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) (k = 1.2 × 10(6) 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 (k = 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) (k = ∼10(7) 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).
The mechanism of the horseradish peroxidase (HRP)-H 2 O 2 -catalysed polymerisation of aniline in the presence of AOT vesicles was investigated. AOT (= bis-(2-ethylhexyl)sulfosuccinate) served as vesicleforming surfactant and dopant for obtaining at pH = 4.3 and room temperature within 24 h under optimal reaction conditions the green emeraldine salt form of polyaniline in 90-95% yield. Based on UV/VIS/NIR and EPR measurements carried out during the polymerisation reaction, and based on changes in aniline and H 2 O 2 concentrations and HRP activity, a mechanism is proposed. According to this ''radical cation mechanism'' chain growth occurs on the vesicle surface through addition of aniline radical cations to the growing polymer chain. H 2 O 2 plays two essential roles, to oxidise the heme group of HRP, and to oxidise the growing polymer chain for allowing the stepwise addition of new aniline radical cations. The entire reaction can be divided into three kinetically distinct phases. In the first rapid phase (5-10 min), the actual polymer formation takes place to yield the emeraldine salt form of polyaniline in its bipolaron state. In the second and third slower phases (1-2 days) the bipolarons transform into polarons with unpaired electrons. During the reaction, the HRP activity is decreasing until the enzyme becomes inactive after polymer formation. Reactions carried out with partially deuterated anilines were analysed by 2 H magic-angle spinning (MAS) NMR spectroscopy to demonstrate the regioselectivity of the chain growth: para-coupling of the aniline units clearly dominates. Association of the formed polyaniline with the vesicle membrane is evident from cryo-TEM and SANS measurements.
The aniline dimer PADPA (= p-aminodiphenylamine = N-phenyl-1,4-phenylenediamine) was polymerized to poly-(PADPA) at 25 °C with Trametes versicolor laccase (TvL)/O 2 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 (J. Phys. Chem. B 2008, 112, 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.
The aniline dimer N-phenyl-1,4-phenylenediamine (= p-aminodiphenylamine, PADPA) was oxidized with horseradish peroxidase isoenzyme C (HRPC) and hydrogen peroxide (H 2 O 2) to oligo(PADPA) in an aqueous suspension of 80-100 nm-sized anionic vesicles at pH = 4.3 and at T 25 °C. The vesicles were formed from AOT (= sodium bis(2-ethylhexyl) sulfosuccinate) and served as templates for obtaining oligo(PADPA) as emeraldine salt form of polyaniline (PANI-ES) in the polaron form. The optimal reaction conditions for obtaining a stable oligo(PADPA)-AOT vesicle suspension with a high conversion and low amounts of HRPC were elaborated by using UV/vis/NIR spectroscopy. The formation of PANI-ES type products was confirmed by in situ UV/vis/NIR, Raman and EPR spectroscopy measurements. However, HPLC-MS analyses indicated that the oligo(PADPA) products obtained are not
Selenium is an essential element that is involved in biological redox processes. The electrode potentials of the selenocysteine half-reactions RSe(*) + e(-) --> RSe-, (RSeSeR)(*)(-) + e(-) --> 2 RSe(-), and RSeSeR + 2 e(-) --> 2 RSe(-) [E degrees' (pH 7)] are +0.43, +0.18, and -0.38 V, respectively, at pH 7. The spectra of RSe(*) and (RSeSeR)(*)(-) are characterized by absorption maxima at 460 nm (epsilon = 560 M(-)(1) cm(-)(1)) and 455 nm (epsilon = 7100 M(-)(1) cm(-)(1)), respectively. The bond dissociation energy of RSe-H has been calculated, and the value of 310 kJ/mol is in agreement with literature values. In comparison with the sulfur analogue cysteine, the more facile accessibility of the radical oxidation state is striking and may have biological implications, such as in mediation of one-electron- and two-electron-transfer processes, as illustrated by catalysis by selenocysteine of the electron transfer between dithiothreitol and benzyl viologen.
The decay of peroxynitrite [O=NOO(-), oxoperoxonitrate(1-)] was examined as a function of concentration (0.050-2.5 mM), temperature (5-45 degrees C), and pH (2.2-10.0). Below 5 degrees C and pH 7, little amounts of the decomposition products nitrite and dioxygen are formed, even when the peroxynitrite concentration is high (2.5 mM). Instead, approximately > or =90% isomerizes to nitrate. At higher pH, decomposition increases at the expense of isomerization, up to nearly 80% at pH 10.0 at 5 degrees C and 90% at 45 degrees C. Much less nitrite and dioxygen per peroxynitrite are formed when the peroxynitrite concentration is lower; at 50 microM and pH 10.2, < or =40% decomposes. In contrast to two other reports (Pfeiffer, S.; Gorren, A. C. F.; Schmidt, K.; Werner, E. R.; Hansert, B.; Bohle, D. S.; Mayer, B. J. Biol. Chem. 1997, 272, 3465-3470, and Coddington, J. W.; Hurst, J. K.; Lymar, S. V. J. Am. Chem. Soc. 1999, 121, 2438-2443), we find that the extent of decomposition is dependent on the peroxynitrite concentration.
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