Direct electrochemistry

The formal potential of the NAD+/NADH redox couple is -0.56 V vs. SCE at pH 7 [15, 17]. However, at platinum and glassy carbon electrodes NADH, oxidation occurs at 0.7 V and 0.6 V vs. SCE, respectively [18]. From these oxidation potentials, it is clear that the direct electrochemical oxidation of NADH requires a substantial overpotential. In nature, NADH oxidation is thought to occur by a one-step hydride transfer. However, on bare electrodes the reaction has been shown to occur via a different and higher energy pathway which produces NAD radicals as intermediates. 

Analytical models of modified electrodes for NADH oxidation 

In conclusion, the direct electrochemical oxidation of NADH is of little practical use in analytical applications due to electrode fouling, and also in biosensor applications because of the low efficiency for the generation of enzymatically active NAD+ due to the different possible side reactions. 

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The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

Determined by direct electrochemistry at a glassy carbon electrode (cyclic, differential pulse, or square-wave voltammetry). 

Zheng W, Li QE, Su L, Yan YM, Zhang J, Mao LQ. 2006. Direct electrochemistry of multicopper oxidases at carhon nanotubes noncovalently functionalized with cellulose derivatives. Electroanalysis 18 587-594. 

Are biomacromolecules of greater mass than that of rubredoxin (6 kDa), in particular enzymes (typically, >50 kDa), capable of such rapid conformational adjustment with decreasing temperature At present the answer appears to be We do not know. Unfortunately, the reduction potentials) of enzymes in solution is not usually determinable with direct electrochemistry, so you are invited to find and explore other molecular properties to probe as a function of temperature, for example, (de) protonations near paramagnetic sites that can be followed both by optical and by EPR spectroscopy. 

Zhou, Y., Hu, N., Zeng, Y. and Rusling, J.F. (2002) Heme protein-clay films Direct electrochemistry and electrochemical catalysis. Langmuir, 18, 211-219. 

R.N. Iyer and W.E. Schmidt, Observations on the direct electrochemistry of bovine copper-zinc superoxide dismutase. Bioelectrochem. Bioenerg. 27, 393 104 (1992). 

K.D. Gleria, H.A.O. Hill, V.J. Lowe, and D.J. Page, Direct electrochemistry of horse-heart cytochrome c at amino acid-modified gold electrodes. J. Electroanal. Chem. 213, 333-338 (1986). 

Z. Dai, E Yan, J. Chen, and H.X. Ju, Reagentless amperometric immunosensors based on direct electrochemistry of horseradish peroxidase for determination of carcinoma antigen-125. Anal. Chem. 75, 5429-5434 (2003). 

Moreover, it has been demonstrated that CNTs promote the direct electrochemistry of enzymes. Dong and coworkers have reported the direct electrochemistry of microperoxidase 11 (MP-11) using CNT-modified GC electrodes [101] and layer-by-layer self-assembled films of chitosan and CNTs [102], The immobilized MP-11 has retained its bioelectrocatalytic activity for the reduction of H202 and 02, which can be used in biosensors or biofuel cells. The direct electrochemistry of catalase at the CNT-modified gold and GC electrodes has also been reported [103-104], The electron transfer rate involving the heme Fe(III)/Fe(II) redox couple for catalase on the CNT-modified electrode is much faster than that on an unmodified electrode or other... 

J. Wang, M. Li, Z. Shi, N. Li, and Z. Gu, Direct electrochemistry of cytochrome c at a glassy carbon electrode modified with single-wall carbon nanotubes. Anal. Chem. 74, 1993-1997 (2002). 

G.C. Zhao, Z.Z. Yin, L. Zhang, and X.W. Wei, Direct electrochemistry of cytochrome c on a multi-walled carbon nanotube modified electrode and its electrocatalytic activity for the reduction of H2O2. Electrochem. Commun. 7, 256-260 (2005). 

G.C. Zhao, L. Zhang, X.W. Wei, Z.S. Yang, Myoglobin on multi-walled carbon nanotubes modified electrode direct electrochemistry and electrocatalysis. Electrochem. Commun. 5, 825—829 (2003). 

M. Wang, Y. Shen, Y. Liu, T. Wang, F. Zhao, B. Liu, and S. Dong, Direct electrochemistry of microperoxidase 11 using carbon nanotube modified electrodes. J. Electroanal. Chem. 578, 121-127 (2005). 

A. Salimi, A. Noorbakhsh, and M. Ghadermarz, Direct electrochemistry and electrocatalytic activity of catalase incorporated onto multiwall carbon nanotubes-modified glassy carbon electrode. Anal. Biochem. 344,16-24 (2005). 

FIGURE 16.9 Principle of reagentless amperometric immunosensor based on immobilized antigen, competitive immunological reaction, and direct electrochemistry of HRP label (adapted from [138]). 

Nowadays, studies of direct electrochemistry of redox proteins at the electrodesolution interface have held more and more scientists interest. Those studies are a convenient and informative means for understanding the kinetics and thermodynamics of biological redox processes. And they may provide a model for the study of the mechanism of electron transfer between enzymes in biological systems, and establish a foundation for fabricating new kinds of biosensors or enzymatic bioreactors. 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

Recently, a novel method of Hb immobilization was achieved by Lu [120], The direct electrochemistry of Hb was successfully achieved by adsorbed Hb onto the surface of a yeast cell through electrostatic attractions on a GC electrode. The bioactivity of Hb immobilized in yeast cell film was retained, and the catalytic reduction of NO and H202 was estimated. 

The determination of H202 is very important in many different fields, such as in clinical, food, pharmaceutical, and environmental analyses [202], Many techniques such as spectrophotometry, chemiluminesence, fluorimetry, acoustic emission, and electrochemistry methods have been employed to determine H202. Electrochemical methods are often used because of their advantages. Among these electrochemical methods, the construction of the mediator-free enzyme-based biosensors based on the direct electrochemistry of redox proteins has been reported over the past decade [203— 204], The enzyme-based biosensors, which use cyt c as biocatalyzer to catalyze H202, were widely studied. 

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