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Die UV/VIS-Spektren des Redoxsystems 1m, (m = 1–5) zeigen fur 1Red und 1ox ahnlichen Verlauf und entsprechen dem Polyentyp. Die Spektren von 1sem unterscheiden sich von diesen stark; die beiden Hauptmaxima wandern pro Vinylengruppe um 110—150 nm nach langeren Wellen. Auch fur andere Radikalionen vom Violentyp gilt λmax ≈m. Sie verhalten sich wie Cyanine, obwohl laut HMO-Rechnungen Ubergange mit Konfigurationswechselwirkung vorliegen. Die Spektren der Azaderivate von 1 werden diskutiert. Eine Methode zur Bestimmung der Semichinonbildungskonstanten aus den spektralen Daten wird beschrieben. Two-step Redox Systems, XIX. – UV/VIS Spectroscopy of Vinylogous and Azavinylogous Redox Systems with 1,3,3-Trimethylindolenine as End Group and a Colour Rule for Violenes The UV/VIS spectra of the redox system 1m, (m = 1 – 5 ) are similar for 1Red and 1ox and display polyene type character. The spectra of 1sem differ sharply. The two main maxima are shifted bathochromically 110–150 nm per vinylene group. Since λmax≈ m is valid for other radical cations of the violene type, they therefore show the behaviour of cyanines, although HMO calculations indicate configuration interaction for the highest electronic transitions. The spectra of the aza derivatives of 1 are discussed and a method for the determination of semiquinone formation constants from spectral data is given.

1. Reversible transfer of a proton from 3,6-di-tert-butyl-2-hydroxyphenoxyl to triethylamine, accompanied by the formation of a semiquinone anion radical, was detected. 2. The equilibrium constant and elementary rate constants of the mutual transition between the anion radical and the radical were determined. 3. Intramolecular migration of a cation between the oxygen atoms of the anion radical was detected and investigated. It was hypothesized that this migration is reduced to an oscillation of the hydrogen bonds between the proton of the triethylammonium cation and the oxygen atoms of the semiquinone.

The role of quinones in photobiological functions and in biochemical electron-transport processes has been well recognized1. The photoreduction of simple 1,4-benzoquinones in solution has been extensively studied and the mechanism can be represented by the following reactions: The intermediate semiquinone radical QH produced by the triplet quinone abstracting a hydrogen atom from the donor solvent RH, has been observed by e.s.r. in CIDEP studies2. In some hydroxylic solvents, it has been demonstrated3–5 the neutral semiquinone radicals convert rapidly to radical anions in alkaline solutions

The cell membrane-associated respiratory electron transport chain of Neisseria gonorrhoeae was examined using electron paramagnetic spectroscopy (EPR) at liquid helium temperatures and optical spectroscopy at liquid nitrogen and room temperatures. EPR spectra of dithionite-reduced particles indicated the presence of centers N-1 and N-3 in the site I region of the respiratory chain, whereas reduction with succinate revealed the existence of center S-1 from the succinate cytochrome c reductase segment. Free radical(s) resembling that due to falvin semiquinone were observed with both reductants. Low temperature (77 K) optical difference spectra indicated the presence of cytochromes with alpha band maxima at 549, 557, and 562. Bands at 567, 535, and 417 nm, characteristic of the CO compound of cytochrome o, were also identified. Cytochromes a1 and a3 were not detected; however, a broad but weak absorbance with an alpha band maximun at 600 nm and a Soret shoulder at 440 nm was observed. Hence the respiratory chain of N. gonorrhoeae appears to contain several nonheme iron centers, cytochrome c, two b cytochromes, with cytochrome o which probably serves as the terminal oxidase.

The complete absorption difference spectrum of the primary electron acceptor of Photosystem II has been measured at room temperature in subchloroplast fragments prepared with deoxycholate. The shape and amplitude of the spectrum indicate that the primary reaction involves the reduction of one bound plastoquinone molecule per reaction center to its semiquinone anion. In addition two small absorbance band shifts occur near 545 (C550) and 685 nm, which may be due to an influence of the semiquinone on the absorption spectrum of a reaction center pigment.

The oxidation of benzidine, o,o ′-tolidine, and o,o ′-dianisidine by the acetate complex of trivalent cobalt was studied chiefly in glacial acetic acid medium, considering the possible formation of semiquinone and quinonediimine compounds. The effect of uv light on the solutions of these compounds in glacial acetic acid and in benzene was followed spectrophotometrically and chromatographically and compared with the effect of Co(III) acetate. Certain possibilities explaining the course of the studied reactions are discussed.

The rate of reaction between superoxide anion (O2) and 1,2-dihydroxybenzene-3,5-disulfonic acid (tiron) was measured with pulse radiolysis-generated O2. A kinetic spectrophotometric method utilizing competition between p-benzoquinone and tiron for O2 was employed. In this system, the known rate of reduction of p-benzoquinone was compared with the rate of oxidation of tiron to the semiquinone. From the concentration dependence of the rate of tiron oxidation, the absolute second order rate constant for the reaction was determined to be 5x10-8 M-minus1-s-minus1. Ascorbate reduced O2 to hydrogen peroxide with a rate constant of 10-8 M-minus1-s-minus1 as determined by the same method. The tiron semiquinone may be used as an indicator free radical for the formation of superoxide anion in biological systems because of the rapid rate of oxidation of the catechol by O2 compared to the rate of O2 formation is most enzymatic systems. Tiron oxidation was used to follow the formation of superoxide anion in swollen chloroplasts. The chloroplasts photochemically reduced molecular oxygen which was further reduced to hydrogen peroxide by tiron. Tiron oxidation specifically required O2 since O2 was consumed in the reaction and tiron did not reduce the P700 cation radical or other components of Photosystem I under anaerobic conditions.

A model (NADH-phenazine methosulfate-O2) formally similar to pyridine nucleotide-dependent flavoprotein hydroxylases catalyzed the hydroxylation of several aromatic compounds. The hydroxylation was maximal at acid pH and was inhibited by ovine Superoxide dismutase, suggesting that perhydroxyl radicals might be intermediates in this process. The stoichiometry of the reaction indicated that a univalent reduction of oxygen was occurring. The correlation between the concentration of semiquinone and hydroxylation, and the inhibition of hydroxylation by ethanol which inhibited semiquinone oxidation, suggested the involvement of phenazine methosulfate-semiquinone. Activation of hydroxylation by Fe3+ and Cu2+ supported the contention that univalently reduced species of oxygen was involved in hydroxylation. Catalase was without effect on the hydroxylation by the model, ruling out H2O2 as an intermediate. A reaction sequence, involving a two-electron reduction of phenazine methosulfate to reduced phenazine methosulfate followed by disproportionation with phenazine methosulfate to generate the semiquinone, was proposed. The semiquinone could donate an electron to O2 to generate O2 which could be subsequently protonated to form the perhydroxyl radical.

This voltammetric and potentiometric study concerns the reduction pre-waves of ninhydrin 1, the one electron cathodic and anodic waves of the pinacol reduction product 3 and the anodic (two electron) of the dihydro derivative, aci-reductone 2. The rate of the electronic transfer depends on the disproportionation and dimerisation reactions of the semiquinone radical 1·. In dilute solution, the redox system 12 is reversible; the value of e′° (+ 30 mV) stands between those of the dehydro ascorbic acid/ascorbic acid and the rhodizonic acid/tetrahydroxyquinone systems previously studied. This value explains the catalytic action of the system 12 in the oxydation of thiol.

The photochemical reactivity of the title compound 3 was studied in the context of a novel acid catalysed photocyclisation reaction recently discovered by Barton et al. [1b]. The following results were obtained by flash photolysis methods and steady state irradiations in the presence and absence of quenchers and sensitisers combined with luminescence measurements: The fluorescence of 3 in benzene is very weak and short-lived (ϕF1 ≤ 10−4, τF1 ≤ 10−9 s) due to efficient intersystem crossing (ϕT = 0.8 ± 0.15). On the other hand, the conjugate acid 4 exhibits an intense green fluorescence (ϕF1 = 0.97 ± 0.1) with a lifetime of (4. ± 1.) · 10−9 s. Acidity constants of 4 in the ground and first excited triplet state were estimated in acetic/trifluoroacetic acid mixtures by comparison with Hammett indicators: pK(S0) = −0.3, pK(T1) = +1. The Forster cycle yields pK(S1) = +6, however, adiabatic protonation of S1 (3) to produce fluorescent 4 is not efficient. The lowest ππ* triplet state of 3 (ET = 45 ± 1 kcal/mol, 3τ = 8 · 10−4 s in dilute, degassed benzene solution) is an intermediate in the photoreduction of 3 by oxidisable substrates. Hydrogen abstraction from 2-propanol by T1 (3) yields the semiquinone methide radical 5 which dimerises to 6. On readmission of air after irradiation, 6 is reoxidised to the starting material 3. Predictions derived from PPP SCF CI calculations are in accord with the observed spectroscopic and photochemical properties of 3.

The ESR spectrum of the fluorescein semiquinone radical (dianion form) produced by illumination of an alkaline aqueous solution of uranine and leuco-uranine at pH between 11 and 14 was observed and its hfs constants were determined by comparing the spectrum with those of the mono- and trianion forms of fluorescein semiquinone and those of eosm semiquinone reported by Leaver. Six protons in the xanthene ring were found to be comprised of three pairs of magnetically equivalent protons, which suggests a certain dynamic mechanism to average the coupling constants. From a temperature dependence of the line width, protonation to the –O− group followed by deprotonation from the –OH group was proposed as a plausible mechanism.

The ESR linewidths of semiquinone anions of xanthene dyes in alkaline protic solvents were studied as functions of the temperature and the viscosity. It was found that the linewidths of phloxine semiquinone, the ESR spectra of which show three hyperfine lines due to two equivalent protons, decrease with a decrease in the temperature or an increase in the viscosity of the solvents, and vary unimolecularly. Such a variation can not be explained either by a usual “chemical exchange” process between different states or by intramolecular motion. We propose that the linewidths may be due to the modulation of the hyperfine coupling constants of H atoms; the modulation results from the dissociation and/or distortion of the hydrogen bonds.

Wheat chloroplasts photochemically reduced molecular oxygen, as a Hill oxidant in the Mehler reaction, to superoxide anion which then oxidized added 1,2-dihydroxybenzene-3,5-disulfonate to its semiquinone, a comparatively stable free radical at pH 7. The last mentioned reaction was rapid in aqueous solution, but the rate of formation of 1,2-dihydroxybenzene-3,5-disulfonate semiquinone by the chloroplast system was calculated as T1 of 0.6 s. The Mehler reaction, or more specifically the univalent reduction of oxygen by Photosystem I, was rate-limiting so that the 1,2-dihydroxybenzene-3,5-disulfonate seniquinone was a useful spin probe for superoxide anion production at room temperature. The ESR signal of 1,2-dihydroxybenzene-3,5-disulfonate semiquinone was proportional to its steady state concentration and decayed in the dark with a T1/2 of 5-6 s. This oxygen-dependent signal was enhanced by mediation of chloroplastic oxygen reduction through methyl viologen. The superoxide anion scavengers ascorbate and L-epinephrine competitively obscured 1,2-dihydroxybenzene-3,5-disulfonate semiquinone formation, butadded superoxide dismutase was not as effective in this role. Partial inhibition by superoxide dismutase was achieved only by preincubation of Photosystem I enriched particles with ten times the endogenous concentration of superoxide dismutase. This and the persistence of a small amount of a 1,2-dihydroxybenzene-3,5-disulfonate (Tiron) oxidizing species in the dark supports the concept of Tiron accessibility but not the superoxide dismutase accessibility of superoxide anion bound in its formative enzyme complex. Benzoquinone and naphthoquinone disulfonate also reacted with superoxide anion, and supported both the Hill reaction and the Mehler reaction as final oxidants of both water and superoxide anion.

An electron-transferring flavoprotein (ETF) which couples the oxidation of NADH to the reduction of butyryl coenzyme A dehydrogenase has been purified in large quantities from the anaerobic bacterium Peptostreptococcus elsdenii. The purified protein appears to be homogeneous on the basis of polyacrylamide disc gel electrophoresis, thin layer isoelectric focusing, sedimentation velocity, and the Sephadex G-100 elution profile. Preliminary evidence suggests that ETF is also found in a complex with another flavoprotein. ETF has a molecular weight of 72,000 to 75,000 by Sephadex G-100 chromatography and is a dimer of two nonidentical subunits of approximate molecular weights of 33,000 and 41,000 determined by sodium dodecyl sulfate disc gel electrophoresis. Based on amino acid analyses, there are 2 moles of FAD per mole of protein of molecular weight 75,600. Possibly each subunit is associated with 1 FAD. The protein-bound FAD is 2.4 times more fluorescent than free FAD. Both tyrosine and tryptophan contribute to the protein fluorescence of ETF. Although the major prosthetic group of ETF is FAD, the isolated protein contains variable amounts of 6-hydroxy-7,8-dimethyl-10-(ribityl-5′-ADP)-isoalloxazine and 7-methyl-8-hydroxy-10-(ribityl-5′-ADP)-isoalloxazine. The isolated protein binds 30% more FAD to give enhanced activity and marked changes in the absorption spectrum. The spectrum of the oxidized isolated ETF is also drastically changed when the protein is maintained for long periods in its reduced form before subsequent reoxidation. In both cases, the major change is a large increase in the absorption around 400 nm. Apo-ETF has been prepared by treatment of the native protein with guanidine HCl. Catalytic activity is restored to the apoprotein by addition of FAD but not FMN. The reconstituted ETF also has increased absorption around 400 nm and decreased absorption at 450 nm compared to the isolated ETF. Unlike many other flavoprotein dehydrogenases which stabilize the neutral, blue flavin semiquinone and do not react with sulfite, ETF stabilizes the red, anionic semiquinone and also reacts with sulfite. Anaerobic titration of ETF with NADH or sodium dithionite resulted in a 2-electron reduction per protein-bound FAD, indicating that there are no colorless oxidation-reduction active groups present.

The electron spin-lattice relaxation times (T 1) of a variety of semiquinone ions in hydrogen bonding solvents have been measured by the pulsed saturation recovery technique as a function of temperature (T) and viscosity (η) of the solvent. Also linewidths (ΔH) have been measured in suitable cases in such solvents at low radical concentrations (∼10−4 M). It is observed that (i) the temperature and viscosity dependence ofT 1 can be fitted to an equation of the form 1/T 1=A(T/η)+Bexp(-ΔE/RT) whereA andB are constants and ΔE is an activation energy of the order of 1 kcal mole−1 for these systems; (ii)T 1 is essentially independent of the radical concentration within the range 10−3 to 5×10−2 M; (iii) the concentration independent part of the linewidth (ΔH) increases linearly with (η/T) at sufficiently low temperatures, and (iv) the (η/T) dependent part ofT 1 is sensitive to the size of the semiquinone as well as that of the solvent molecule, whereas the linewidth which is proportional to (η/T) at high viscosity, low temperature region is not sensitive to the size of the semiquinone and that of the solvent. Based on these observations, it is postulated that in hydrogen bonding solvents, three types of motion contribute significantly to electron spin relaxation: The fact thatT 1 is not sensitive to the concentration of the radicals, is ascribed to the formation of the solvent cage that prevents the close approach of radicals, thereby rendering radical-radical interactions to be weak mechanisms for relaxation, even at relatively high radical concentrations.

The Diels-Alder reaction of fluoranil with cyclopentadiene, 1,3-butadiene, and 1-acetoxy-1,3-butadiene gave 1,4, 5, 8-bis(methylene)-4a, 8a, 9a, 10a-tetrafluoro-1, 4, 4a, 5, 8, 8a, 9a, 10a-octahydroanthraquinone (I), 2, 3, 4a, 8a-tetrafluoro-4a, 5, 8, 8a-tetrahydro-1,4-naphthoquinone (III), and 5-acetoxy-2, 3, 4a, 8a-tetrafluoro-4a, 5, 8, 8a-tetrahydro-1,4- naphthoquinone (VI), respectively. Hydrogenation of I gave the expected saturated diketone(II). Hydrogenation of III afforded, with elimination of the two tertiary fluorines, 2,3-difluoro-5, 6, 7, 8-tetrahydro-1, 4- dihydroxynaphthalene (IV). In hydrogenation of VI, acetic acid and two moles of hydrogen fluoride were eliminated to give 2,3-difluoro-1, 4-dihydroxynaphthalene(VII). Both dihydroxy compounds IV and VII yielded on oxidation with ferric chloride the corresponding quinones, 2, 3- difluoro-5, 6, 7, 8-tetrahydro-1, 4-naphthoquinone (V) and 2, 3-difluoro-1, 4-naphthoquinone (VIII), respectively. Equivalent amounts of compounds IV and V gave a red-brown semiquinone IX, and a mixture of VI and VIII gave a dark-violet semiquinone X.

Diphenylthallium hydroxide reacts very smoothly in organic solvents with catechols, pyrogallols and 1,2-diketones to stable paramagnetic diphenylthalliumsemiquinone and diphenylthalliumsemidione complexes respectively. This reaction has been investigated with 43 different ligands in numerous solvents. Therefore its general application is proofed. The ESR-spectra of these solutions show the hydrogen hyperfine structure of the ligands and an unusual large coupling which we assign to magnetic interaction of the free electron with the 203Tl and 205Tl nuclei. The thallium splittings observed depend remarkably on the ligands used. We refer this to different solvation phenomena. Semiquinones of the pyrogallol type show on principle two different radical types. Small concentrations of diphenylthallium hydroxide produce 1 1 complexes. These can be converted into 1 2 complexes by addition of further amounts of diphenyl thallium hydroxide. The ESR-spectra of the semiquinone-and the semidione-thallium complexes exhibit an unusually strong solvent dependence of the thallium coupling and of the g-factor. It is possible to explain both variations uniformly by the different donor ability of the solvents used. All complexes investigated indicate remarkable temperature dependence of the thallium splitting and the g-factor. The thallium couplings show a positive temperature gradient whereas the g-factor decrease with increasing temperature as expected. Based on the sum of the observed effects, we assume that the radicals observed by us are ion pairs in which a diphenylthallium cation interacts with the semiquinone- or semidione-anion, respectively.

The reaction between a hydroquinone aqueous solution and manganese dioxide has been investigated by electron paramagnetic resonance spectroscopy. A very stable p-benzosemiquinone anion is formed when an aqueous solution of hydroquinone is passed through a column of manganese dioxide. The kinetics for the reaction between hydroquinone and manganese dioxide follow the equation where A0 is the initial concentration of hydroquinone, C is the concentration of p-benzosemiquinone anions, W is the amount of manganese dioxide, F is the feed rate of a hydroquinone aqueous solution, and k10 and k20 are constants; k10 = 3.1 × 10−2 (cm3/g MnO2· min), k20 = 3.2 (cm3/g MnO2·min). A mechanism for the reaction is proposed such that Mn(IV) in manganese dioxide is reduced to Mn(II) with hydroquinone from which the semiquinone anion is formed via a neutral semiquinone. The formation of semiquinone anions from several substituted hydroquinones is also discussed.

Lith development is produced by a combination of two primary mechanisms; the dependence of induction time on exposure and the autocatalytic continuation phase of development by semiquinone. Each mechanism may be modified in its action by other components of the system. The induction time is influenced principally by bromide ion, both that which is initially present and that which is released from adjacent developing grains and adsorbates to the grain SUI face, advantageously by polyethylene oxides, disadvantageously by some dyes. Semiquinone development is affected by the sulphite ion concentration and system components whose reactions lead to more rapid formation of the semiquinone such as hydroxyl ion and gelatin (arginine).Additionally, bleaching of the latent image in the low exposure region may play some part in producing a sharp toe to the characteristic curve.

2H-Benz[f]isoindol-4,9-chinone werden innerhalb des Hohlraumresonators durch Elektrolyse an einer Quecksilberelektrode zu Semichinon-Anionradikalen reduziert und ESR-Spektren aufgenommen. Der Einflus der Substituenten auf die Hyperfeinstruktur wird diskutiert. Die Kopplungskonstanten der Methylprotonen in 1,3-Stellung und des Pyrrolstickstoffs sind mit groser Annaherung der Summe der Hammettschen Substituentenkonstanten proportional. Diyne Reaction, XXXIII. E.S.R. Examinations of 2H-Benzol[f]isoindole-4,9-quinones 2H-Benzo[f]isoindole-4,9-quinones are reduced to semiquinone radical anions by electrolysis on a mercury electrode within the cavity of a resonator and the e.s.r. spectra are measured. The influence of the substituents on the hyperfine structure is discussed. The coupling constants of the methyl protons in 1,3-position and of the pyrrole nitrogen are with a great approximation proportional to the sum of Hammett's constants of the substituents.

P -Hydroquinone and catechol catalytically promote the oxidation of oxyhemoglobin and Oxymyoglobin to the ferriform. Kinetic data for oxyhemoglobin oxidation indicates a first-order dependence upon the hemeprotein concentration and half-order dependence upon diphenol; however at high catalyst concentration, saturation is observed with similar V values for both diphenols despite the difference in reactivity. It is proposed that initially formed quinone oxidizes the hemeprotein with oxygen release; in turn, the semiquinone oxidizes a second molecule of hemeprotein and regenerates the quinone, with the bound oxygen acquiring two electrons. Except for the more reactive oxymyoglobin, the reduced form of the catalyst must be present to oppose semiquinone disappearance by dismutation. Since the expected release of O 2 for water formation is observed, the system may be considered a model for terminal oxidase, the couple Q S replacing a Fe 2+ Fe 2+ or a Cu + Cu 2+ system. It is tentatively inferred that oxyhemoglobin has the structure HbFe 2+ ---O 2 and that the rate of the catalyzed oxidation is limited by the rate of generation of the true reacting form, the superoxide ferri structure, HbFe 3+ ---O 2 − .

Summary Binding of 8α-(S-peptidyl)flavins, the type of covalently attached flavin within the active site of mitochondrial monoamine oxidase, can occur within the FMN-binding site of Azotobacter flavodoxin. Association results in typical hypochromicity of the visible absorbance and a bathochromic shift. There is a decrease in fluorescence of protein but not flavin and a generation of negative ellipticity in the near-UV band in the CD spectra. Bound peptidyl flavins are reduced with EDTA and light to blue semiquinone complexes. Reduction to the hydroquinone level can be effected with excess free flavin during photoreduction or with excess dithionite. No significant monoamine oxidase activity could be detected. In common with other flavodoxins, the FMN-binding site of Azotobacter flavoprotein affixes the coenzyme within a flexible tryptophanyl-containing pocket that allows the benzenoid edge of the isoalloxazine ring to project toward solvent.

The oxidation enthalpy of reduced flavin mononucleotide at pH 7.0 in 0.2 m phosphate buffer has been studied by determining the heat associated with the reaction: FMNH 2 + 2 Fe(CN) −3 6 ⇌ FMN + 2 Fe(CN) −4 6 + 2 H + . (a) (The quinone, semiquinone, and hydroquinone forms of FMN are represented as FMN, FMNH, and FMNH 2 , respectively.) Calorimetric experiments were performed in a flow microcalorimeter which was modified to prevent sample contamination by oxygen. The enthalpy observed for reaction (a), after correction for dilution and buffer effects, was −39.2 ± 0.4 kcal (mole FMNH 2 ) −1 at 25 °C. The potential difference, ΔE ′, developed by reaction (a) was determined potentiometrically and corresponded to a free energy change, ΔG ′, of −30.3 kcal (mole FMNH 2 ) −1 . The resulting entropy change, ΔS ′, was thus calculated to be −29.8 e.u. Reaction (a) was also studied at temperatures of 7 °C and 35.5 °C. ΔC p ′ for the reaction was calculated as −155 ± 18 cal deg −1 (mole FMNH 2 ) −1 at 20 °C. ΔH ′ for the reaction (b), FMNH 2 ⇌ FMN + H 2 , (b) was calculated as +14.2 ± 0.7 kcal mole −1 at 25 °C, relative to the enthalpy of the hydrogen electrode being identically equal to zero at all values of pH and temperature. The free energy at pH 7.0 for reaction (b), calculated from the potential was found to be −9.7 kcal mole −1 , which resulted in an entropy for reaction (b) of 80.2 e.u. A thermal titration of reaction (a) was used to calculate the thermodynamic parameters for the formation of semiquinone dimer according to the reaction FMNH 2 + FMN ⇌ (·FMNH) 2 . (c) The free energy, enthalpy, and entropy changes for reaction (c) were estimated to be −6.1 kcal mole −1 , −7 kcal mole −1 , and −3 e.u., respectively.

Die vinylogen Verbindungen N1–3, O1–2, S1–3 sowie ihre Azaderivate N2a2 und S2a4 erweisen sich als zweistufige Redoxsysteme, deren Semichinonbildungskonstante Ksem mit wachsender Kettenlange stark fallt (ca. 105 101). Fur die planaren vinylogen Systeme gilt log KSem ˜ JSemmm (Coulombsches Repulsionsintegral). Azasubstitution erhoht KSem stark und verschiebt den gesamten Potentialbereich positiv. N3a4, O3a4 und S3a4 zeigen keine reversible Oxidationswelle. Die schwach basischen Amidrazone AS und ASe bilden bei der elektrochemischen Oxidation Tetrazene (z. B. S2a4). Soweit moglich, werden die Elektronenspektren der Redoxsysteme mitgeteilt. Nur die Radikalkationen zeigen einen Vinylensprung von 100–140 nm. Two Step Redox Systems, XVI1). – Spectroscopy and Polarography of Vinylogous Bibenzimidazoles, Bibenzoxazoles and Bibenzothiazoles and their Aza Derivatives The vinylogous compounds N1–3, O1–3, S1–3 and their aza derivatives N2a2 and S2a4 prove to be two step redox systems of which the semiquinone formation constant KSem decreases drastically with growing chain length (circa 105 101). For the planar vinylogous systems a linear correlation between log KSem and JSemmm (Coulomb repulsion integral) is found. On aza substitution all potentials are shifted positively and KSem is strongly enhanced. N3a4, O3a4 and S3a4 display no reversible oxidation waves. The weakly basic amidrazones AS and ASe form tetrazenes during electrochemical oxidation (e. g. S2a4). Wherever possible the electronic spectra of the redox systems are given. The radical cations only show vinylene shifts of 100–140 nm.

The oxidation of benzidine, o,o′-tolidine and o,o′-dianisidine with cobalt(III) perchlorate was studied. The oxidation of benzidine and o,o′-tolidine involves two-electron exchange, is reversible and the appropriate quinonediimine is formed. No semiquinone is formed during the reaction in the studied medium.

An electron density map of the oxidized form of flavodoxin from Clostridium strain MP has been calculated to a resolution of 1.9 A employing isomorphous and anomalous scattering data from two heavy atom derivatives, Smiii and Aui. In the region from 3.0 to 1.9 A, the largest structure amplitudes (85% of the total | F | s) have been included in the computation. The over-all figure of merit is 0.808. The molecular model resulting from interpretation of this map has a chain length of 138 residues, in accord with the sequence (Tanaka, M., Haniu, M., Yasunobu, K. T., and Mayhew, S. G. (1974) J. Biol. Chem. 249, 4393–4396). As reported earlier, the molecule possesses a high degree of secondary structure, including a central parallel sheet and four helices. More than 70% of the residues appear to participate in backbone hydrogen bonding. Atomic coordinates, incorporating the sequence data, have been optimized by use of Diamond's model-building program. The flavin mononucleotide prosthetic group adopts a somewhat extended conformation. As expected for the oxidized state, the isoalloxazine ring is planar. It is partly shielded from solvent by a tryptophan residue and interacts on its inner surface with methionine. Protein atoms are also in appropriate positions for interaction with atoms N-1, O-2, and N-3 of the flavin ring, but N-5 appears too far removed to form a hydrogen bond with the apoprotein. The ribityl phosphate moiety also makes multiple contacts with protein groups. Two of the three O' oxygens are in positions where they may participate in hydrogen bonding; the phosphate is closely surrounded by four hydroxyamino acids and four backbone NH groups. However, the prosthetic group is accessible on one side to solvent. Peaks likely to represent solvent can be observed near the dimethylbenzene end of the isoalloxazine ring and adjacent to the 3'-ribityl oxygen. While the mutual arrangement of protein and ribityl phosphate is nearly identical with that found by Watenpaugh et al. (Watenpaugh, K. D., Sieker, L. C., Jensen, L. H., LeGall, J., and Dubourdieu, M. (1972) Proc. Nat. Acad. Sci. U. S. A. 69, 3185–3188) in Desulfovibrio vulgaris flavodoxin, the orientation and interactions of the flavin ring differ strikingly in the two flavodoxins. The structure of the oxidized form of Clostridium MP flavodoxin is very similar to that deduced earlier for the semiquinone form at 3.25 A, despite large differences in the x-ray intensities (Ri = 0.66).

A quantitative test of the ENDOR theory for free radicals in solution has been developed experimentally using the semiquinone (SQ) radical anions of parabenzo‐ (PBSQ), duro‐ (DSQ), and 2,5‐dimethyl‐para‐benzo‐ (2,5‐DPMBSQ) dissolved in ethyl alcohol (EtOH) and dimethoxyethane (DME) solvents. It is shown that, in general, an ENDOR signal arising from a molecule containing four or less equivalent nuclei, such as PBSQ, can be analyzed rigorously whereas a molecule containing more than four equivalent nuclei, such as DSQ, which, in principle, could be analyzed rigorously, practically is best analyzed using approximate forms of the ENDOR theory. It is shown that 2,5‐DMPBSQ may be analyzed using a combination of both the rigorous and approximate forms of the ENDOR theory. The analysis involves calculation of the ENDOR relaxation parameters, T2n, Ωn, and Ωe,n, from the experimental ENDOR percent enhancement and linewidth studies. A comparison was made by performing the ESR linewidth and saturation studies to obt...

Summary Reductive titrations and rapid kinetic studies are reported on extensively or completely activated, particulate and solubilized succinate dehydrogenase (SD) preparations. There is one iron-sulfur (Fe−S) center of the ferredoxin type present per flavin which is reduced by succinate, but even in activated preparations at most 60% of these centers were reduced within the turnover time of the enzymes. Flavin semiquinone formation does not precede or significantly lag behind the reduction of the Fe−S centers. On reduction of the soluble enzymes an accumulation of semiquinone is observed. No qualitative difference between the behavior of preparations containing 4 Fe or 8 Fe per flavin was found. Succinate-ubiquinone reductase (Complex II) contains an Fe−S component with properties of high-potential Fe−S proteins (See Ruzicka and Beinert, Biochem. Biophys. Res. Communs. this issue). It occurs at a concentration close to that of the bound flavin and has been observed to be reduced by succinate at approximately the same rate as the ferredoxin type (g=1.94) component. With dithionite, reduction of additional Fe−S groups (0.2 to 0.5 per flavin) is observed but the significance of this is uncertain.

The microsomal flavoprotein, NADPH-cytochrome c reductase, has been reexamined to determine: (1) the nature of the flavine bound to the enzyme and (2) the oxidation-reduction state of the "half-reduced" form of the flavoprotein. Iyanagi and Mason (Iyanagi, T., and Mason, H.S. (1973), Biochemistry 12, 2297) have recently proposed that NADPH-cytochrome c reductase contains both FAD and FMN as prosthetic groups in lieu of FAD as the sole constituent, as suggested by all previous studies of this enzyme. The data presented herein, utilizing the recently published fluorometric procedure of Faeder and Siegel (Faeder, E. J., and Siegle, L. M. (1973), Anal. Biochem. 53, 332) for the determination of FAD and FMN in mixtures, confirm the conclusions of Iyanagi and Mason for both rat and pig liver reductase preparations. Data for other flavoproteins are also presented. Iyanagi and Mason have also concluded that the air-stable "semiquinone" is a form of NADPH-cytochrome c reductase reduced by one electron per two falvines (F-FH). The present studies, however, do not agree with this conclusion, but instead support our previous results which indicate that both the aerobic and anaerobic half-reduced states of this flavoprotein exist in the two-electron reduced form (FH-FH). Removal of NADP+ does not affect the spectrum of the air-stable half-reduced form of the flavoprotein, nor does it affect the back titration of this intermediate by potassium ferricyanide. The possible implications of these observations on the catalytic cycle of the flavines of NADPH-cytochrome c reductase are discussed.

Reductive photobleaching of eosin and uranin in ethanol solutions was studied by the flash technique. Transient spectra of semiquinone with a lifetime of millisecond order was established. In addition, another transient spectra with a life of the same order was observed in the case of eosin. This was interpreted to be due to a certain complex intermediate between ethanol and eosin, formed prior to semiquinone. The aerated alcoholic solution and the evacuated aqueous solution, when illuminated by a flash, showed no transient spectra in the wavelength region of the above two intermediates. The fluorescence of the aerated ethanol solution produced by the flash had about twenty times as large intensity as that of the evacuated solution induced under otherwise similar condition. A scheme for the primary processes of the reductive photobleaching involving the above complex can interpret all these results satisfactorily.

The redox potential of •O2− was determined based on the dependence of the electron transfer reaction from •O2− upon the known redox potential of various acceptors A (including a range of quinones, dyes and ferricyanide). The efficiencies and the rates of these electron transfer processes were determined, using the technique of pulse radiolysis, by monitoring the formation kinetics of the semiquinone radical anions at the appropriate wavelength. From the percentage efficiency versus Eo′ plot, an Eo′ value of + 0.15 ± 0.01 V at pH 7.0 and 22°C, or Eo = + 0.57 ± 0.01 V, for the • O 2 − O 2 couple was obtained. The rate k(•O2− + A → O2 + •A−) = 9.8 × 108 M−1 sec−1 where A = p-benzoquinone. The Eo value for the •HO2 radical is > 1.0 V. It was also found that hydroquinone can quantitatively reduce •O2− to H2O2, k(•O2− + QH2 → QH• + HO2−) = 1.6 ± 0.1 × 107 M−1 sec−1 at pH 7.0 and 22°C.

Die Polarographie der heterocyclischen Redoxsysteme 6–10, 14 und 15 zeigt reversible zweistufige Einelektronenubergange mit Semichinonbildungskonstanten K ≈ 107–1010 (6–10) bzw. ≈ 104 (14) und 120 (15) [Abbildung 6]. Der Substituenteneinflus auf K und die Lage der Potentiale werden besprochen. Die Elektronenspektren von 6–10, 14 und 15 in ihren Oxydationsstufen RED, SEM und OX werden verglichen (Tabelle 3). Two-step Redox Systems, VII1). Polarography and Spectroscopy of Heterocyclic Azines and their Higher Oxidation Levels The polarography of the heterocyclic redox systems 6–10, 14 and 15 reveals reversible one-electron transfers in two steps. Semiquinone formation constants K are ≈ 107–1010 (6–10), ≈ 104 (14) and 120 (15) [figure 6]. The influence of substituents on K and the position of the potentials are discussed. Electron spectra of 6–10, 14 and 15 on the oxidation levels RED, SEM and OX are compared (table 3).

Peptostreptococcus elsdenii flavodoxin is completely reduced by 1 mole of sodium dithionite per mole of proteinbound flavin mononucleotide; 0.5 mole of dithionite generates almost maximum flavin semiquinone. The amount of semiquinone formed is dependent on pH. Semiquinone formation constants determined at pH 8.3 and 9.26, were 910 and 110, respectively. Fully reduced flavodoxin reacts rapidly with oxidized flavodoxin to form semiquinone. The rate of this reaction is pH-dependent and decreases from 105 m-1 min-1 at pH 6.7 to 1.6 x 104 m-1 min-1 at pH 9.26. The oxidation-reduction potential (E2) for the couple oxidized flavodoxin/flavodoxin semiquinone was determined by equilibrium experiments with indigo dyes. E2 was -0.115 volt at pH 7. The oxidation-reduction potential (E1) for the couple semiquinone/fully reduced flavodoxin was determined from the equilibrium concentration of these components after treating flavodoxin with hydrogen and hydrogenase. E1 was -0.373 volt at pH 7. A value for E1 was also obtained from the concentrations of fully reduced flavodoxin and flavodoxin semiquinone formed in equilibrium with excess TPNH in the presence of spinach ferredoxin-TPN reductase. By this method E1 was -0.371 volt at pH 7. The effect of pH on E1 suggests an ionization in fully reduced flavodoxin with a pK = 5.8. When reduced flavodoxin is mixed with air, the semiquinone is formed rapidly. Air oxidation of flavodoxin semiquinone is slow; the observed rate is markedly dependent on pH.

A spectrographic study of the oxidation of catechol showed that in a rigid medium (96° alcohol at 100° K), after brief exposure to intense irradiation with the wavelength absorbed in u.v. light, a transitional configuration existed, the properties of which were those of the corresponding free radical (semiquinone). The same observation can be made when catechol is exposed to the action of pure polyphenol oxidase of mushroom, under similar experimental conditions.

The proton hyperfine structure in a series of methylsubstituted semiquinone ions has been shown to arise in a general sense from configuration interaction. A crude calculation of the proton hfs to be expected in a hypothetical solution of C2H4+ ions indicates that the σ—π interaction is responsible for the hfs of protons attached directly to an aromatic ring. The molecular orbital theory of hyperconjugation accounts for the detailed hfs associated with the methyl protons.

The flash photolysis of many quinones in solution produces a number of transient intermediates, each with its characteristic absorption spectrum. A preliminary study of these showed that the appearance of intermediates is clearly related to the efficiency of the quinone as a photosensitizer. A detailed study of the transient spectra of duroquinone as a function of solvent and of pH enabled an unequivocal assignment to be made to all the principal absorptions. The main intermediates are (1) The semiquinone radical with first absorption maximum at 410 mμ which appears in all solvents containing abstractable hydrogen at pH 7 and below. (2) The semiquinone radical ion with first absorption maximum at 430 mμ which appears only at pH 7 and above. (3) The triplet state which is only observed strongly in the viscous solvent liquid paraffin and has a relatively short life. Analogous spectra of similar intermediates are reported for benzoquinone, toluquinone, xyloquinone, naphthaquinone and anthraquinones.

— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization by chlorophyll of hydroquinone oxidation in a pyridine-water solvent. The rate of radical decay was found to be second order with respect to the radical concentration, with a rate constant of 6.7 × 105 l./mole sec at -30°C and an activation energy of 6900 cal/mole. The rate of radical formation was recombination-limited and, through the use of β-carotene as a quencher, the rate constant was determined to be 8.81 × 105 l./mole sec at -30°C. The effect of light intensity and hydroquinone concentration on the rate of semiquinone radical formation and on the steady state radical concentration was also investigated and possible mechanisms to explain the results are discussed.

The effect of pH on the position of equilibrium in the p-phenylenediamine–ferricyanide and p-aminophenol–ferricyanide redox systems is discussed. The equilibria are established very rapidly at all pH. At high pH (>7), the equilibria lie in favour of the quinone imine and the instability of the systems is due to hydrolysis of these imines. At low pH (

The mechanism of photodegradation of cotton cellulose sensitised by sodium anthraquinone-1- and -2-su!phonates (A) has been examined. Measurements of diffuse reflectance indicate that the sensitisers are present in monodisperse form on the cotton cellulose and interact, on irradiation (λ, 365 nm), to give the corresponding anthrahydroquinones (AH2). A hydrogen-abstraction reaction, involving the nπ*-excited state (A*), leads to the formation of the semiquinone (AH·), which produces the anthrahydroquinone, probably by a further abstraction process: A* + Cell AH· + Cell radical The accompanying decrease in degree of polymerisation of the cotton cellulose is equivalent to the production of AH2. The photoinduced changes in the sensitiser and substrate are first-order in sensitiser concentration. Oxygen and water vapour are necessary for the cyclic regeneration of the sensitiser, which proceeds at the same rate for both 1- and 2-sulphonates. However, sodium anthraquinone-2-sulphonate is 16 times more efficient as a sensitiser than the 1-sulphonate, a result which is in agreement with the quantum yields for the production of AH2 from these sensitisers in solution.

Evidence is presented disproving the previously suggested formation of stable semiquinone esters during the oxidation of the sulphuric acid esters of leuco vat dyes (1). Under appropriate conditions it appears most likely that the leuco compound which may be produced by hydrolysis of the original ester will disproportionate to yield the vat dye and a compound at a lower oxidation level than the leuco compound.

The electron spin resonance (e.s.r.) spectrum of potassium durosemiquinone in tetrahydrofuran has been studied as a function of temperature and found to exhibit line-width alternation indicating the presence of an intramolecular migration of the potassium ion between the oxygens of the semiquinone. The addition of potassium tetraphenylboride to the system produces intermolecular scrambling of potassium, the spectral effects of which are superimposed upon those of the intramolecular exchange. From the spectral line-shapes approximate values of the activation energies and frequency factors were derived. The tetraphenylboride anion did not play a detectable role in the various exchange mechanisms.

The extinction coefficients of the different oxidation states of the chloroplast flavoprotein, ferredoxin-NADP reductase, have been determined in the spectral region from 600 to 400 nm. The photoreduction of the flavoprotein by chloroplast fragments produces the semiquinone, fp · FADH•, and not the fully reduced form of the enzyme, fp · FADH2. The photoreduction of the flavoprotein, and of NADP, is inhibited by inorganic pyrophosphate. This metabolite does not affect the photoreduction of ferredoxin, and therefore it is a specific inhibitor at the level of the oxidation of reduced ferredoxin in the electron transport system of chloroplasts. These observations are discussed in relation to the function of ferredoxin, flavoprotein, and pyrophosphate in photosynthesis.

Soweit bisher nicht bekannt, wird die Synthese der Diquartarsalze 3a–e, 3Ba–c und 4 sowie der zugehorigen Heterocyclen beschrieben. Diese Quartarsalze werden, zusammen mit 5a–c, polarographisch untersucht. Alle Verbindungen nehmen in Wasser, Acetonitril und DMF ein Elektron reversibel auf. Reversible Aufnahme eines zweiten Elektrons wird nur bei 3a, 3c und 4 beobachtet, jedoch nicht in Wasser. Die Semichinonbildungskonstanten liegen bei 105 bis 108. HMO-Berechnungen korrelieren gut mit dem N V1-Ubergang, jedoch nicht mit dem Redoxpotential. Two-Step Redox Systems, XII1). – Synthesis and Polarography of Quaternary Salts Derived from Phenanthrolines, 2,7-Diazapyrene and from Diazoniapentaphenes As far as unknown the synthesis of the diquaternary salts 3a–e, 3Ba–c and 4 is described together with that of the parent heterocycles. Salts 3, 4 and 5a–c are investigated by polarography. In water, acetonitrile and DMF all compounds trap one electron reversibly. Reversible uptake of a second electron is observed with 3a, 3c and 4, however not in water. Semiquinone formation constants are 105 to 108. HMO calculations correlate well with N V1 transitions but not with redox potentials.

The staining of amino groups by formazan when dehydrated paraffin sections are incubated in a mixture of hydroquinone and 3-(4,5-dimethyl thiazolyl-2)-2 ,5-diphenyl-2H-tetrazolium bromide at an acid pH is reported. The mechanism of this reaction and of the cytoplasmic deposition of formazan in fresh frozen sections incubated under similar conditions is investigated. It is shown that the oxidation of hydroquinone to semiquinone is responsible for the reaction, the tetrazole acting as electron acceptor. The tissue amino groups, exposed by dehydration and thermal shrinkage, and the nitrogen groupings of phosphobipid behave as "catalysts." The relevant properties of the hydroquinone-benzoquinone oxidation-reduction system are described, and the reactions between benzoquinone and tissue constituents are reviewed.

A cell, producing measurable currents, is used as a model in further studies on the inhibitory action of polyethylene oxides (PEO) on development by hydroquinone (Hq). The cell reaction is found to be inhibited when PEO is in contact with the anode (silver), but not when in contact with the cathode (silver bromide). There is in general a parallel between the behaviour of substances in the cell and in a photographic emulsion test. Polymers and substituted hydroquinones which do not show the PEO-Hq effect with emulsion are inactive in the cell. The resistance of some Hq derivatives to PEO inhibition may be associated with their demonstrated adsorption to silver, but an exception suggests an alternative, that they may have more active semiquinones. The results of the cell experiments are believed to be applicable to the case of photographic development. The hypothesis that PEO inhibits Hq development by interfering with the adsorption of developing agent (semiquinone) at latent image specks thus rece...

— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization of reduction or oxidation, by chlorophyll, pheophytin or hematoporphyrin, of several quinone-hydroquinone pairs in various solvents. The rate of radical decay was found to be second order with respect to the radical concentration in all systems. Radical formation rates were determined by the initial production rate minus the decay rate. The kinetic constants for single electron transfer between triplet porphyrins and quinones or hydroquinones were determined usingβ-carotene as a quencher in aqueous pyridine, and by measuring the initial rate of radical formation at various concentrations of quinones and hydroquinones in methanol and ethanol. These constants were found to be approximately the same in a given solvent for benzoquinone and hydroquinone with all porphyrins, though the rates differed in different solvents: pyridine-water ∼ 106I./mole sec, and methanol and ethanol ∼ 5X 104l./mole sec. Trimethylquinone and its hydroquinone also give similar rate constants for radical formation in pyridine-water, ∼ 106 l./mole sec. The second order radical decay constants for both benzoquinone and hydroquinone in pyridine-water were the same, ∼ 105 I./mole sec, with either chlorophyll, pheophytin or hematoporphyrin as sensitizer. The same activation energy, 6900 cal/mole, was found for chlorophyll-benzoquinone and hydroquinone in aqueous pyridine; 5500 cal/mole was obtained for these systems in ethanol. In methanol and ethanol solutions of chlorophyll, the same radical decay rate constants, ∼106 I./mole sec, were observed for both benzoquinone and hydroquinone. Also, the same decay constants, ∼ 106 I./mole sec, were found for trimethylquinone and its hydroquinine in pyridine-water. These latter two compounds gave extremely small steady-state ESR signals in ethanol compared with aqueous pyridine. We have also observed that the steady-state signal obtained with chlorophyll-menadione in ethanol-water was much enhanced by the presence of NADH. In contrast, NAD+ was found to decrease radical production, by increasing the decay rate, in the chlorophyll-hydroquinone system in aqueous pyridine. These results are discussed in terms of possible mechanisms for radical formation and disappearance. The most likely possibility is considered to be a one-electron oxidation or reduction of the porphyrin triplet, followed by radical disproportionation and redox reactions between the disproportionation products.

1. The course of the oxidative titration of the reduced form of polyarylenequinones was investigated. The occurrence of this process is related to the structure of the investigated polymers. The oxidative process probably includes a stage of formation of the semiquinone form. 2. Sulfonic acid-substituted polyarylenequinones possess increased normal potentials in comparison with p-benzoquinone. 3. The electron exchange capacity of the polyarylenequinones was measured. In all cases it was lower than the theoretical values; this difference decreased from swollen sulfonic acid-substituted polymers to polymers without sulfonic acid groups. 4. Variation of the ratio of the initial reagents (p-benzoquinone and diamine) does not lead to any appreciable change in the ion exchange capacity of polyarylenequinones.

The naphthazarin semiquinone cation in sulphuric acid exhibits an electron spin resonance spectrum which varies with the temperature. This variation is ascribed to rotational isomerization and the shape of the spectrum at different temperatures is reconstructed using a theory based on the Bloch equations. From the temperature dependence of the rate of isomerization, the potential barrier to rotation is estimated to be 4 kcals. The mechanism of isomerization probably involves the breaking of an intramolecular hydrogen bond.

The addition of Phenidone to a hydroquinone developer does not increase the rate of autoxidation of the hydroquinone when the solution is exposed to air or to oxygen. The rate of autoxidation does not appear to be limited by the rate of solution of the gas because it can be varied over a wide range by changing the pH. The Phenidone has no effect at any pH, but when present in high concentration its rate of loss can become appreciable through dismutation of its semiquinone and it can compete with the hydroquinone. When the hydroquinone is present in concentrations above 2 g per litre its rate of autoxidation is insensitive to change in its concentration and is proportional to the area of the gas-solution interface. The evidence suggests that the critical stage of the autoxidation takes place at the gas-solution interface.