Reduction of cytochrome c at a lipid bilayer modified electrode

The reduction of horse heart cytochrome c has been investigated at a platinum electrode modified with a lipid bilayer membrane (BLM) which immobilized vinyl ferrocene as an electron mediator. The current—voltage curves show that the direct electrochemistry of cytochrome c at the metal electrode occurs quite efficiently. An adsorption equilibrium constant for cytochrome at the BLM surface, as well as an electron transfer rate constant between the protein and the modified electrode have been estimated from these results. The values of both parameters are much higher than those reported with other types of electrode modifications, indicating that a lipid bilayer-modified platinum electrode system using vinyl ferrocene as a mediator provides substantial improvements in electrochemical activity of cytochrome c at metal electrodes. The potential for modifying and utilizing this new class of “biomembrane-like” electrode surface for metalloprotein electrochemistry is briefly discussed.     

Direct electrochemistry of bacterial cytochrome c551 at surface-modified gold electrodes

The direct electrochemistry of the single heme cytochrome c₅₅₁ from the bacterium Pseudomonas aeruginosa has been investigated at gold electrodes surface-modified through chemisorption of polyfunctional organic molecules. The results have been compared and contrasted with those obtained under the same conditions for the eukaryotic cytochrome c from horse heart. Both cytochromes give a quasi-reversible electrode reaction at pH 6.0 at a modified interface presenting only 4-pyridyl groups to the solution suggesting the occurrence, in both cases, of a hydrogen bonding interaction from lysine side-chains on the protein to pyridyl-nitrogens on the electrode surface. However, in contrast, gold electrodes modified by Pyridine-n-AldehydeThioSemicarbazones (n = 2, 3, 4) give electrochemistry which is strongly isomer-dependent in the case of horse heart cytochrome c but completely isomer-independent in the case of cytochrome c₅₅₁. It is suggested that interaction of the eukaryotic protein with surfaces is dominated by its lysine residues only, but that interaction of the bacterial cytochrome is through hydrogen bonding from the surface to both lysines and carboxylate groups of aspartate residues. This is supported by observation of the loss of cytochrome c₅₅₁ electrochemistry at 4-pyridyl-only modified gold at pH 9.0 compared with the good, quasi-reversible electrochemistry maintained under the same conditions at PATS-4 modified gold. It is concluded that, while the two cytochromes show many similarities with respect to their structures and functions, they have quite different interfacial electron transfer reactions, particularly at PATS-modified electrodes. This may correlate with the known large differences between the two proteins in net electrostatic charge and surface charge distribution.     

Interfacial electrochemistry of cytochrome c at tin oxide,indium oxide,gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10^?2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.     

Surface modifiers for the promotion of direct electrochemistry of cytochrome c

Fifty-four bifunctional organic compounds were studied to assess their ability to promote the direct electrochemistry of horse heart cytochrome c at a modified gold electrode. From the results of the survey it was possible to identify those features important for successful promotion of the electrochemical activity. It is suggested that it is necessary to provide groups on the electrode surface which can hydrogen bond or form salt bridges to the positively charged lysine side chain groups around the heme crevice on cytochrome c. The functional groups, Y, may be anionic or weakly basic and can be attached to the electrode using a bifunctional compound X ～ Y. The group, X, adsorbs or binds to the gold surface through nitrogen, phosphorus, or sulphur. A “pre-activation” step for the adsorption of some surface modifiers has been discovered. The molecular structure of a compound which promotes cytochrome c electrochemistry can be either conformationally rigid or flexible, aromatic or aliphatic, but it should direct Y out from the electrode. The length of the molecule does not appear to affect the rate of electron transfer. The presence of a hydrophobic zone in the structure is neither necessary nor sufficient for successful promotion of cytochrome c electrochemistry.     

Electrochemistry and biosensing activity of cytochrome c immobilized in macroporous materials

An amperometric biosensor for hydrogen peroxide (H₂O₂) has been constructed by immobilizing cytochrome c on an indium/tin oxide (ITO) electrode modified with a macroporous material. Cyclic voltammetry showed that the direct and quasi-reversible electron transfer of cytochrome c proceeds without the need for an electron mediator. A surface-controlled electron transfer process can be observed with an apparent heterogeneous electron-transfer rate constant (k_s) of 29.2?s^?1. The biosensor displays excellent electrocatalytic responses to the reduction of H₂O₂ to give amperometric responses that increase steadily with the concentration of H₂O₂ in the range from 5???M to 2?mM. The detection limit is 0.61???M at pH?7.4. The apparent Michaelis-Menten constant (K_m) of the biosensor is 1.06?mM. This investigation not only provided a method for the direct electron transfer of cytochrome c on macroporous materials, but also established a feasible approach for durable and reliable detection of H₂O₂.

Electrochemistry and biosensing activity of cytochrome c immobilized on a mesoporous interface assembled from carbon nanospheres

We report on an amperometric biosensor for hydrogen peroxide. It is obtained via layer-by-layer assembly of ordered mesoporous carbon nanospheres and poly(diallyldimethylammonium) on the surface of an indium tin oxide (ITO) glass electrode and subsequent adsorption of cytochrome c. UV–vis absorption spectroscopy was applied to characterize the process of forming the assembled layers. Cyclic voltammetry revealed a direct and quasi-reversible electron transfer between cytochrome c and the surface of the modified ITO electrode. The surface-controlled electron transfer has an apparent heterogeneous electron-transfer rate constant (k _s ) of 5.9?±?0.2?s^?1 in case of the 5-layer electrode. The biosensor displays good electrocatalytic response to the reduction of H₂O₂, and the amperometric signal increase steadily with the concentration of H₂O₂ in the range from 5?μM to 1.5?mM. The detection limit is 1?μM at pH 7.4. The apparent Michaelis-Menten constant (K _m ) of the sensor is 0.53?mM. We assume that the observation of a direct electron transfer of cytochrome c on mesoporous carbon nanospheres may form the basis for a feasible approach for durable and reliable detection of H₂O₂.

The indirect coulometric titration of cytochrome c oxidase with cytochrome c

The indirect coulometric titration of cytochrome c oxidase and dioxygen using cytochrome c as a mediator is described. Results of both the indirect coulometric titrations and the cyclic voltammetric experiments reported herein verify that the reaction mechanism involves the catalytic regeneration of the electroactive species, the cytochrome c mediator, with the selective reduction of cytochrome c oxidase alone. During the indirect coulometric titrations dioxygen is reduced to water only by cytochrome c oxidase and not by either direct reduction at the electrode surface or reaction with cytochrome c. This system utilizes the electron transfer selectivity of cytochrome c for cytochrome c oxidase over dioxygen and offers a means by which the reaction of cytochrome c oxidase and dioxygen can be examined.     

Interfacing cytochrome c to electrodes with a DNA – carbon nanotube composite film

Multi-walled carbon nanotube (MWCNT) is successfully immobilized on the surface of platinum electrode by mixing with DNA. The DNA/MWCNT modified electrodes are characterized by electrochemical impedance spectroscopy and cyclic voltammetry. Further research indicates that cytochrome c can strongly adsorbed on the surface of the modified electrode, and forms an approximate monolayer. The immobilized MWCNT can promote the redox of horse heart cytochrome c which gives reversible redox peaks with a formal potential of 81 mV vs SCE.     

UV-ozone dry-cleaning process for indium oxide electrodes for protein electrochemistry

The present study reports, for the first time, on electrochemical responses of cytochrome c at a UV-ozone treated indium oxide electrode. Results from surface tension measurements indicate that UV-ozone treatment is an efficient cleaning procedure to remove organic species contamination on surfaces. Well-defined redox responses for cytochrome c were observed at a UV-ozone treated fully hydrophilic indium oxide electrode. Electrochemical parameters, including the diffusion coefficient, the heterogeneous electron transfer rate constant and the redox potential, were in good agreement with those previously reported. However, decrease in peak current for cytochrome c and [Fe(CN)₆]⁴⁻ were observed at a UV-ozone treated electrode. From XPS results, this behavior would be understood to indicate a decrease in homogeneous active electrode surface area by a decrease in conductivity of the indium oxide surface by UV-ozone treatment. Simple and effective UV-ozone treatment methods are useful for surface contamination sensitive electrochemistry.     

Surface functions of 2-mercaptopyridine, 2-mercaptopyrazine and 2-mercaptoquinoxaline modified Au(1 1 1) electrodes for direct rapid electron transfer of cytochrome c

Surface functions of modified electrodes for the rapid electron transfer of cytochrome c have been examined by using new surface modifiers. 2-Mercaptopyrazine (2-PyZSH) and 2-mercaptoquinoxaline (or 2-mercaptobenzopyrazine, 2-MQ) modified Au(1 1 1) electrodes gave well-defined cyclic voltammmograms of cytochrome c, while a 2-mercaptopyridine (2-PySH) modified electrode gave no response. The STM images of 2-PySH and 2-PyZSH modified surfaces were similar to each other, suggesting 2-PySH and 2-PyZSH adsorbed at both thiolate S and pyridine (or pyrazine) N atoms with pyridine (or pyrazine) ring being perpendicular to the electrode surface. The 2-PyZSH modified surface has another pyrazine N atom faced to the solution, through which cytochrome c can interact, and the double layer capacitance data of the electrode gave more hydrophilic nature than the 2-PySH modified surface. On the other hand, although 2-MQ was suggested to adsorb on the electrode in a similar manner to 2-PySH and 2-PyZSH to give N atom at the solution side, the 2-MQ modified surface showed less hydrophilicity than the 2-PySH modified surface due to the quinoxaline ring. These results clearly reveal that the pyridine and pyrazine N atoms faced to the solution (rather than the hydrophilicity of the electrode surface) are important for the rapid electron transfer of cytochrome c on these modified electrodes.     

Remarkable Electrochemical Responses of Ferrocene/NaY Zeolite Composite modified Electrode Based on Hydrophobic Ionic Liquid

The ferrocene/NaY zeolite composites (Fc/NaY) are introduced on the surface of a glassy carbon electrode together with the hydrophobic ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆). The modified electrode thus constructed exhibits a pair of reversible redox peaks corresponding to ferrocene. Additionally the peak separation remains almost constant (58–75 mV) and the value of the ratio i_pa/i_pc is close to 1 for scan rates in the range from 10 to 1000 mV s^?1. The effects of the scan rate, aqueous supporting electrolytes, hydrophobic ionic liquid and the contents of ferrocene encapsulated by electrochemistry are investigated. The extrazeolite electron transfer process is discussed. Furthermore, the Fc/NaY/IL‐modified electrode shows good mediation towards oxidation of ascorbic acid, dopamine, hydroquinone, and catechol.     

Self-assembly of bilayer lipid membrane at multiwalled carbon nanotubes towards the development of photo-switched functional device

Bilayer lipid membrane (BLM) was self-assembled on a uniquely fabricated hydrophilic surface, containing N atoms from the carbon source of ethylene amine, of the multi-walled carbon nanotubes (MWNTs) to form the BLM/MWNTs nanocomposites. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and photoelectric experiments were taken to study the properties of the BLM/MWNTs nanocomposites. The thickness of the BLM, which was calculated from the CV data obtained at BLM/MWNTs electrode, turned out to be 4.38 nm, suggesting that the lipid self-assembled at the nanotubes surface was consistent with a bilayer structure. C₆₀-incorporated BLM could also be self-assembled at the nanotubes surface (C₆₀-BLM/MWNTs). The formation of BLM on the MWNTs surface blocked the diffusion of [Fe(CN)₆]^3/4− redox ions across BLM to the MWNTs electrode as no redox current was observed by CV measurement, whereas the incorporation of the electron mediator, C_60, resumed a pair of redox peaks at C₆₀-BLMs/MWNTs electrode. Moreover, the incorporation of C₆₀ led to a four order of magnitude reduction of the resistance of C₆₀-BLM/MWNTs (369.3 Ω) than that of BLM/MWNTs (3.238 × 10⁶ Ω). MWNTs electrode exhibited an intrinsic cathodic photocurrent (166 μA cm⁻²) while BLM/MWNTs electrode blocked photocurrent response of the MWNTs. Interestingly, C₆₀-BLM/MWNTs electrode resumed partial photoelectric properties (photo current: 65 μA cm⁻²) due to the electron mediation effect of C₆₀ incorporated into the lipid membrane. As a result, the novel self-assembled BLM/MWNTs nanocomposites provided a simple yet useful model to study the C₆₀-mediated photoelectric properties of the BLM/MWNTs which may be applicable to develop new biosensors and molecular devices.     

Immobilization and direct electrochemistry of cytochrome <Emphasis Type="BoldItalic">c</Emphasis> at a single-walled carbon nanotube-modified electrode

A single-walled carbon nanotube (SWNT)-modified electrode was fabricated and characterized by SEM and ac impedance techniques. The direct electrochemistry of cytochrome c (Cyt c), which was adsorbed on the surface of the SWNT, was studied by cyclic voltammetry. The results from cyclic voltammetry and infrared spectroscopy indicated that Cyt c remained in its original structure and did not undergo structural change after its immobilization on the SWNT. Further results demonstrated that the SWNT had promotional effects on the direct electron transfer of Cyt c and also indicated that the immobilized Cyt c retained its electrocatalytic activity to the reduction of H₂O₂. This modified electrode might be used in development of new biosensors and the biofuel cells.     

Direct Electrochemistry of Cytochrome c Based on Poly(diallyldimethylammonium Chloride)‐ Graphene Nanosheets/Gold Nanoparticles Hybrid Nanocomposites and Its Biosensing

A novel biosensor was developed by entrapping cytochrome c (Cyt c) in thin films of the room temperature ionic liquid (RTIL) containing nanocomposites of poly(diallyldimethylammonium chloride)‐graphene nanosheets‐gold nanoparticles (PDDA‐Gp‐AuNPs) at a 11‐mercaptoundecanoic acid‐6‐mercapto‐1‐hexanol modified gold electrode. The synthesized PDDA‐Gp‐AuNPs hybrid nanocomposites were characterized by UV‐vis spectroscopy, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The PDDA‐Gp‐AuNPs nanocomposites could increase the effective surface of the electrode, enhance the fixed amount of Cyt c on the electrode surface, promote the electron transfer and facilitate the catalytic activity of Cyt c. The RTIL could provide a biocompatible microenvironment to keep Cyt c biological activities, act as an effective mediator to immobilize a large number of Cyt c on the electrode and have good conductivity to improve electron transfer. Therefore, the resultant electrode exhibited good electrochemical performance and electrocatalytic activity. It could be used for electrochemical detection of H₂O₂ with rapid response, high sensitivity, wide linear range and low detection limit, as well as good stability, repeatability and selectivity. The sensor might be promising for practical application.     

Immobilization and electrochemistry of cytochrome c on amino-functionalized mesoporous silica thin films

The immobilization and electrochemistry of cytochrome c (cyt c) on amino-functionalized mesoporous silica thin films are described. The functionalized silica films with an Im3m cubic phase structure were deposited on conducting ITO substrate by co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) in the presence of Pluronic F127 under acidic conditions. The high specific surface area, large pore size and functional inner surface of mesoporous silica thin films result in a high cyt c loading, and the cyt c immobilization on this silicate framework is stable. After adsorption of cyt c, the ordered cubic structure of mesoporous silica and the redox activity of immobilized cyt c are retained as demonstrated by X-ray diffraction (XRD), Transmission electron microscope (TEM) and cyclic voltammetry. The redox behavior of the cyt c/silica film-modified ITO electrode is a surface-controlled quasi-reversible process for the experimental conditions used in this work and the electron transfer rate constant is calculated is 1.33 s⁻¹. The ITO electrode modified by cyt c/silica film possesses a high stability; even cyt c retains its redox activity following immobilization for several months. Furthermore, the electrocatalytic activities of the modified ITO electrode to hydrogen peroxide and ascorbic acid have been studied. Since these behaviors are quite pronounced, the modified electrode can be used for detection of hydrogen peroxide and ascorbic acid.     

Direct electrochemical behavior of cytochrome c, and its determination on phytic acid modified electrode

Phytic acid (PA) with its unique structure was attached to a glassy carbon electrode (GCE) to form PA/GCE modified electrode which was characterized by electrochemical impedance. The electrochemical behavior of cytochrome c (Cyt c) on the PA/GCE modified electrode was explored by cyclic voltammetry and differential pulse voltammetry. The Cyt c displayed a quasi-reversible redox process on PA modified electrode pH 7.0 phosphate buffer solution with a formal potential (E ^0′) of 57 mV (versus Ag/AgCl). The peak currents were linearly related to the square root of the scan rate in the range of 20–120 mV·s^?1. The electron transfer rate constant was determined to be 12.5 s^?1. The PA/GCE modified electrode was applied to the determination of Cyt c, in the range of 5?×?10^?6 to 3?×?10^?4 M, the currents increase linearly to the Cyt c concentration with a correlation coefficient 0.9981. The detection limit was 1?×?10^?6 M (signal/noise?=?3).     

Analysis of a high redox potential heme in tetraheme cytochrome c3 by direct electrochemistry

Cytochrome c₃ from Desulfovibrio vulgaris (Miyazaki F), a redox protein, contains four bis-histidine-coordinated hemes and has lower redox potential than other heme proteins. Direct electrochemical measurements of cytochrome c₃ were carried out using a pyrolytic graphite edge (PGE) electrode. A low redox potential, already measured by redox titration, and a high redox potential (− 245 mV vs. Ag/AgCl) were observed at room temperature. The high redox potential of cytochrome c₃ was similar to that observed for the loss of an axial ligand at heme. To investigate the loss of the histidine ligand, we explored the electrochemistry of four cytochrome c₃ mutants, in which the sixth coordinated histidine was replaced by methionine. The electrochemistry of the cytochrome c₃ mutants indicated that only Heme III undergoes loss of its axial histidine ligand.     

Metal ions and complexes as modulators of protein-interfacial electron transport at graphite electrodes

An extensive investigation of the direct (unmediated) electrochemical activity of various redox proteins at pyrolytic graphite electrodes has been undertaken. With the exception of the “blue” copper protein azurin, a profound preference for the hydrophilic “edge” over the hydrophobic “basal” plane orientation of the graphite surface is observed. This may be identified with the presence of various oxidised (CO) functionalities at the polished “edge” surface which, most probably in a random manner, constitute reversible and productive binding domains for the proteins. Conditions under which the rates and reversibility of heterogenous electron transport may be optimised depend upon the protein under examination. Well-behaved electrochemistry, indicate of diffusion-dominated heterogeneous electron transport, is modulated by electrode surface protonation (pK = 5.6) and levels of redox-inert multivalent cations, including Mg²⁺ and Cr(NH₃)³⁺₆. The electrochemistry of several proteins which have negatively charged interaction domains, including plastocyanin, and chloroplast and bacterial ferrodoxins, is promoted and stabilised by electrode surface protonation, and interfacial binding of multivalent cations which is attenuated at high ionic strength. Coversely, the electrochemistry of horse-heart cytochrome c, for which the region around the exposed heme edge carries a net positive charge, is inhibited by electrode surface protonation and destablished by the presence of multivalent cations. These patterns of behaviour may be rationalised in terms of a heterogeneous electrode surface which comprises regions of hydrophilic polar groups at which proteins may associate reversibly if resultant coulombic interactions are favourable, and regions of extensive hydrophobicity at which less reversible and (probably) degradative adsorption occurs. Within this basic model, there is considerable scope for domain selectivity which may arise from variations in medium and short range order and distribution of CO functionalities. Implications for the control of in vivo electron-transport processes are discussed.     

Evaluation of a glassy carbon electrode modified by a bilayer lipid membrane with incorporated DNA

The objective of the present work was the evaluation and characterization of a glassy carbon (GC) electrode modified by a bilayer lipid membrane (BLM) with incorporated single-stranded deoxyribonucleic acid fss DNA). Various procedures were developed and tested for the incorporation of ss DNA at the electrode modified by the lipidic membrane: Differential pulse voltammetry (i.e. oxidation of guanine and adenine residues) was used to monitor the incorporation of ss DNA at the GC electrode modified by the BLM. The results have shown that the lipid membrane enhances the stability of ss DNA during a "medium-exchange" of the electrode and prohibits its diffusion from the electrode surface. The third scheme was proven to be the most appropriate as both electrode modification by the BLM and DNA adsorption occur in one stage and much faster (as no BLM thinning process is required) as compared to the former two techniques; furthermore, maximized loading of DNA in BLMs is achieved which reduces by ca. 10-fold the DNA amounts that can be detected electrochemically. Conventional planar "free-suspended" and self-assembled metal supported BLMs were used to monitor in situ the incorporation of ss DNA in these membranes. The results have shown that the adsorption of ss DNA at lipid membranes (as a medium for DNA incorporation on an electrode surface) can occur much faster, using milder conditions and smaller amounts of DNA than by previously described techniques.     

Direct electrochemistry of cytochrome c on a phosphonic acid terminated self-assembled monolayers

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS-(CH₂)₃-PO₃H₂, MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5 s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and -PO₃H₂ groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the -PO₃H₂ terminated SAMs with short chain is nearly reversible. Its formal potential (E⁰′ = 18 ± 3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E⁰′ = 18-22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on -PO₃H₂ terminated SAMs is relatively slow as compared to -SO₃H and -COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.