exerts a considerable influence on flavin semiquinone reaction rates. A biphasic dependence was observed, with the rate constant being virtually independent of dielectric at low values and sharply increasing at high values. This was interpreted in terms of a change in... 

Table 3. Rate constants for flavin semiquinone reactions...

The reaction of ferricyanide with the semiquinone forms of flavodoxins is more rapid than is the oxygen reaction. Second-order rate constants with A. vinelandii, C. pasteurianum and D. vulgaris flavodoxins are 8.3 x 10 M" s , 1.1 x 10 " s", and 8.3 x lO M s" respectively This is to be compared with a value of... 

Verdazyls (111) can also transfer an electron to o -quinones to give the verdazylium cation (113) and a semiquinone anion (114) (80IZV2785), or to tetranitromethane to give the cation (113) and the tetranitromethane anion radical (115) (74MI22100). Rate constants and activation parameters for the electron transfer from triphenylverdazyl to tetracyanoethylene have been determined by Soviet chemists (79ZOR2344). 

According to recent data, the property of dithionite as an electron donor for nitrogenase is different from that of the natural donor flavodoxin (Burgess and Lowe, 1996). Flavodoxin from Azotobacter vinelandii has the redox potential equal to -0.515 V for the reversible transition between the semiquinone and hydroquinone forms of flavodoxin. Unlike dithionite, flavodoxin can reversibly reduce the [Fe4S4]+l cluster Av2 by one electron to the [Fe4S4]° state in which all iron ions exist in the ferrous form. It is assumed that, under natural conditions, two electrons can transfer from Av2 to Avl. Flavodoxin reduces both Av2 bound to Avl and free Av2 in a solution. The apparent rate constants of these reactions are 400 s 1 and > 1000 s"1, respectively (Duyvis et al. 1998). 

Micellar catalysis of the photobleaching of riboflavin and riboflavin-5-phosphate was investigated in a recent e.s.r. study of the effects of polyoxyethylene(20) sorbitan monooleate and sodium dodecyl sulfate on the rate of formation and decay of an intermediate semiquinone radical (Kowarski, 1969). In the photodegradation of riboflavin-6-phosphate, both the rate of formation of the semiquinone radical and the rate constant for its decay were appreciably enhanced by the anionic and the non-ionic surfactant (Table 19). Similarly, the catalysis of the photobleaching of riboflavin by sodium dodecyl sulfate was found to be related to an increased rate of formation of the semiquinone radical. Hence, the micellar catalysis of the photodegradation of riboflavin and riboflavin-5-phosphate is the consequence of a combined effect of an increased rate of semiquinone radical formation and an accelerated rate of its decay (Kowarski, 1969). 

Inasmuch as T+ continously disappears from the system, being consumed in an acid-irreversible hydrolysis, as shown in Section IV, G, it is not possible to derive K from S+ concentration measurements. Therefore, the comparison was restricted to the rate constants of semiquinone decay. This reaction has second-order kinetics and the rate constants may be correlated with the influence of the substituents (see also Section IV,D). 

FIGURE 9. Marcus analysis of electron transfer reactions between MADH and amicyanin. Values of kn Twere determined for the reactions of different redox forms of MADH with amicyanin shown in Figure 8 0-quinol (A), O-semiquinone ( ), N-semiquinone ( ). Rate constants were also obtained for the reverse reactions of the O-quinol (A) and O-semiquinone ( ). The solid line represent fits to Eqs. 4 and 5, which are superimposible. 

FIGURE 3. The Catalytic Cycle for Flavocytochrome 62 F, flavin H, heme Cyt c, cytochrome c. Electrons are shown as hlack dots and are used to indicate the two-electron reduced flavin (hydroquinone), F with two dots one-electron reduced flavin (semiquinone), F with one dot reduced heme, H with one dot reduced cytochrome c, Cyt c with one dot. The rate constants shown are for S. cerevisiae flavocytochrome hi at 25 C, pH 7.5, I = O.IOM. The whole catalytic cycle turns over at approximately 100 s ". The details of the cycle are described in the main text. 

The reaction of H0 with 9,10-anthraquinone in MeCN produces an adduct (stable at -20 which reacts further at room temperature to yield the semiquinone anion radical (AQ ). The equilibrium constants for the formation of the adducts and the rate constants for the reaction of the adduct with a second quinone molecule are given in Table 18 (see Scheme 18). 

Here, k, ifcb, and k b correspond to the relaxation rates due to the alkyl radical (R ), and the ketyl or semiquinone radicals (XHCO ), and their dipole-dipole interaction, respectively. These rate constants are given by... 

The electron transfer reactivity of Ceo has been compared with that of p-benzoquinone which has a slightly more negative one-electron reduction potential ( °red relative to the SCE = -0.50 V) [44] than Ceo (E°red —0.43 V). The rate constants of electron transfer from Cgo and Ceo to electron acceptors such as allyl halides and manganese(III) dodecaphenylporphyrin [45] correlate well with those from semiquinone radical anions and their derivatives. Linear correlations are obtained between logarithms of rate constants and the oxidation potentials of... 

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