A mediated thin-layer voltammetry method for the study of redox protein electrochemistry

Redox mediators boost NOx reduction via trade-off electron charges using a cube-shaped (cMn@rGO) catalyst; mechanism and electrochemical study

Gaseous pollutants like sulfur dioxide and nitrogen oxide(s) (SO2, NOx) have been increasing exponentially for the last two decades, which have had adverse effects on human health, aquatic life, and the environment. Recently, for air pollution taming, manganese/oxide (Mn/MnO) has become a very promising heterogeneous catalyst due to its environment-friendly, low-price, and remarkable catalytic abilities for toxic gases. In this work, cube-shaped Mn nanoparticles (cMn NPs) were decorated on the surface of reduced graphene oxide (rGO) by the solvothermal method. The resulting cMn@rGO composite was employed for electrochemical NOx reduction. However, the microscopic (TEM/HRTEM) and structural analysis were utilised to investigate the morphology and characteristics of the cMn@rGO composite. This electrochemical-based treatment for NOx reduction is employed by using electron shuttle or redox mediators. Here, four distinct redox mediators are used to address electrochemical obstacles, which effectively facilitate electron transportation and promoted NOx reduction on the electrode surface. These mediators not only significantly enhanced the NOx conversion into valuable products, i.e., N2 and N2O, but also made the process smooth with high performance. Among these mediators, neutral red (N.R) exhibited extraordinary potential in enhancing NOx reduction. The obtained results indicated that the remarkable catalytic performance (∼93%) of the cMn@rGO can be attributed to several factors, including the catalyst's three-dimensional architecture structure and abundant active sites. The designed catalyst (cMn@rGO) is not only cost-effective and sustainable but also exhibits excellent potential in effectively reducing NOx, which could be beneficial for large-scale NOx abatement.

Electrochemical analyses of redox-active iron minerals: a review of nonmediated and mediated approaches

Redox-active minerals are ubiquitous in the environment and are involved in numerous electron transfer reactions that significantly affect biogeochemical processes and cycles as well as pollutant dynamics. As a consequence, research in different scientific disciplines is devoted to elucidating the redox properties and reactivities of minerals. This review focuses on the characterization of mineral redox properties using electrochemical approaches from an applied (bio)geochemical and environmental analytical chemistry perspective. Establishing redox equilibria between the minerals and working electrodes is a major challenge in electrochemical measurements, which we discuss in an overview of traditional electrochemical techniques. These issues can be overcome with mediated electrochemical analyses in which dissolved redox mediators are used to increase the rate of electron transfer and to facilitate redox equilibration between working electrodes and minerals in both amperometric and potentiometric measurements. Using experimental data on an iron-bearing clay mineral, we illustrate how mediated electrochemical analyses can be employed to derive important thermodynamic and kinetic data on electron transfer to and from structural iron. We summarize anticipated methodological advancements that will further contribute to advance an improved understanding of electron transfer to and from minerals in environmentally relevant redox processes.

Cyclic voltammetric study of the redox system of glutathione using the disulfide bond reductant tris(2-carboxyethyl)phosphine

The stabilization of the reduction state of proteins and peptides is very important for the monitoring of protein-protein, protein-DNA and protein-xenobiotic interactions. The reductive state of protein or peptide is characterized by the reactive sulfhydryl group. Glutathione in the reduced (GSH) and oxidized (GSSG) forms was studied by cyclic voltammetry. Tris(2-carboxyethyl)phosphine (TCEP) as the disulfide bond reductant and/or hydrogen peroxide as the sulfhydryl group oxidant were used. Cyclic voltammetry measurements, following the redox state of glutathione, were performed on a hanging mercury drop electrode (HMDE) in borate buffer (pH 9.2). It was shown that in aqueous solutions TCEP was able to reduce disulfide groups smoothly and quantitatively. The TCEP response at -0.25 V vs. Ag/AgCl/3 M KCl did not disturb the signals of the thiol/disulfide redox couple. The origin of cathodic and anodic signals of GSH (at -0.44 and -0.37 V) and GSSG (at -0.69 and -0.40 V) glutathione forms is discussed. It was shown that the application of TCEP to the conservation of sulfhydryl groups in peptides and proteins can be useful instrument for the study of peptides and proteins redox behavior.

Elucidating thermodynamic parameters for electron transfer proteins using isothermal titration calorimetry: application to the nitrogenase Fe protein

Establishing thermodynamic parameters for electron transfer reactions involving redox proteins is essential for a complete description of these important reactions. While various methods have been developed for measuring the Gibbs free energy change (Delta G(HR) or E(m)) for the protein half-reactions, deconvolution of the respective contributions of enthalpy (Delta H(HR)) and entropy (Delta S(HR)) changes is much more challenging. In the present work, an approach is developed using isothermal titration calorimetry (ITC) that allows accurate determination of all of these thermodynamic parameters for protein electron transfer half-reactions. The approach was validated for essentially irreversible and reversible electron transfer reactions between well-characterized mediators and between mediators and the protein cytochrome c. In all cases, the measured thermodynamic parameters were in excellent agreement with parameters determined by electrochemical methods. Finally, the calorimetry approach was used to determine thermodynamic parameters for electron transfer reactions of the nitrogenase Fe protein [4Fe-4S](2+/+) couple in the absence or presence of MgADP or MgATP. The E(m) value was found to change from -290 mV in the absence of nucleotides to -381 mV with MgATP and -423 mV with MgADP, consistent with earlier values. For the first time, the enthalpy (Delta H(HR)) and entropy (Delta S(HR)) contributions for each case were established, revealing shifts in the contribution of each thermodynamic parameter induced by nucleotide binding. The results are discussed in the context of current models for electron transfer in nitrogenase.

Chemical and on-line electrochemical reduction of metalloproteins with high-resolution electrospray ionization mass spectrometry detection

The observation of the reduced forms of several metal-containing proteins using electrospray ionization (ESI) is reported for the first time. High-resolution mass analysis using Fourier transform ion cyclotron resonance mass spectrometry allows the oxidized and reduced forms of the proteins to be distinguished. The metalloproteins are reduced both chemically and electrochemically. Under normal sample handling conditions, the proteins that are reduced in solution appear in their oxidized form in their ESI mass spectra. Rigorous exclusion of oxygen from the solution of the reduced protein allows the observation of the reduced form in the gas phase. The metal centers investigated include heme and non-heme iron proteins, copper, and a manganese-substituted iron-sulfur cluster of the form [3FeMn-4S]. The electrochemical method is shown to provide several advantages over chemical reduction. The oxidation state of the metal center is stable with respect to electrospray ionization in both positive and negative ionization modes.

Use of stopped-flow spectrophotometry to establish midpoint potentials for redox proteins

Stopped-flow spectrophotometry was examined as a tool to assign midpoint potentials to protein redox half-reactions. The method involves the rapid mixing of protein and electron transfer mediator solutions and the determination of the absorbance of at least one of the reacting species or products at equilibrium. The utility of the method was demonstrated with two different redox proteins (nitrogenase iron protein and cytochrome c) with very different midpoint potentials. The overall errors ranged from about +/-0.5 to 3 mV. Advantages of the method include short times required for the experiments, the precision and accuracy of the method in comparison to other methods, the conservative use of valuable protein in the experiments and the ease of obtaining midpoint potentials for redox protein half-reactions, and the potential range covered by a single electron transfer mediator when the method involves mediated electron transfer. It is concluded that the stopped-flow spectrophotometry should be considered the method of choice for determining protein midpoint potentials.

Protein redox potential measurements based on kinetic analysis with mediated continuous-flow column electrolytic spectroelectrochemical technique. Application to TTQ-containing methylamine dehydrogenase

Kinetic determination of protein redox potentials with a mediated continuous-flow column electrolytic spectroelectrochemical technique (CFCESET) is described. In this method, the redox state of the mediator is completely regulated by the continuous-flow column electrolysis, and the homogeneous redox reaction between the mediator and a protein sample in the column is monitored spectroscopically at the downstream of the column. The protein/mediator reaction is in the pseudo-first-order kinetics, and then the rate equation is analytically solved. The kinetic analysis provides the protein redox potential as well as the homogeneous rate constant. In the kinetic measurements, equilibration of the system within the column is not required, which allows the use of increased kinds of mediators. This method was successfully applied to quinoprotein methylamine dehydrogenase containing tryptophan tryptophylquinone (TTQ) as a prosthetic group. The kinetic aspect is also valuable for the thermodynamic analysis with the mediated CFCESET. The half-life time of the kinetics can be utilized to optimize the system for the attainment of the equilibrated state within the column and can provide the assurance that the system is in equilibrium.

Entropies of redox reactions between proteins and mediators: the temperature dependence of reversible electrode potentials in aqueous buffers

The temperature dependencies of the reversible electrode potentials for a number of charge transfer reactions of redox mediators were used to evaluate the corresponding charge transfer entropies in Tris-HCl (pH 8) buffer. The redox mediator thermodynamic data, along with reaction enthalpy data for mediator redox protein electron transfer, were used to evaluate the charge transfer entropy for the cytochrome c redox couple [(cytc)ox/(cytc)red] in Tris-HCl (pH 8) buffer and were found to be equal to -16 cal/degrees K mol. Reversible electrode potentials at 298 degrees K for the redox mediator half-reactions were observed to vary from -528 to +657 mV (vs NHE). Charge transfer entropies were observed to depend upon the structure of the redox mediators and to vary from -13.8 to -29.7 cal/degrees K mol for a closely related series of organic dications (viologens) and a value of -43.6 cal/degrees K mol was observed for the [Fe(CN)6]3-/[Fe(CN)6]4- couple under the same conditions. A procedure for determining charge transfer entropies of protein redox couples which cannot be studied by direct electrochemical methods is outlined. The factors contributing to the magnitude of the charge transfer entropies are discussed.

Determination of rate and equilibrium constants for the reactions between electron transfer mediators and proteins by linear sweep voltammetry

Redox proteins undergo measurable charge transfer at electrodes only under special circumstances, while they readily take part in electron transfer reactions with mediators in solution. Advantage was taken of the latter fact to develop a new method to study the kinetics and equilibria of protein-mediator electron transfer reactions. It was shown that rate and equilibrium constants for the electron exchange between electron transfer mediator and the protein can be obtained from the analysis of the perturbation of the linear sweep voltammetry (LSV) response of the mediator due to the presence of the protein. The experiments were carried out under conditions where the protein does not interact with the electrode. Theoretical data obtained by digital simulation are presented to show the conditions under which rate and equilibrium constants are accessible by the LSV technique. The electron transfer reactions between ferri- and ferrocytochrome c and N,N,N',N'-tetramethylphenylenediamine and the corresponding radical cation in phosphate-buffered saline (0.04 M phosphate, pH 7.4, 0.1 M NaCl) buffer were selected to demonstrate the technique. These studies resulted in an equilibrium constant equal to 1.0 and forward and reverse rate constants equal to 1.6 x 10(4) M-1 s-1. The data available from this method include forward and reverse rate constants for electron transfer and the formal potential for the protein redox couple.