Reaction of Ferrate(VI) with ABTS and Self-Decay of Ferrate(VI): Kinetics and Mechanisms

Reactions of ferrate­(VI) during water treatment generate perferryl­(V) or ferryl­(IV) as primary intermediates. To better understand the fate of perferryl­(V) or ferryl­(IV) during ferrate­(VI) oxidation, this study investigates the kinetics, products, and mechanisms for the reaction of ferrate­(VI) with 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and self-decay of ferrate­(VI) in phosphate-buffered solutions. The oxidation of ABTS by ferrate­(VI) via a one-electron transfer process produces ABTS^•+ and perferryl­(V) (<i>k</i> = 1.2 × 10⁶ M^–1 s^–1 at pH 7). The perferryl­(V) mainly self-decays into H₂O₂ and Fe­(III) in acidic solution while with increasing pH the reaction of perferryl­(V) with H₂O₂ can compete with the perferryl­(V) self-decay and produces Fe­(III) and O₂ as final products. The ferrate­(VI) self-decay generates ferryl­(IV) and H₂O₂ via a two-electron transfer with the initial step being rate-limiting (<i>k</i> = 26 M^–1 s^–1 at pH 7). Ferryl­(IV) reacts with H₂O₂ generating Fe­(II) and O₂ and Fe­(II) is oxidized by ferrate­(VI) producing Fe­(III) and perferryl­(V) (<i>k</i> = ∼10⁷ M^–1 s^–1). Due to these facile transformations of reactive ferrate­(VI), perferryl­(V), and ferryl­(IV) to the much less reactive Fe­(III), H₂O₂, or O₂, the observed oxidation capacity of ferrate­(VI) is typically much lower than expected from theoretical considerations (i.e., three or four electron equivalents per ferrate­(VI)). This should be considered for optimizing water treatment processes using ferrate­(VI)

Electrode Potentials of l‑Tryptophan, l‑Tyrosine, 3‑Nitro‑l‑tyrosine, 2,3-Difluoro‑l‑tyrosine, and 2,3,5-Trifluoro‑l‑tyrosine

Electrode potentials for aromatic amino acid radical/amino acid couples were deduced from cyclic voltammograms and pulse radiolysis experiments. The amino acids investigated were l-tryptophan, l-tyrosine, <i>N</i>-acetyl-l-tyrosine methyl ester, <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester, <i>N</i>-acetyl-2,3-difluoro-l-tyrosine methyl ester, and <i>N</i>-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester. Conditional potentials were determined at pH 7.4 for all compounds listed; furthermore, Pourbaix diagrams for l-tryptophan, l-tyrosine, and <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester were obtained. Electron transfer accompanied by proton transfer is reversible, as confirmed by detailed analysis of the current waves, and because the slopes of the Pourbaix diagrams obey Nernst’s law. <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH) at pH 7 are 0.99 ± 0.01 and 0.97 ± 0.01 V, respectively. Pulse radiolysis studies of two dipeptides that contain both amino acids indicate a difference in <i><i>E</i>°</i>′ of approximately 0.06 V. Thus, in small peptides, we recommend values of 1.00 and 0.96 V for <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH), respectively. The electrode potential of <i>N</i>-acetyl-3-nitro-l-tyrosine ethyl ester is higher, while because of mesomeric stabilization of the radical, those of <i>N</i>-acetyl-2,3-difluoro-l-tyrosine methyl ester and <i>N</i>-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester are lower than that of tyrosine. Given that the electrode potentials at pH 7 of <i>E</i>°′(Trp^•,H⁺/TrpH) and <i><i>E</i>°</i>′(TyrO^•,H⁺/TyrOH) are nearly equal, they would be, in principle, interchangeable. Proton-coupled electron transfer pathways in proteins that use TrpH and TyrOH are thus nearly thermoneutral

Redox Properties and Activity of Iron–Citrate Complexes: Evidence for Redox Cycling

Iron in iron overload disease is present as non-transferrin-bound iron, consisting of iron, citrate, and albumin. We investigated the redox properties of iron citrate by electrochemistry, by the kinetics of its reaction with ascorbate, by ESR, and by analyzing the products of reactions of ascorbate with iron citrate complexes in the presence of H₂O₂ with 4-hydroxybenzoic acid as a reporter molecule for hydroxylation. We report −0.03 V < <i>E</i>°′ > +0.01 V for the (Fe³⁺–cit/Fe²⁺–cit) couple. The first step in the reaction of iron citrate with ascorbate is the rapid formation of mixed complexes of iron with citrate and ascorbate, followed by slow reduction to Fe²⁺–citrate with <i>k</i> = ca. 3 M^–1 s^–1. The ascorbyl radical is formed by iron citrate oxidation of Hasc^– with <i>k</i> = ca. 0.02 M^–1 s^–1; the majority of the ascorbyl radical formed is sequestered by complexation with iron and remains EPR silent. The hydroxylation of 4-hydroxybenzoic acid driven by the Fenton reduction of iron citrate by ascorbate in the presence of H₂O₂ proceeds in three phases: the first phase, which is independent of the presence of O₂, is revealed as a nonzero intercept that reflects the rapid reaction of accumulated Fe²⁺ with H₂O₂; the intermediate oxygen-dependent phase fits a first-order accumulation of product with <i>k</i> = 5 M^–1 s^–1 under aerobic and <i>k</i> = 13 M^–1 s^–1 under anaerobic conditions; the slope of the final linear phase is ca. <i>k</i> = 5 × 10^–2 M^–1 s^–1 under both aerobic and anaerobic conditions. Product yields under aerobic conditions are greater than predicted from the initial concentration of iron, but they are less than predicted for continuous redox cycling in the presence of excess ascorbate. The ongoing formation of hydroxylated product supports slow redox cycling by iron citrate. Thus, when H₂O₂ is available, iron–citrate complexes may contribute to pathophysiological manifestations of iron overload diseases

Efficient Polymerization of the Aniline Dimer <i>p</i>‑Amino­diphenyl­amine (PADPA) with Trametes versicolor Laccase/O₂ as Catalyst and Oxidant and AOT Vesicles as Templates

The aniline dimer PADPA (= <i>p</i>-amino­diphenyl­amine = <i>N</i>-phenyl-1,4-phenyl­ene­diamine) was polymerized to poly­(PADPA) at 25 °C with Trametes versicolor laccase (TvL)/O₂ as catalyst and oxidant and in the presence of vesicles formed from sodium bis­(2-ethylhexyl) sulfosuccinate (AOT) as templates. In comparison to the previously studied polymerization of aniline with the same type of enzyme–vesicle system, the polymerization of PADPA is much faster, and considerably fewer enzymes are required for complete monomer conversion. Turbidity measurements indicate that PADPA strongly binds to the vesicle surface before oxidation and polymerization are initiated. Such binding is confirmed by molecular dynamics (MD) simulations, supporting the assumption that the reactions which lead to poly­(PADPA) are localized on the vesicle surface. The poly­(PADPA) obtained resembles the emeraldine salt form of polyaniline (PANI-ES) in its polaron state with a high content of unpaired electrons, as judged from UV/vis/NIR, EPR, and FTIR absorption measurements. There are, however, also notable spectroscopic differences between PANI-ES and the enzymatically prepared poly­(PADPA). Poly­(PADPA) appears to be similar to a chemically synthesized poly­(PADPA) as obtained in a previous work with ammonium peroxydisulfate (APS) as the oxidant in a mixture of 50 vol % ethanol and 50 vol % 0.2 M sulfuric acid (<i>J. Phys. Chem. B</i> <b>2008</b>, <i>112</i>, 6976–6987). ESI-MS measurements of early intermediates of the reaction with TvL and AOT vesicles indicate that the presence of the vesicles decreases the extent of formation of unwanted oxygen-containing species in comparison to the reaction in the absence of vesicles. This is the first information about the differences in the chemical composition of early reaction intermediates when the reaction carried out in the presence of vesicles under optimal conditions is compared with a template-free system

Enzymatic Synthesis of Highly Electroactive Oligoanilines from a <i>p</i>‑Aminodiphenylamine/Aniline Mixture with Anionic Vesicles as Templates

Oligoanilines with characteristic properties of the electrically conductive emeraldine salt form of polyaniline (PANI-ES) are promising molecules for various applications. A mixture of such oligoanilines can be obtained, for example, enzymatically under mild conditions from the linear aniline dimer <i>p</i>-aminodiphenylamine (PADPA) with hydrogen peroxide (H₂O₂) and low amounts of horseradish peroxidase (HRP) in an aqueous pH = 4.3 suspension of anionic vesicles formed from AOT, the sodium salt of bis­(2-ethylhexyl)­sulfosuccinate. However, the simultaneous formation of undesired side products containing phenazine-type units or oxygen atoms is unsatisfactory. We have found that this situation can be improved considerably by using a mixture of PADPA and aniline instead of PADPA only but otherwise nearly identical conditions. The PANI-ES-like oligoaniline products that are obtained from the PADPA and aniline mixture were not only found to have much lower contents of phenazine-type units and not contain oxygen atoms but also were shown to be more electroactive in cyclic voltammetry measurements than the PANI-ES-like products obtained from PADPA only. The AOT vesicle suspension remained stable without product precipitation during and after the entire reaction so that it could be analyzed by in situ UV/visible/near-infrared, in situ electron paramagnetic resonance, and in situ Raman spectroscopy measurements. These measurements were complemented with ex situ high-performance liquid chromatography analyses of the deprotonated and reduced products formed from mixtures of PADPA and either fully or partially deuterated aniline. On the basis of the results obtained, a reaction mechanism is proposed for explaining this improved HRP-triggered, vesicle-assisted synthesis of electroactive PANI-ES-like products. The oligomeric products obtained can be further used, without additional special workup, for example, to coat electrodes for their possible application in biosensor devices