Successive One-electron Transfer Research Articles

Radical and diradical states of bis(molybdenocene dithiolene) complexes.

The synthesis and characterization of two bis(dithiolene) proligands involving heteroatomic linkers such as 1,4-dithiine and dihydro-1,4-disiline between the two protected dithiolene moieties are described. Two bimetallic complexes involving these heteroatomic bridges between two redox active bis(cyclopentadienyl)molybdenum dithiolene moieties have been synthesized and characterized by electrochemistry, spectroelectrochemistry, and their properties rationalized with (TD-)DFT. Cyclic voltammetry experiments show sequential oxidation of the two redox centers with ΔE values between successive one-electron transfers varying according to the nature of the bridge. Depending on the nature of the heteroatomic bridge, the bis-oxidized complexes exhibit either a diradical character with both radicals essentially localized on the metallacycles, or a closed-shell dicationic state.

Photocatalytic Cross-Pinacol Coupling Promoted by Carbon Dioxide.

Cross-pinacol coupling of two different carbonyl compounds was achieved through successive one-electron transfer processes under photocatalytic conditions. In the reaction, an umpoled anionic carbinol synthon was generated in situ to react nucleophilically with a second electrophilic carbonyl compound. We revealed that a CO2 additive promoted the photocatalytic generation of the carbinol synthon to suppress undesired radical dimerization. A wide variety of aromatic and aliphatic carbonyl substrates underwent the cross-pinacol coupling to afford the corresponding unsymmetric vicinal 1,2-diols, where even a combination of carbonyl reactants with similar structures such as two aldehydes and two ketones were also well tolerated with high cross-coupling selectivity.

Electrochemical behaviour of 2,2′-bibenzimidazoles: Voltammetric, in situ UV–vis- and EPR-spectroelectrochemical and computational studies

• The electrochemical properties of 2,2′-bibenzimidazole and its substituted analogues have been investigated by cyclic voltammetry, square-wave voltammetry and in situ UV–vis- and EPR-spectroelectrochemistry. • The nature of the formed in the electrochemical process radicals has been elucidated. • The formation of 2,2′- bibenzoimidazolylidene species and the mechanism of the electrochemical reduction process have been postulated. The electrochemical properties of 2,2′-bibenzimidazole and its substituted analogues 5,6-dimethyl-2,2′-bibenzimidazole, 5,5′,6,6′-tetramethyl-2,2′-bibenzimidazole as well as 1,1′-dimethyl-2,2′-bibenzimidazole were investigated by cyclic voltammetry (CV) and square-wave voltammetry (SWV) combined with mechanistic studies by means of in situ UV–vis- and EPR-spectroelectrochemistry. The data obtained for N H derivatives suggest the formation of 2,2′-bibenzoimidazolylidene species via ECEC reaction mechanism consisting of two successive one-electron transfers accompanied by proton migration. The resulting species undergo further one-electron reduction with the formation of stable anion-radicals, whose structures have been identified by in situ UV–vis- and EPR-spectroelectrochemical measurements and supported by the computational study using density functional theory (DFT) calculations. In contrast to N H derivatives, the electrochemical reduction of N,N -dimethyl substituted derivative leads to radical anion and dianion species.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster.

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K2{[UV(OSi(OtBu)3)3]2(μ-O)(μ-η2:η2-N2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K6{(OSi(OtBu)3)2UIV}3{(OSi(OtBu)3)2UVI}(μ4-N)3(μ3-N)(μ3-O)2], 1, a novel example of N2 cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N═UVI═N core analogue of UO22+. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N24- ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium(V) bis-nitride [K4{[UV(OSi(OtBu)3)3]2(μ-O)(μ-N)2}], X. Additionally, cooperative potassium binding to the U-bound N24- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess 13CO to yield K13CN and KN13CO. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Peroxidase evolution in white-rot fungi follows wood lignin evolution in plants

A comparison of sequenced Agaricomycotina genomes suggests that efficient degradation of wood lignin was associated with the appearance of secreted peroxidases with a solvent-exposed catalytic tryptophan. This hypothesis is experimentally demonstrated here by resurrecting ancestral fungal peroxidases, after sequence reconstruction from genomes of extant white-rot Polyporales, and evaluating their oxidative attack on the lignin polymer by state-of-the-art analytical techniques. Rapid stopped-flow estimation of the transient-state constants for the 2 successive one-electron transfers from lignin to the peroxide-activated enzyme (k2app and k3app ) showed a progressive increase during peroxidase evolution (up to 50-fold higher values for the rate-limiting k3app ). The above agreed with 2-dimensional NMR analyses during steady-state treatments of hardwood lignin, showing that its degradation (estimated from the normalized aromatic signals of lignin units compared with a control) and syringyl-to-guaiacyl ratio increased with the enzyme evolutionary distance from the first peroxidase ancestor. More interestingly, the stopped-flow estimations of electron transfer rates also showed how the most recent peroxidase ancestors that already incorporated the exposed tryptophan into their molecular structure (as well as the extant lignin peroxidase) were comparatively more efficient at oxidizing hardwood (angiosperm) lignin, while the most ancestral "tryptophanless" enzymes were more efficient at abstracting electrons from softwood (conifer) lignin. A time calibration of the ancestry of Polyporales peroxidases localized the appearance of the first peroxidase with a solvent-exposed catalytic tryptophan to 194 ± 70 Mya, coincident with the diversification of angiosperm plants characterized by the appearance of dimethoxylated syringyl lignin units.

Effect of Manganese Oxide Aging and Structure Transformation on the Kinetics of Thiol Oxidation.

The kinetics and mechanism of thiol oxidation by Mn oxides undergoing dynamic structural transformation under environmentally relevant conditions remain poorly understood. In this study, thiol/disulfide pair concentrations were simultaneously determined in situ using voltammetric microelectrodes during the interaction of four common thiols (cysteine, homocysteine, cysteamine, and glutathione) with fresh and aged δ-MnO2 at pH 7.0. The reaction kinetics was first order with respect to thiol and zero order with respect to Mn oxides. A transient intermediate sulfur surface species observed during the reaction provides evidence for a mechanism involving two successive one-electron transfer steps. The reaction kinetics of fresh and aged δ-MnO2 was investigated with cysteine and compared to that of manganite, a Mn(III) oxyhydroxide phase. The reactivity of aged δ-MnO2 decreased as a result of structural transformation to cryptomelane but remained higher than that of manganite, suggesting the potential roles of transient Mn(III) surface intermediate in promoting the reduction of Mn(IV) in δ-MnO2 and cryptomelane (compared to Mn(III) in manganite). This study demonstrates the importance of correlating Mn oxide mineral structure and redox reactivity and extends the potential for thiols commonly found in sedimentary environments to be utilized as electron shuttles during dissimilatory Mn reduction.

Unraveling a Single-Step Simultaneous Two-Electron Transfer Process from Semiconductor to Molecular Catalyst in a CoPy/CdS Hybrid System for Photocatalytic H2 Evolution under Strong Alkaline Conditions.

Electron transfer processes from semiconductor to molecular catalysts was studied in a model hybrid photocatalytic hydrogen evolution system composed of [Co((III))(dmgH)2PyCl] (CoPy) and CdS under different pH conditions. Thermodynamic and kinetic studies revealed that photocatalytic H2 evolution under high pH conditions (pH 13.5) can only account for the thermodynamically more favorable single-step simultaneous two-electron transfer from photoirradiated CdS to Co(III)Py to produce unavoidable intermediate Co(I)Py, rather than a two-step successive one-electron transfer process. This finding not only provides new insight into the charge transfer processes between semiconductors and molecular catalysts but also opens up a new avenue for the assembly and optimization of semiconductor-molecular catalyst hybrid systems processed through multielectron transfer processes.

Electrochemical Explorations of 9,10-Anthraquinone/Hafnium(IV) Ion Interactions in Nonaqueous Solvents

Introduction The well-defined electrochemical behavior of quinones such as 9,10-anthraquinone (AQ) in nonaqueous solvents (1,2) suggests its use as a probe for interactions of metal cations with the AQ carbonyl oxygens. In previous work from this laboratory, the interaction of AQ with added Al(III) trifluoromethanesulfonate [Al(TfO)3] in adiponitrile has been shown to produce a shift in the initial AQ reduction process to more positive potentials (3). In contrast to the behavior seen for 9-fluorenone (4), however, the second AQ reduction process disappears even at the 0.4 : 1.0 Al3+ : AQ point. This observation suggests that AQ reduction products are good ligands for the Al3+ cation, with approximately three reduced AQ ligands coordinated to one Al3+. The present work explores whether this effect extends to other metals, particularly Hf4+. The general effect of Hf4+ on the cyclic voltammetry of AQ in adiponitrile is also treated. Experimental Adiponitrile (AN) and 9,10-anthraquinone (AQ) were obtained from Aldrich Chemical Co. Hafnium trifluoromethansulfonate [hafnium triflate, Hf(TfO)4] was obtained from Alfa-Aesar. Tetraethylammonium tetrafluoroborate [TEA BF4] was purchased from Southwestern Analytical Chemicals (SACHEM). Voltammograms were acquired with a PAR283 potentiostat using PowerSuiteTM software. Potentials are reported with respect to a Ag/AgCl (0.1M EMICl in EMI BF4) reference electrode (Cypress Systems). Vitreous carbon electrodes were obtained from Cypress Systems. All experiments were carried out in a Vacuum Atmospheres drybox. Results and Discussion A cyclic voltammogram of AQ in adiponitrile without added Hf(TfO)4 is shown in Figure 1a, which shows the usual two uncomplicated successive one-electron transfers. Upon addition of Hf(TfO)4 at the 0.4 : 1.0 Hf4+ : AQ level, however, a substantial positive potential shift is observed for AQ reduction (Figure 1b). This shift is due to complexation of one of the AQ carbonyl oxygens by Hf4+. As in the case of Al3+ addition, there is none of the original second AQ redox process at this point. This observation implies that Hf4+ is complexed by multiple AQ reduction products, as was also seen for Al3+ addition. Further additions of Hf(TfO)4 give only the shifted reduction process at the 1:1 Hf4+ : AQ point, with substantially more of the small reduction peak at +0.1 V in Figure 1b. The latter observation suggests the possibility that both AQ carbonyl oxygens can undergo complexation by Hf4+ in this system. The present results indicate extensive interaction of Hf4+ with AQ, including the role of reduced AQ species as multiple ligands for Hf4+. References N. A. Macıas-Ruvalcaba and D. H. Evans, J. Phys. Chem. C, 114 , 1285 (2010).L. Jeftic and G. Manning, J. Electroanal. Interfacial Echem, 26, 195 (1970).Electrochemical Society 226th Meeting, Fall 2014, Cancum, Mexico, ECS H1-1345.D. Canby, E. Sanders,and G. T. Cheek, J. Electrochem. Soc., 160(7), G3159 (2013). Figure 1

How can multielectron transfer be realized? A case study with keggin-type polyoxometalates in acetonitrile.

Theoretical consideration and computational simulation have been performed on the voltammetric properties of Keggin polyoxometalates (POMs), and the conversion from successive one-electron transfer in unacidified media to four-electron transfer (through two-electron transfer) in acidified media has been discussed. Perfect simulation of the cyclic voltammograms of POMs could be achieved using the standard formal potentials and the protonation constants, systematically evaluated by the equations, in which “simple (intrinsic)” and “synergistic (extrinsic)” electron-withdrawing effects of the μ4-oxygen were taken into consideration. In the proposed model, the formal potential of the one-electron redox waves for the ith reduction step is presented by Ei°(z0, s) = Ei** + 0.51(z0 – i + 1) + 1.067s (i = 1, 2, 3, 4; E1** = E2** = 0.577 V; E3** = E4** = 0.377 V), where z0 is the initial ionic charge of a Keggin POM and s is the mean bond valence of the μ4-O–W bonds in the POM. The values of Ei**s are related to the energy levels of the two lowest unoccupied molecular orbitals (LUMOs) of a hypothetical Keggin POM with null charge and null bond valence. Then it was revealed that the LUMOs have small on-site repulsion, which may be an important factor that makes multielectron transfer feasible. These findings would give a big clue in developing novel redox materials exhibiting multielectron transfers.

Combinatorial Synthesis of Gold-Based Thin Films for Improved Electrocatalytic Conversion of CO2 to CO

Carbon dioxide electroreduction (CO2ER) was studied on 35 gold-based thin film catalysts sputtered on glassy carbon disks. A sputter-down setup was used for the deposition of these thin films in a combinatorial manner. Zinc and silver were employed in combination with gold in order to obtain new catalysts. Linear sweep voltammetry was employed to obtain the selectivity profile of each catalyst. Among the 35 catalysts, three ternary combinations (Au14Ag34Zn52, Au32Ag51Zn17, and Au16Ag10Zn74) and one binary combination (Au80Zn20) were identified as being active catalysts, reducing the dissolved CO2 in favor of proton reduction. Two ternary catalysts (Au16Ag10Zn74 and Au32Ag51Zn17) exhibited higher selectivity and lower overpotential for CO2ER than the pure metals. The Au80Zn20 binary catalyst exhibited the longest potential range where the selectivity for CO2ER remains constant. A decrease of the gold loading between 18 and 88 % was obtained for these catalysts, suggesting that they are promising candidates for an industrial application of CO2ER. Electrochemical impedance spectroscopy proved that the mechanism of CO2ER, involving two successive one-electron transfers, is identical on pure metals and combined sputtered catalysts.

Electrochemical Investigation of Fluorenone Complexation by Lewis Acids in Ionic Liquids

The electrochemical characteristics of 9-fluorenone have been investigated in various ionic liquids as a means of determining the extent of interaction of representative Lewis acids with this ketone. The ionic liquids studied, with Lewis acids in a particular medium, are : 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI BF4) [BF3 etherate], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (BMPY TfO) [Hf(TfO)4 and Sc(TfO)3], and aluminum chloride : 1-ethyl-3-methylimidazolium chloride (AlCl3 : EMICl) [aluminum chloride]. In EMI BF4 and BMPY TfO, 9-fluorenone undergoes two successive one-electron transfers; however, addition of a Lewis acid produces a new reduction process shifted to a more positive potential. The extent of this shift is taken as an indication of the strength of the Lewis acid interaction with 9-fluorenone. The strongest Lewis acid : 9-fluorenone interaction, judging from the magnitude of the potential shift, is found in the AlCl3 : EMIC ionic liquid, followed by that of Hf(TfO)4 in BMPY TfO. The interaction of Sc(TfO)3 with 9-fluorenone in BMPY TfO is considerably less than that of Hf(TfO)4. In EMI BF4, however, Sc(TfO)3 interacts quite strongly with 9-fluorenone, indicating that the triflate anion interacts extensively enough with Sc3+ in BMPY TfO to restrict its complexation of 9-fluorenone.

Microbial Mn(IV) reduction requires an initial one-electron reductive solubilization step

Mn(IV) and Mn(II) are the most stable and prevalent forms of manganese in natural environments. The occurrence of Mn(III) in minerals and the detection of soluble Mn(III) in natural waters, however, suggest that Mn(III) is an intermediate in both the oxidation of Mn(II) and the reduction of Mn(IV). Mn(III) has recently been proposed as an intermediate during the oxidation of Mn(II) by Mn-oxidizing bacteria but has never been considered as an intermediate during the bio-reduction of Mn(IV). Here we show for the first time that microbial Mn(IV) reduction proceeds step-wise via two successive one-electron transfer reactions with production of soluble Mn(III) as transient intermediate. Incubations with mutant strains demonstrate that the reduction of both solid Mn(IV) and soluble Mn(III) occurs at the outer membrane of the cell. In addition, pseudo-first order rate constants obtained from these incubations indicate that Mn(IV) respiration involves only one of the two potential terminal reductases (c-type cytochrome MtrC and OmcA) involved in Fe(III) respiration. More importantly, only the second electron transfer step is coupled to production of dissolved inorganic carbon, suggesting that the first electron transfer reaction is a reductive solubilization step that increases Mn bioavailability. These findings oppose the long-standing paradigm that microbial Mn(IV) reduction proceeds via a single two-electron transfer reaction coupled to organic carbon oxidation, and suggest that diagenetic models should be revised to correctly account for the impact of manganese reduction in the global carbon cycle.

Fully Delocalized (Ethynyl)(vinyl)phenylene Bridged Triruthenium Complexes in up to Five Different Oxidation States

Triruthenium [(dppe)(2)Ru{-C≡C-1,4-C(6)H(2)-2,5-R(2)-CH═CH-RuCl(CO)(P(i)Pr(3))(2)}(2)](n+) (4a, R = H; 4b, R = OMe) containing unsymmetrical (ethynyl)(vinyl)phenylene bridging ligands and displaying five well-separated redox states (n = 0-4) are compared to their bis(alkynyl)ruthenium precursors (dppe)(2)Ru{-C≡C-1,4-C(6)H(2)-2,5-R(2)-C≡CR'} (2a,b: R' = TMS; 3a,b: R' = H) and their symmetrically substituted bimetallic congeners, complexes {Cl(dppe)(2)Ru}(2){μ-C≡C-1,4-C(6)H(2)-2,5-R(2)-C≡C} (A(a), R = H; A(b), R = OMe) and {RuCl(CO)(P(i)Pr(3))(2)}(2){μ-CH═CH-1,4-C(6)H(2)-2,5-R(2)-CH═CH} (V(a), R = H; V(b), R = OMe) as well as the mixed (ethynyl)(vinyl)phenylene bridged [Cl(dppe)(2)Ru-C≡C-1,4-C(6)H(4)-CH═CH-RuCl(CO)(P(i)Pr(3))(2)] (M(a)). Successive one-electron transfer steps were studied by means of cyclic voltammetry, EPR and UV-vis-NIR-IR spectroelectrochemistry. These studies show that the first oxidation mainly involves the central bis(alkynyl) ruthenium moiety with only limited effects on the appended vinyl ruthenium moieties. The second to fourth oxidations (n = 2, 3, 4) involve the entire carbon-rich conjugated path of the molecule with an increased charge uniformly distributed between the two arms of the molecules, including the terminal vinyl ruthenium sites. In order to assess the charge distribution, we judiciously use (13)CO labeled analogues to distinguish stretching vibrations due to the acetylide triple bonds and the intense and charge-sensitive Ru(CO) IR probe in different oxidation states. The comparison between complex pairs 4a,b(n+) (n = 0-3), A(a,b)(n+) and V(a,b)(n+) (n = 0-2) serves to elucidate the effect of the methoxy donor substituents on the redox and spectroscopic properties of these systems in their various oxidation states and on the metal/ligand contributions to their frontier orbitals.

Single Two-Electron Transfers and Successive One-Electron Transfers in Biferrocenyl−Indacene Isomers

Novel biferrocenyl complexes of s- and as-dihydroindacenes have been prepared and the charge transfer properties of their mono- and dicationic derivatives, selectively generated by one-electron and...

“Chain-Like” Trimetallic Ruthenium Complexes with C7Carbon-Rich Bridges: Experimental and Theoretical Investigations of Electronic Communication Tuning in Five Distinct Oxidation States

In this work, we report the synthesis and the electronic properties of the unique highly conjugated molecular wires trans-[Cl-(dppe)(2)Ru=C=C=(Ph)C-CH=(CH(3))C-C[triple bond]C-(X)(2)Ru-C=C-C(CH(3))=CH-C(Ph)=C=C=Ru(dppe)(2)Cl](n+) (n = 2, X = dppe ([3a](OTf)(2)) and dppm ([3b](OTf)(2)) with three similar metal centers spanned by two odd-numbered unsaturated C(7) chains providing a 28 A long conjugated path and displaying five well-separated redox states (n = 0-4). Successive one-electron transfer steps were studied by means of cyclic voltammetry, EPR and UV-vis-NIR-IR spectroelectrochemistry. The electronic and physical properties of the different states were further rationalized with the help of DFT-based calculations. Upon one-electron reduction (n = 1), the single electron is delocalized over the two carbon chains through the central metal atom to an extent driven by the rotations within and between the chains. The second reduction (n = 0) involves the whole carbon-rich conjugated path of the molecule in a spin polarized scheme: one electron is delocalized over each chain, and the two electrons are antiferromagnetically coupled with a coupling on the order of kT. Interestingly, oxidation processes strongly involve both the metal atoms and the bridging ligands. The combined investigations reveal that the mono-oxidized system (n = 3) presents a spin density uniformly distributed between the metal atoms and the carbon atoms of the chains, whereas in the second oxidation state (n = 4) the compounds show a strong antiferromagnetic coupling on the order of 4 kT between the two single electrons localized in two distinct delocalized spin orbitals implying all the carbon atoms of the bridges and the three metal atoms. Thus, for the first time, electronic communication was fully evidenced and tuned in homonuclear trimetallic oligomeric carbon-rich systems in either an oxidation or a reduction process.

NrdI, a flavodoxin involved in maintenance of the diferric-tyrosyl radical cofactor in Escherichia coli class Ib ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is essential in all organisms. Class I RNRs consist of two homodimeric subunits: alpha2 and beta2. The alpha subunit contains the site of nucleotide reduction, and the beta subunit contains the essential diferric-tyrosyl radical (Y*) cofactor. Escherichia coli contains genes encoding two class I RNRs (Ia and Ib) and a class III RNR, which is active only under anaerobic conditions. Its class Ia RNR, composed of NrdA (alpha) and NrdB (beta), is expressed under normal aerobic growth conditions. The class Ib RNR, composed of NrdE (alpha) and NrdF (beta), is expressed under oxidative stress and iron-limited growth conditions. Our laboratory is interested in pathways of cofactor biosynthesis and maintenance in class I RNRs and modulation of Y* levels as a means of regulating RNR activity. Our recent studies have implicated a [2Fe2S]-ferredoxin, YfaE, in the NrdB diferric-Y* maintenance pathway and possibly in the biosynthetic and regulatory pathways. Here, we report that NrdI is a flavodoxin counterpart to YfaE for the class Ib RNR. It possesses redox properties unprecedented for a flavodoxin (E(ox/sq) = -264 +/- 17 mV and E(sq/hq) = -255 +/- 17 mV) that allow it to mediate a two-electron reduction of the diferric cluster of NrdF via two successive one-electron transfers. Data presented support the presence of a distinct maintenance pathway for NrdEF, orthogonal to that for NrdAB involving YfaE.

Treatment of phenylmercury salts by heterogeneous photocatalysis over TiO 2

UV/TiO 2 photocatalysis of phenylmercury salts in aqueous solutions has been performed starting from both acetate (C 6H 5HgCH 3CO 2, PMA) and chloride (C 6H 5HgCl, PMC) salts, in the presence or the absence of oxygen at acidic pH. Removal of Hg(II) in solution took place with the simultaneous deposit of dark or pale gray solids on the photocatalyst, identified as metallic Hg (when starting from PMA) or mixtures of Hg(0) and Hg 2Cl 2 (when starting from PMC). Partial mineralization of the organic part of both compounds has also been achieved. Hg(II) removal and mineralization were enhanced in the absence of oxygen. PMA photocatalysis followed a saturation kinetics, going from first order at low concentrations to zero order at higher concentrations (>0.5 mM). For PMA, reaction was faster at high pH (11) with formation of mixtures of Hg and HgO. Phenol was detected as a product of the reaction in both cases, PMA and PMC, and no formation of dangerous methyl- or ethylmercury species was observed in the first case. A mechanism for the photocatalytic reaction has been proposed. The fact that calomel was found as a deposit when starting from PMC under nitrogen suggests that the mechanism of Hg(II) transformation proceeds through successive one-electron transfer reactions passing by mercurous forms.

Electrochemical study of Hypocrellin B in acetonitrile and aqueous micellar media. Comparison with Hypocrellin A

The electrochemical behaviour of Hypocrellin B (HBH2) has been investigated in acetonitrile and in aqueous micellar media. In organic medium, HBH2 behaves similarly to Hypocrellin A (HAH2) since two successive reversible one-electron transfers are observed in the cathodic region. Exhaustive electrolyses have led to the formation of different reduced forms of HBH2, which have been characterized by spectroscopic investigations. Contrary to HAH2, EPR experiments demonstrated that the one-electron transfer is weakly coupled with protonation phenomenon, leading mainly to the formation of HBH2˙−. Added protons in the medium are required to obtain HBH3˙. In aqueous micellar medium, the first reduction step, whose potential depends on the pH value, is equally observed. This has allowed a diagram of E1/2vs. pH to be established and the determination of the two pKa values of HBH2, whereas the pKa value of the one-electron reduced form of HBH2 has been estimated. The ability of this radical species to produce superoxide anion is discussed and compared with what was previously described for the one-electron reduced forms of HAH2.

Electron-transfer chemistry of the iron-molybdenum cofactor of nitrogenase: delocalized and localized reduced states of FeMoco which allow binding of carbon monoxide to iron and molybdenum.

The electron-transfer chemistry of the isolated iron-molybdenum cofactor of nitrogenase (FeMoco) has been studied by electrochemical and spectroelectrochemical methods. Two interconverting forms of the cofactor arise from a redox-linked ligand isomerism at the terminal iron atom; this is attributed to rotamerism of an anionic N-methyl formamide ligand bound at this site. FeMoco in its EPR-silent oxidised state is shown to undergo three successive one-electron transfer steps. We argue that the first and second redox processes are associated with electron-transfer delocalised over the iron-sulfur core of the cofactor, whilst the third irreversible process is localised on molybdenum. This is strongly reinforced by spectroelectrochemical studies under (12)CO and (13)CO which reveal two independent carbon monoxide binding sites that are specifically associated with the second (iron core) and third (molybdenum) electron-transfer processes and which give rise to terminal nu((12)CO) bands at 1885 and 1920 cm(-1) respectively. Moreover, in parallel with earlier studies on the enzyme system, it is shown that at low CO concentration, carbon monoxide binds to the cofactor in bridging modes, with nu(CO) bands at 1835 and 1808 cm(-1) that are interconverted by single-electron transfer. Importantly we show that the contentious overall 2e difference in the assignment of the metal oxidation levels in the resting state of the enzyme-bound cofactor, arising from analysis of (57)Fe ENDOR and Mössbauer data, can be resolved in the light of the electron-transfer chemistry of the isolated cofactor described herein.

Isoprenoid biosynthesis through the methylerythritol phosphate pathway: the (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase (GcpE) is a [4Fe-4S] protein.

One of the last bottlenecks in the elucidation of the methylerythritol phosphate pathway for isoprenoid biosynthesis has been solved. A [4Fe–4S] cluster is the prosthetic group of the GcpE protein from E. coli. This enzyme is involved in the conversion of 1 into 2, through two successive one-electron transfers.