Heme Release in Myoglobin−DDAB Films and Its Role in Electrochemical NO Reduction

Controlling the Selectivity of Electrocatalytic NO Reduction through pH and Potential Regulation on Single-Atom Catalysts

Electrocatalytic nitrogen oxide reduction (NOxRR) emerges as an effective way to bring the disrupted nitrogen cycle back into balance. However, efficient and selective NOxRR is still challenging partly due to the complex reaction mechanism, which is influenced by experimental conditions such as pH and electrode potential. Here, we have studied the enzyme-inspired iron single-atom catalysts (Fe-N4-C) and identified that the selectivity roots in the first step of the nitric oxide reduction. Combining the constrained molecular dynamics (MD) simulations with the quasi-equilibrium approximation, the effects of electrode potential and pH on the reaction free energy were considered explicitly and predicted quantitatively. Systematic heat maps for selectivity between single-N and N-N-coupled products in a wide pH-potential space are further developed, which have reproduced the experimental observations of NOxRR. The approach presented in this study allows for a realistic simulation of the electrocatalytic interfaces and a quantitative evaluation of interfacial effects. Our results in this study provide valuable and straightforward guidance for selective NOx reduction toward desired products by precisely designing the experimental conditions.

Electrocatalytic Systems for NOx Valorization in Organonitrogen Synthesis

Inorganic nitrogen oxide (NOx ) species, such as NO, NO2 , NO3 - , NO2 - generated from the decomposition of organic matters, volcanic eruptions and lightning activated nitrogen, play important roles in the nitrogen cycle system and exploring the origin of life. Meanwhile, excessive emission of NOx gases and residues from industry and transportation causes troubling problems to the environment and human health. How to efficiently handle these wastes is a global problem. In response to the growing demand for sustainability, scientists are actively pursuing sustainable electrochemical technologies powered by renewable energy sources and efficient utilization of hydrogen energy to convert NOx species into high-value organonitrogen chemicals. In this minireview, recent advances of electrocatalytic systems for NOx species valorization in organonitrogen synthesis are classified and described, such as amino acids, amide, urea, oximes, nitrile etc., that have been widely applied in medicine, life science and agriculture. Additionally, the current challenges including multiple side reactions and complicated paths, viable solutions along with future directions ahead in this field are also proposed. The coupling electrocatalytic systems provide a green mode for fixing nitrogen cycle bacteria and bring enlightenment to human sustainable development.

How to Turn an Electron Transfer Protein into a Redox Enzyme for Biosensing

Cytochrome c is a small globular protein whose main physiological role is to shuttle electrons within the mitochondrial electron transport chain. This protein has been widely investigated, especially as a paradigmatic system for understanding the fundamental aspects of biological electron transfer and protein folding. Nevertheless, cytochrome c can also be endowed with a non-native catalytic activity and be immobilized on an electrode surface for the development of third generation biosensors. Here, an overview is offered of the most significant examples of such a functional transformation, carried out by either point mutation(s) or controlled unfolding. The latter can be induced chemically or upon protein immobilization on hydrophobic self-assembled monolayers. We critically discuss the potential held by these systems as core constituents of amperometric biosensors, along with the issues that need to be addressed to optimize their applicability and response.

Quantification of Active Site Density and Turnover Frequency: From Single-Atom Metal to Nanoparticle Electrocatalysts

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe-N-C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe-N x sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1-13). The SD and TOF quantification can afford an improved understanding of the structure-activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst

Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron-nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm-2 h-1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.

Revisiting the nitrite reductase activity of hemoglobin with differential pulse voltammetry

Nitric oxide (NO) is an omnipresent signalling molecule in all vertebrates. NO modulates blood flow and neural activity. Nitrite anion is one of the most important sources of NO. Nitrite is reduced to NO by various physiological mechanisms including reduction by hemoglobin in vascular system. In this study, nitrite reductase activity (NRA) of hemoglobin is reported using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in a wide potential window from +0.3 V to -1.3 V (vs. Ag/AgCl). To the best of our knowledge, a detailed look into NRA of hemoglobin is proposed here for the first time. Our results indicated two different regimes for reduction of nitrite by hemoglobin in its Fe(II) and Fe(I) states. Both reactions showed a reversible behaviour in the time scale of the experiments. The first reduction displayed a normal redox behaviour, while the latter one had the characteristics of a catalytic electro-reduction/oxidation. The reduction in Fe(II) state was selected as a tool for comparing the NRA of hemoglobin (Hb) and hemoglobin-S (Hb-S) under native-like conditions in a didodecyldimethyl ammonium bromide (DDAB) liquid crystal film. These investigations lay the prospects and guidelines for understanding the direct electrochemistry of hemoglobin utilizing a simplified mediator-free platform.

Supramolecular electrode assemblies for bioelectrochemistry

For more than three decades, the field of bioelectrochemistry has provided novel insights into the catalytic mechanisms of enzymes, the principles that govern biological electron transfer, and has elucidated the basic principles for bioelectrocatalytic systems. Progress in biochemistry, bionanotechnology, and our ever increasing ability to control the chemistry and structure of electrode surfaces has enabled the study of ever more complex systems with bioelectrochemistry. This feature article highlights developments over the last decade, where supramolecular approaches have been employed to develop electrode assemblies that increase enzyme loading on the electrode or create more biocompatible environments for membrane enzymes. Two approaches are particularly highlighted: the use of layer-by-layer assembly, and the modification of electrodes with planar lipid membranes.

Effect of Bioconjugation on the Reduction Potential of Heme Proteins

The modification of protein surfaces employing cationic and anionic species enables the assembly of these biomaterials into highly sophisticated hierarchical structures. Such modifications can allow bioconjugates to retain or amplify their functionalities under conditions in which their native structure would be severely compromised. In this work, we assess the effect of this type of bioconjugation on the redox properties of two model heme proteins, that is, cytochrome c (CytC) and myoglobin (Mb). In particular, the work focuses on the sequential modification by 3-dimethylamino propylamine (DMAPA) and 4-nonylphenyl 3-sulfopropyl ether (S1) anionic surfactant. Bioconjugation with DMAPA and S1 are the initial steps in the generation of pure liquid proteins, which remain active in the absence of water and up to temperatures above 150 °C. Thin-layer spectroelectrochemistry reveals that DMAPA cationization leads to a distribution of bioconjugate structures featuring reduction potentials shifted up to 380 mV more negative than the native proteins. Analysis based on circular dichroism, MALDI-TOF mass spectrometry, and zeta potential measurements suggest that the shift in the reduction potentials are not linked to protein denaturation, but to changes in the spin state of the heme. These alterations of the spin states originate from subtle structural changes induced by DMAPA attachment. Interestingly, electrostatic coupling of anionic surfactant S1 shifts the reduction potential closer to that of the native protein, demonstrating that the modifications of the heme electronic configuration are linked to surface charges.

Structure and Modification of Electrode Materials for Protein Electrochemistry

The interactions between proteins and electrode surfaces are of fundamental importance in bioelectrochemistry, including photobioelectrochemistry. In order to optimise the interaction between electrode and redox protein, either the electrode or the protein can be engineered, with the former being the most adopted approach. This tutorial review provides a basic description of the most commonly used electrode materials in bioelectrochemistry and discusses approaches to modify these surfaces. Carbon, gold and transparent electrodes (e.g. indium tin oxide) are covered, while approaches to form meso- and macroporous structured electrodes are also described. Electrode modifications include the chemical modification with (self-assembled) monolayers and the use of conducting polymers in which the protein is imbedded. The proteins themselves can either be in solution, electrostatically adsorbed on the surface or covalently bound to the electrode. Drawbacks and benefits of each material and its modifications are discussed. Where examples exist of applications in photobioelectrochemistry, these are highlighted.

Electrochemistry of mammalian cytochrome P450 2B4 indicates tunable thermodynamic parameters in surfactant films

Electrochemical methods continue to present an attractive means for achieving in vitro biocatalysis with cytochromes P450; however understanding fully the nature of electrode-bound P450 remains elusive. Herein we report thermodynamic parameters using electrochemical analysis of full-length mammalian microsomal cytochrome P450 2B4 (CYP 2B4) in didodecyldimethylammonium bromide (DDAB) surfactant films. Electronic absorption spectra of CYP 2B4-DDAB films on silica slides reveal an absorption maximum at 418nm, characteristic of low-spin, six-coordinate, water-ligated Fe(III) heme in P450. The Fe(III/II) and Fe(II/I) redox couples (E1/2) of substrate-free CYP 2B4 measured by cyclic voltammetry are -0.23V and -1.02V (vs. SCE, or 14mV and -776mV vs. NHE) at 21°C. The standard heterogeneous rate constant for electron transfer from the electrode to the heme for the Fe(III/II) couple was estimated at 170s(-1). Experiments indicate that the system is capable of catalytic reduction of dioxygen, however substrate oxidation was not observed. From the variation of E1/2 with temperature (18-40°C), we have measured entropy and enthalpy changes that accompany heme reduction, -151Jmol(-1)K(-1) and -46kJmol(-1), respectfully. The corresponding entropy and enthalpy values are less for the six-coordinate low-spin, imidazole-ligated enzyme (-59Jmol(-1)K(-1) and -18kJmol(-1)), consistent with limited conformational changes upon reduction. These thermodynamic parameters are comparable to those measured for bacterial P450 from Bacillus megaterium (CYP BM3), confirming our prior reports that the surfactant environment exerts a strong influence on the redox properties of the heme.

Investigation of electrocatalytic pathway for hemoglobin toward nitric oxide by electrochemical approach based on protein controllable unfolding and in-situ reaction

An electrochemical approach based on protein controllable unfolding was developed and applied in combination with in-situ reaction in order to investigate the electrocatalytic pathway for hemoglobin (Hb) toward nitric oxide (NO). Hb was entrapped in a dimethyldidodecylammonium bromide (DDAB) film modified glassy carbon electrode (DDAB/Hb/GCE). Two typical denaturants of acid and urea were synergistically utilized to control the incorporated Hb to a most unfolded state without losing heme groups. Under optimal conditions, the unfolded DDAB/Hb/GCE exhibited accelerated direct electron transfer. The sensitivities for the detection of ascorbic acid (AA), NaNO(2) and NO were improved as 3, 10 and 12 times higher than those on the native DDAB/Hb/GCE, and the limits of detection (LODs) for AA, NaNO(2) and NO were down to 0.33, 0.83 and 0.063 μM, respectively. The unfolded DDAB/Hb/GCE was further applied for the investigation of Hb-NO interaction in NaNO(2) solution. With successive additions of AA, NO was generated in situ on DDAB/Hb/GCE. A new reduction peak of the intermediate HbFe(II)-HN(2)O(2) was successfully revealed near -0.65 V. The whole electrocatalytic mechanism was proposed and verified by density functional theory. The method can be a promising platform for facile study of the interaction between NO and heme proteins.

Combining voltammetry with HPLC: application to electro-oxidation of glycerol

The combination of cyclic voltammetry and "online" chromatographic techniques for product detection is limited by the typically long analysis times in chromatographic columns. Therefore, traditionally, product analysis is performed offline after long bulk electrolysis experiments. To overcome the limitation of the inherently different time scales of voltammetry and high-performance liquid chromatography (HPLC), we suggest here to adopt rapid online sample collection with a micrometer-sized sampling tip placed close to the working electrode, followed by offline analysis of the sample fractions in an HPLC system. To demonstrate this concept, we applied online fraction collection and offline HPLC analysis to the glycerol electro-oxidation on Au and Pt electrodes in alkaline media and show that we can successfully follow the concentration changes of glycerol and its reaction products in good correspondence with the current profile obtained simultaneously with voltammetry. Moreover, the method allows for a detailed discrimination of the different mechanisms of glycerol oxidation on both electrodes. Therefore, this simple approach enables the monitoring of soluble reaction products during voltammetry with an HPLC system and thereby allows for new insights into the mechanisms of complex multistep electrode reactions.

The influence of solution-phase HNO2 decomposition on the electrocatalytic nitrite reduction at a hemin-pyrolitic graphite electrode

The mechanism of nitrite electroreduction by hemin adsorbed at pyrolitic graphite is investigated. Two main issues are addressed: the effect of the medium pH and the selectivity of the reaction, which was determined by the combined use of the rotating ring disk electrode (RRDE) and online electrochemical mass spectroscopy (OLEMS). In acidic media, the behavior observed is indicative of the presence of NO, as the main reactant, generated from the solution-phase decomposition of HNO(2). Reduction of the NO-heme complex shows a Tafel slope of 59 mV/dec(-1) and a pH dependence of 42 mV/pH, indicative of a so-called EC mechanism. In acidic media, HNO(2) and NO are reduced to hydroxylamine (NH(2)OH) with almost 100% selectivity at low potentials, nitrous oxide (N(2)O) being only a minor side product. In neutral media, the hemin is largely unresponsive to the presence of nitrite, giving only a very small reduction current. The comparison of our simple heme catalyst to the behavior of the naturally occurring heme-containing nitrite reductases, which operate under biological conditions, suggests that these enzymes dissociate nitrite at neutral pH either via a complexation step favored by a specific ligating environment or by locally regulating the pH to induce HNO(2) dissociation.

A comparison of the higher order harmonic components derived from large-amplitude Fourier transformed ac voltammetry of myoglobin and heme in DDAB films at a pyrolytic graphite electrode

A debate as to whether heme remains bound or is released in myoglobin molecules incorporated into a didodecyldimethylammonium bromide (DDAB) film adhered to a pyrolytic graphite electrode has prompted a comparison of their electrochemistry by the highly sensitive large-amplitude Fourier transformed ac voltammetric method. The accessibility of third, fourth, and higher harmonic components that are devoid of background current and the enhanced resolution relative to that available in dc voltammetry have allowed a detailed comparison of the Fe(III)/Fe(II) and Fe(II)/Fe(I) redox processes of myoglobin and heme molecules to be undertaken as a function of buffer composition and pH and in the presence and absence of NaBr in the buffer and/or film. Under most conditions examined, only very subtle differences, in the Fe(III)/Fe(II) process were found, implying this process cannot be used to indicate the intactness or otherwise of myoglobin in myoglobin-DDAB films. In contrast, higher order ac harmonics obtained from myoglobin-DDAB and heme-DDAB films reveal pH dependent differences with respect to the Fe(II)/Fe(I) couple. Analysis of the ac harmonics, and with the hypothesis that the Fe(II)/Fe(I) process reflects the myoglobin state, suggests that the majority of the iron heme is released from myoglobin-DDAB (pH 5.0, no NaBr) films in contact with pH 5.0 (0.1 M sodium acetate) buffer solution devoid of or containing NaBr. However, myoglobin films prepared with pH 5.0 buffer containing NaBr shows significant difference in the higher harmonic shapes and midpoint potentials in the Fe(II)/Fe(I) process relative to the case when heme molecules are used, although as noted in other studies, a significant fraction of the Mb is rendered electroinactive in the presence of NaBr. The voltammetric responses of myoglobin and heme-DDAB (pH 5.0) films in contact with pH 7.0 (0.1 M) phosphate buffer solution also exhibit significant differences in the Fe(II)/Fe(I) redox couple in the higher harmonics in contrast to a report [de Groot, M.T.; Merkx, M.; Koper, M. T. M. J. Am. Chem. Soc. 2005, 127, 16224] that claimed identical midpoint potentials apply to both films under conditions of dc cyclic voltammetry. The FT-ac voltammetric data therefore suggest that a substantial fraction of myoglobin in myoglobin-DDAB (pH 5.0) films in contact with pH 7.0 phosphate buffer solution remains intact. No evidence of a catalytic effect that enhanced the released of heme from myoglobin was found at the pyrolytic graphite electrode surface. In summary, higher harmonic ac voltammetric data indicate that the Fe(II)/Fe(I) process but not the Fe(III)/Fe(II) reflects the state of myoglobin in DDAB films. On this basis, films prepared at pH 5.0 should include NaBr, or else films should be prepared at neutral pH to achieve films with myoglobin remains in its intact near native state when a myoglobin-DDAB film is confined to a graphite electrode surface. Otherwise, the release of heme in myoglobin molecules incorporated into a DDAB film is likely to be a dominant reaction pathway.

Cerium phosphate nanotubes: synthesis, characterization and biosensing

Cerium phosphate (CeP) nanotubes have been synthesized and confirmed by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The 1D nanomaterial has a monoclinic crystal structure with a mean width of 15-20 nm and a length up to several micrometers. The nanotubes have been employed as electrode substrates for immobilization and direct electrochemistry of heme proteins/enzymes with myoglobin (Mb) as a model. The electrochemical characteristics of the Mb-CeP/GC electrode were studied by voltammetry. After being immobilized on the nanotubes, Mb can keep its natural structure and undergo effective direct electron transfer reaction with a pair of well-defined redox peaks at -(367 +/- 3) mV (pH 7.5). The apparent electron transfer rate constant is (9.1 +/- 1.4) s(-1). The electrode displays good features in the electrocatalytic reduction of H(2)O(2), and thus can be used as a biosensor for detecting the substrate with a low detection limit (0.5 +/- 0.05 microM), a wide linear range (0.01-2 mM), high sensitivity (14.4 +/- 1.2 microA mM(-1)), as well as good stability and reproducibility. CeP nanotubes can become a simple and effective biosensing platform for the integration of heme proteins/enzymes and electrodes, which can provide analytical access to a large group of enzymes for a wide range of bioelectrochemical applications.

Apoferritin as a bionanomaterial to facilitate the electron transfer reactivity of hemoglobin and the catalytic activity towards hydrogen peroxide

In this report, apoferritin as a stable bionanomaterial was modified with hemoglobin on pyrolytic graphite electrode. Rapid electron transfer reactions of hemoglobin were achieved with the help of apoferritin in a large pH range. Moreover, hemoglobin as an enzyme exhibits fine electrocatalytic activity towards the reaction of hydrogen peroxide, and a wide concentration range of linear relationship between the reduction peak current and the concentration of hydrogen peroxide has been obtained with a higher upper detection limit, which may be further developed for a hydrogen peroxide biosensor. Therefore, a new property of apoferritin is explored, in which apoferritin works as a bionanomaterial to be an accelerant of the electron transfer of Hb and a stabilizer to retain the catalytic ability of the protein under mal-condition.

Evidence for heme release in layer-by-layer assemblies of myoglobin and polystyrenesulfonate on pyrolitic graphite

Layer-by-layer assemblies of myoglobin and polystyrenesulfonate (PSS) on pyrolitic graphite have been investigated with the goal of determining the origin of the voltammetric response of these films. From the similar midpoint potential, coverage and electron transfer behavior compared with those of adsorbed free heme, it was concluded that the observed voltammetric peak is due to heme adsorbed at the electrode surface. This suggests that the interactions between the pyrolitic graphite electrode, PSS and myoglobin can result in heme release from the protein followed by heme adsorption on the electrode.