Formal Reduction Potential of 3,5-Difluorotyrosine in a Structured Protein: Insight into Multistep Radical Transfer

The reversible Y–O•/Y–OH redox properties of the α[subscript 3]Y model protein allow access to the electrochemical and thermodynamic properties of 3,5-difluorotyrosine. The unnatural amino acid has been incorporated at position 32, the dedicated radical site in α[subscript 3]Y, by in vivo nonsense codon suppression. Incorporation of 3,5-difluorotyrosine gives rise to very minor structural changes in the protein scaffold at pH values below the apparent pK (8.0 ± 0.1) of the unnatural residue. Square-wave voltammetry on α[subscript 3](3,5)F[subscript 2]Y provides an E°′(Y–O•/Y–OH) of 1026 ± 4 mV versus the normal hydrogen electrode (pH 5.70 ± 0.02) and shows that the fluoro substitutions lower the E°′ by −30 ± 3 mV. These results illustrate the utility of combining the optimized α[subscript 3]Y tyrosine radical system with in vivo nonsense codon suppression to obtain the formal reduction potential of an unnatural aromatic residue residing within a well-structured protein. It is further observed that the protein E°′ values differ significantly from peak potentials derived from irreversible voltammograms of the corresponding aqueous species. This is notable because solution potentials have been the main thermodynamic data available for amino acid radicals. The findings in this paper are discussed relative to recent mechanistic studies of the multistep radical-transfer process in Escherichia coli ribonucleotide reductase site-specifically labeled with unnatural tyrosine residues.National Institutes of Health (U.S.) (Grant GM29595

Characterization of the Surface Electrode Reaction in the Presence of Uniform Interaction: The Case of Mo(VI) Reduction in the Presence of Phenanthroline and an Excess of Fulvic Acids

cited By 1International audienceSurface electrode reactions involving lateral uniform interactions between adsorbed species is studied by means of square-wave voltammetry (SWV). Interactions are represented by the interaction product aθ, were a is the Frumkin interaction parameter (a is positive for attraction and negative for repulsion forces) and θ is the surface coverage. The properties of the SW voltammetric response enable detection of interactions and recognition of the type of interaction forces by a simple procedure. The influence of the interactions on the apparent electrochemical reversibility of the surface electrode reaction is studied in detail. Utilizing "quasireversible maximum" the simple methodology for estimation of the standard redox rate constant without knowing the exact value of the interaction product aθ is developed. Theoretical predictions are illustrated and confirmed by experiments with Mo(VI) in the presence of phenantroline and an excess of fulvic acids

Catalytic adsorptive stripping voltammetry of molybdenum: Redox kinetic measurements

cited By 10International audienceThe electrode mechanism of Mo(VI) reduction was studied under catalytic adsorptive stripping mode by means of square-wave voltammetry (SWV). Mo(VI) creates a stable surface active complex with mandelic acid. The electrode reaction of Mo(VI)-mandelic acid system undergoes as one-electron reduction, exhibiting properties of a surface electrode process. In the presence of chlorate, bromate, and hydrogen peroxide, the electrode reaction is transposed into a catalytic mechanism. The experimental results are compared with the recent theory for surface catalytic reaction, enabling qualitative characterization of the electrode mechanism in the presence of different catalytic agents. Utilizing both the method of "split SW peaks" and "quasireversible maximum" the standard redox rate constant of Mo(VI)-mandelic acid system was estimates as ks = 150 ± 5 s-1. By fitting the experimental and theoretical results, the following catalytic rate constants have been estimated: (8.0 ± 0.5) × 104 dm3 s-1, (1.0 ± 0.1) × 105 mol-1 dm3 s-1, and (3.2 ± 0.1) × 106 mol-1 dm3 s -1, for hydrogen peroxide, chlorate, and bromate, respectively

The role of adsorption in the catalytic electrode mechanism studied by means of square-wave voltammetry

cited By 6International audienceA complex electrode mechanism coupled with adsorption of the redox couple and an irreversible homogeneous chemical reaction that regenerates the electroactive reactant is studied both theoretically and experimentally under conditions of square-wave voltammetry. The homogeneous regenerative redox reaction, i.e., the so-called catalytic reaction, takes place as a surface or volume process, depending on whether it includes the dissolved or adsorbed form of the electrode product, respectively. A rigorous theoretical model is presented considering simultaneously all relevant phenomena affecting the voltammetric response, such as mass transport, adsorption equilibria and kinetics of two different catalytic reactions, i.e., volume and surface catalytic reactions. The solutions for the surface concentration of the electroactive species are presented in the form of integral equations, thus they are general and valid for any chronoamperometric and voltammetric technique. The model enables systematic study of the role of adsorption in a complex catalytic electrode mechanism. The properties of the voltammetric response show remarkable discrepancies between the effect of the volume and surface catalytic reactions, thus enabling their recognition and separate characterization. From an analytical point of view it is demonstrated that the catalytic mechanism of moderate adsorption is superior in comparison with the pure volume and surface catalytic mechanisms. The theoretical predictions are confirmed by the voltammetric behaviour of the redox couple azobenzene/hydrazobenzene at the mercury electrode by using a mixture of water + acetonitrile as a solvent. © 2005 Elsevier B.V. All rights reserved

Lutetium bis(tetra-tert-butylphthalocyaninato): A superior redox probe to study the transfer of anions and cations across the water|nitrobenzene interface by means of square-wave voltammetry at the three-phase electrode

cited By 34International audienceThe redox properties of lutetium bis(tetra-tert-butylphthalocyaninato) (LBPC) have been studied in nitrobenzene that is deposited as a microfilm on the surface of highly oriented pyrolytic graphite electrodes. The behavior of the modified electrode, which is immersed in an aqueous electrolyte solution, is typical for the three-phase electrode (Scholz, F.; Komorsky-Lovrić, Š.; Lovrić, M. Electrochem. Comm. 2000, 2, 112-118). LBPC can be both oxidized and reduced in one electron reversible processes. The oxidation and the reduction of LBPC at the graphite|nitrobenzene interface is accompanied by the transfer of anion or cation, respectively, from the aqueous phase into the organic layer. Thus, using LBPC as a redox probe for the three-phase electrode, the transfer of both anions and cations across the water|nitrobenzene interface can be studied in a single experiment. The hydrophobicity of LBPC is so high that it enables inspection of cations and anions with ΔwnbGCat+θ ≤ 43 kJ/mol and Δwnb ≤ 50 kJ/mol, respectively. The direct transfer of Na+ and Li+ from water to nitrobenzene, mutually saturated, is achieved for the first time at a macroscopic water|nitrobenzene interface

Kinetics of anion transfer across the liquid | liquid interface of a thin organic film modified electrode, studied by means of square-wave voltammetry

cited By 44International audienceThe electrochemical oxidation of lutetium bis(tetra-tert- butylphthalocyaninato) (LBPC) and decamethylferrocene (DMFC), as well as the reduction of LBPC, lutetium bis(phthalocyaninato) (LPC), and lutetium (tetra-tert-butylphthalocyaninato hexadecachlorphthalocyaninato) (LB-PCl), has been studied in a thin nitrobenzene (NB) film deposited on the surface of a graphite electrode (GE) by means of square-wave voltammetry (SWV). The organic film-modified electrode was immersed in an aqueous (W) electrolyte solution and used in a conventional three-electrode configuration. When the aqueous phase contains ClO4-, NO3-, or Cl - (ClO4-, or NO3- only, in the case of DMFC), both LBPC and DMFC are oxidized to stable monovalent cations in the organic phase. The electron transfer at the GE | NB interface is accompanied by a simultaneous anion transfer across the W | NB interface to preserve the electroneutrality of the organic phase. LBPC, LPC, and LBPCl are reduced to stable monovalent anions accompanied by expulsion of the anion of the electrolyte from the organic into the aqueous phase. In all cases, the overall electrochemical process comprises simultaneous electron and ion transfer across two separate interfaces. Under conditions of SWV, the overall electrochemical process is quasireversible, exhibiting a well-formed "quasireversible maximum" that is an intrinsic property of electrode reactions occurring in a limiting diffusion space. For all the redox compounds that have been studied, the kinetics of the overall electrochemical process is controlled by the rate of the ion transfer across the liquid | liquid interface. Based on the quasireversible maximum, a novel and simple methodology for measuring the rate of ion transfer across the liquid liquid interface is proposed. A theoretical background explaining the role of the ion-transfer kinetics on the overall electrochemical process at the thin organic film modified electrode under conditions of SWV is presented. Comparing the positions of the theoretical and experimental quasireversible maximums, the kinetics of ClO4 -, NO3-, and Cl- across the W | NB interface was estimated. The kinetics of the overall process at the thin organic film modified electrode, represented by the second-order standard rate constant, is 91 ± 8, 90 ± 4, and 133 ± 10 cm4 s-1 mol-1, for the transfer of ClO4 -, NO3-, and Cl- respectively. © 2005 American Chemical Society