Alexander D. Ryabov

FeIII–TAML-catalyzed green oxidative degradation of the azo dye Orange II by H2O2 and organic peroxides: products, toxicity, kinetics, and mechanisms

Oxidation of Orange II ([4-[(2-hydroxynaphtyl)azo]benzenesulfonic acid], sodium salt) by hydrogen peroxide catalyzed by iron(III) complexed to tetra amido macrocyclic ligands (FeIII–TAML activators) in aqueous solutions at pH 9–11 leads to CO2, CO, phthalic acid and smaller aliphatic carboxylic acids as major mineralization products. The products are non-toxic according to the Daphnia magna test. Several organic intermediates have been identified by HPLC and GC-MS that allowed the detailed description of Orange II degradation. The catalytic oxidation can also be performed by organic oxidants such as benzoyl peroxide, tert-butyl and cumyl hydroperoxides. Kinetic studies of the catalyzed oxidation indicated that FeIII–TAML activators react first with ROOR′ to form an oxidized catalyst (kI), which then oxidizes Orange II (kII). Neglecting the reversibility of the first step, the rate equation is rate = kIkII[FeIII][ROOR′][Dye]/(kI[ROOR′] + kII[Dye]); here FeIII and ROOR′ represent the catalyst and peroxide, respectively. The rate constant kI equals (74 ± 3) × 103, (1.4 ± 0.1) × 103, 24 ± 2, and 11 ± 1 M−1 s−1 for benzoyl peroxide, H2O2, t-BuOOH, and cumyl hydroperoxide at pH 9 and 25 °C, respectively. An average value of kII equals (3.1 ± 0.9) × 104 M−1 s−1 under the same conditions. The unraveling of the kinetic mechanism allows the comprehension of the robust reactivity, and this is discussed in detail using the representative results of DFT calculations.

New bioorganometallic ferrocene derivatives as efficient mediators for glucose and ethanol biosensors based on PQQ-dependent dehydrogenases

One known and two new ferrocene-containing mediators incorporating the organometallic moiety and the fragments of natural substrates of oxidative enzymes, viz. 4-ferrocenylphenol (FP), 2-ferrocenyl-4-nitrophenol (FNP), and N -(4-hydroxybenzylidene)-4ferrocenylaniline (HBFA), were studied as electron transfer mediators between the coenzyme pyrroloquinoline quinone (PQQ) of glucose (GDH) and alcohol (ADH) dehydrogenases and the carbon electrode surface. A screen-printed carbon electrode (SPCE) suitable for ADH and GDH immobilization served as a transducer. The electrodes were integrated into a flow-through amperometric cell. All data were obtained at a flow rate of 1 ml min � 1 . The maximal currents (jmax) obtained from the calibration curves for the oxidation of ethanol and D-glucose by ADH and GDH of 2.3 and 3.0 mA, respectively, were obtained when SPCE was modified with HBFA, i.e. with a mediator with a longer arm and a high degree of conjugation. The biosensors were used for ethanol and D-glucose measurements in beverages. There was a good correspondence (r � /0.978 for D-glucose and r � /0.920 for ethanol) between the data obtained by using the biosensors, on one hand, and by the refractometric or hydrometric methods, on the other. The operational stability of biosensors is determined by the inactivation of the immobilized enzymes rather than by leakage of a mediator from an electrode. # 2002 Elsevier Science B.V. All rights reserved.

4-Ferrocenylphenol as an electron transfer mediator in PQQ-dependent alcohol and glucose dehydrogenase-catalyzed reactions

Abstract Enzyme electrodes containing pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenase (ADH) and glucose dehydrogenase (GDH) as a biological component in combination with 4-ferrocenylphenol ( 1 ) as an electron transfer mediator between PQQ and a carbon electrode were constructed and used for measurements of ethanol and d -glucose. Analysis of the current response of the carbon electrodes modified with 1 at different pH and potentials demonstrated that 1 participates in the bioelectrocatalytic oxidation of d -glucose or ethanol. The biosensors showed the highest response at pH 5.5 and the working potentials of 0.3 and 0.4 V (versus Ag|AgCl) for ADH and GDH, respectively. The electrocatalytic processes under such conditions at these electrodes are characterized by the apparent values of the Michaelis constants K M app of 7.1 and 13 mM and the maximal current density j max 40 and 26 μA cm −2 for ethanol and d -glucose, respectively. No electrocatalysis was found when glucose oxidase from Aspergillus niger was used instead of GDH.

Design of More Powerful Iron-TAML Peroxidase Enzyme Mimics

Environmentally useful, small molecule mimics of the peroxidase enzymes must exhibit very high reactivity in water near neutral pH. Here we describe the design and structural and kinetic characterization of a second generation of iron(III)-TAML activators with unprecedented peroxidase-mimicking abilities. Iterative design has been used to remove the fluorine that led to the best performers in first-generation iron-TAMLs. The result is a superior catalyst that meets a green chemistry objective by being comprised exclusively of biochemically common elements. The rate constants for bleaching at pH 7, 9, and 11 of the model substrate, Orange II, shows that the new Fe(III)-TAML has the fastest reactivity at pHs closer to neutral of any TAML activator to date. Under appropriate conditions, the new catalyst can decolorize Orange II without loss of activity for at least 10 half-lives, attesting to its exceptional properties as an oxidizing enzyme mimic.

On the reactivity of mononuclear iron(V)oxo complexes.

Ferric tetraamido macrocyclic ligand (TAML)-based catalysts [Fe{C(6)H(4)-1,2-(NCOCMe(2)NCO)(2)CR(2)}(OH(2))]PPh(4) [1; R = Me (a), Et (b)] are oxidized by m-chloroperoxybenzoic acid at -40 °C in acetonitrile containing trace water in two steps to form Fe(V)oxo complexes (2a,b). These uniquely authenticated Fe(V)(O) species comproportionate with the Fe(III) starting materials 1a,b to give μ-oxo-(Fe(IV))(2) dimers. The comproportionation of 1a-2a is faster and that of 1b-2b is slower than the oxidation by 2a,b of sulfides (p-XC(6)H(4)SMe) to sulfoxides, highlighting a remarkable steric control of the dynamics. Sulfide oxidation follows saturation kinetics in [p-XC(6)H(4)SMe] with electron-rich substrates (X = Me, H), but changes to linear kinetics with electron-poor substrates (X = Cl, CN) as the sulfide affinity for iron decreases. As the sulfide becomes less basic, the Fe(IV)/Fe(III) ratio at the end of reaction for 2b suggests a decreasing contribution of concerted oxygen-atom transfer (Fe(V) → Fe(III)) concomitant with increasing electron transfer oxidation (Fe(V) → Fe(IV)). Fe(V) is more reactive toward PhSMe than Fe(IV) by 4 orders of magnitude, a gap even larger than that known for peroxidase Compounds I and II. The findings reinforce prior work typecasting TAML activators as faithful peroxidase mimics.

Mechanistic considerations on the reactivity of green FeIII-TAML activators of peroxides

Publisher Summary This chapter introduces FeIII- tetra-amido-macrocyclic-ligand (TAML) activators, a family of low-molecular weight green catalysts for the activation of H2O2 and other peroxides to oxidize a wide spectrum of targeted substrates including toxic polychlorophenols, thiophosphate pesticides and nitrophenols, azo dyes, dibenzothiophenes, an anthrax surrogate, and natural or synthetic estrogens. These iron catalysts contain a central ferric iron coordinated to the cavity of a tetraanionic TAML. FeIII-TAML activators of H2O2 display peroxidase-like activities and longevities that are remarkable for low-molecular weight synthetic catalysts. They are functional analogs of catalase-peroxidase enzymes. This chapter summarizes the results of mechanistic studies on various aspects of FeIII-TAML peroxide activation obtained in the laboratories over the past decade. It also describes kinetic properties that are essential for understanding intrinsic reactivity features to build a solid mechanistic picture of FeIII-TAML catalysis and to show that they are a family of successful artificial enzymes.

Electrochemically and Catalytically Active Reconstituted Horseradish Peroxidase with Ferrocene‐Modified Hemin and an Artificial Binding Site

A procedure for modification of hemin chloride by FcCH2NH2 (Fca ferrocenyl) in the presence of 1-(dime- thylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccini- mide affords two main products 1 and 2 with mono- and bis-amidated propion- ic acid residues. Monoamidated conju- gate 1 was loaded into the apoenzyme of horseradish peroxidase (HRP) to afford an electrochemically and catalytically active reconstituted enzyme Fc-HRP with remarkably altered substrate spe- cificity. With ABTS as substrate, the reactivity of Fc-HRP drops threefold compared with native HRP as a result of a lowering of the maximal rate Vm. Compared with HRP the reactivity of Fc-HRP towards water-soluble ferro- cenes is even higher at low concentra- tions of the latter, the rate increase being accompanied by a change in rate law: in contrast to first-order kinetics in ferro- cenes for native HRP, there is a Michae- lis dependence for Fc-HRP. Molecular modeling suggests creation of an artifi- cial hydrophobic binding site within a triangle confined by the ferrocenyl res- idue and the two phenyl rings of Phe 68 and 179. The site is believed to be responsible for the kinetically meaning- ful binding between ferrocene substrates and Fc-HRP which manifests in the saturation kinetics.

Coordinative Approach to Mediated Electron Transfer: Ruthenium Complexed to Native Glucose Oxidase

Catalytically and electrocatalytically very active and stable are the complexes Ru(LL)-GO, which are extremely readily accessible from glucose oxidase (GO) and the RuII complexes cis-[Ru(LL)2 Cl2 ] (LL=bpy, phen). These provide an unprecedentedly high amplification coefficient I/Io (see cyclic voltammograms) even at high scan rates and, correspondingly, very high rates of intramolecular electron transfer.

Enantioselectivity in Enzyme-Catalyzed Electron Transfer to and from Planar Chiral Organometallic Compounds

Asymmetric cyclopalladation of dimethylaminomethylferrocene in the presence of N-acetyl-(R)- or (S)- leucine afforded enantiomerically en- riched palladacycles (S)- and (R)- (Pd{C5H3(CH2NMe2)FeC5H5}(m-Cl))2, respectively. Carbonylation of each enantiomer followed by iodomethyla- tion and reduction by sodium amalgam gave (S)- and (R)-2-methylferrocene carboxylic acid (1) with an optical purity of 80 and 93 %, respectively. (S)- and (R)-1 readily undergo one-electron (1e) oxidation to form the corresponding ferricenium cations by hydrogen perox- ide, catalyzed by horseradish peroxidase (HRP) and chloroperoxidase (CLP) from Caldariomyces fumago (258C, pH 5 - 8 and 2.75, respectively). In the case of HRP, the reaction is strictly first- order with respect to (S)- and (R)-1 (ratea k(HRP)(1)), whereas Michae- lis - Menten kinetics are observed for CLP. The strongly pH-dependent kinetic enantioselectivity is, however, only ob- served in the case of HRP. HRP-gener- ated cations (S)-1 a and (R)-1 a have been used to demonstrate that their enzymat- ic reduction by reduced glucose oxidase (GO) is also enantioselective; the (S)-1 a enantiomer is more reactive than (R)-1 a by a factor of 1.54. The existence of the planar chiral enantioselectivity in the GO catalysis was also confirmed by the cyclic voltammetry study of (S)-1 and (R)-1 in the presence of GO and b-d- glucose with glassy carbon and pyrolytic graphite electrodes. The corresponding enantioselectivity factors k(S)-1 a /k(R)- 1 a are 1.7 and 1.6, respectively. Based on the known X-ray structural data for the active site of GO, it has been tentatively suggested that the enantioselectivity originates from the hydrophobic contact between the enzyme tyr-68 residue and the h 5 -C5H5 ring of 1 a , and a hydrogen bond network formed by his-516 and/or his-559 residues and the carboxylic group of the ferrocene derivative. The findings reported confirm the existence of enantioselective electron transfer be- tween oxidoreductases and organome- tallic compounds with a planar chirality. The lack of kinetic enantioselectivity may be a result of i) the incorrect rate- limiting step, ii) unfavorable pH region, and iii) the deficit of charged groups attached to ferrocenes.

The exchange of cyclometallated ligands: II. Attempts to prepare six-membered palladocycles via the ligand exchange reaction

Abstract Reactions of the cyclopalladated N , N -dimethylbenzylamine chloro-bridged dimer with 2-benzylpyridine (bpH), 2-benzoylpyridine (bopH), and acetanilide have been studied in acetic acid/chloroform at 50°C. 2-Benzylpyridine readily undergoes ligand exchange to afford the six-membered cyclopalladated complex [PdCl(bp)] 2 in a high yield. In contrast to bpH, 2-benzoylpyridine reacts in two time-resolved steps. A bis-adduct, trans -[PdCl 2 (bopH) 2 ], having no palladium—carbon bond, is formed first, followed by precipitation of the cyclopalladated species. Acetanilide does not react with the N , N -dimethylbenzylamine complex at all. The reasons for such different behaviour of these three ligands are discussed taking into account some of the 1 H NMR spectral data.