Electrochemical reduction of NAD+ on a polycrystalline gold electrode

Review on Microreactors for Photo-Electrocatalysis Artificial Photosynthesis Regeneration of Coenzymes

In recent years, with the outbreak of the global energy crisis, renewable solar energy has become a focal point of research. However, the utilization efficiency of natural photosynthesis (NPS) is only about 1%. Inspired by NPS, artificial photosynthesis (APS) was developed and utilized in applications such as the regeneration of coenzymes. APS for coenzyme regeneration can overcome the problem of high energy consumption in comparison to electrocatalytic methods. Microreactors represent a promising technology. Compared with the conventional system, it has the advantages of a large specific surface area, the fast diffusion of small molecules, and high efficiency. Introducing microreactors can lead to more efficient, economical, and environmentally friendly coenzyme regeneration in artificial photosynthesis. This review begins with a brief introduction of APS and microreactors, and then summarizes research on traditional electrocatalytic coenzyme regeneration, as well as photocatalytic and photo-electrocatalysis coenzyme regeneration by APS, all based on microreactors, and compares them with the corresponding conventional system. Finally, it looks forward to the promising prospects of this technology.

In situ electrochemical regeneration of active 1,4-NADH for enzymatic lactic acid formation via concerted functions on Pt-modified TiO2/Ti

Nicotinamide adenine dinucleotide (NAD+) and its reduced form (NADH) are key cofactors serving as essential hydrogen acceptors and donors to facilitate energy and material conversions under mild conditions. We demonstrate direct electrochemical conversion to achieve highly efficient regeneration of enzymatically active 1,4-NADH using a Pt-modified TiO2 catalyst grown directly on a Ti mesh electrode (Pt-TOT). Spectral analyses revealed that defects formed by the inclusion of Pt species in the lattice of TiO2 play a critical role in the regeneration process. In particular, Pt-TOT containing approximately 3 atom% of Pt exhibited unprecedented efficiency in the electrochemical reduction of NAD+ at the lowest overpotential to date. This exceptional performance led to the production of active 1,4-NADH with a significantly high yield of 86 ± 3% at -0.6 V vs. Ag/AgCl (-0.06 V vs. RHE) and an even higher yield of 99.5 ± 0.4% at a slightly elevated negative potential of -0.8 V vs. Ag/AgCl (-0.2 V vs. RHE). Furthermore, the electrochemically generated NADH was directly applied in the enzymatic conversion of pyruvic acid to lactic acid using lactate dehydrogenase.

Experimental insights into electrocatalytic [Cp*Rh(bpy)Cl]+ mediated NADH regeneration

NADH plays a crucial role in many enzymatically catalysed reactions. Due to the high costs of NADH a regeneration mechanism of this cofactor can enlarge the applications of enzymatic reactions dramatically. This paper gives a thorough system analysis of the mediated electrochemical regeneration of active NADH using cyclic voltammograms and potentiostatic measurements with varying pH, electrode potential, and electrolyte solution, highlighting the system's limiting conditions, elucidating optimal working parameters for the electrochemical reduction of NAD+, and bringing new insight on the oxidation of inactive reduction products. Using [Cp*Rh(bpy)Cl]+ as an electron mediator dramatically increases the percentage of enzymatically active electrochemically reduced NADH from 15% (direct) to 99% (mediated) with a faradaic efficiency of up to 86%. Furthermore, investigations of the catalytic mechanisms of [Cp*Rh(bpy)Cl]+ clarifies the necessary conditions for its functioning and questions the proposed reaction mechanism by two-step reduction where first the mediator is reduced and then brought in contact with NAD+.

Electrocatalytic NAD+ reduction via hydrogen atom-coupled electron transfer

Nicotinamide adenine dinucleotide cofactor (NAD(P)H) is regarded as an important energy carrier and charge transfer mediator. Enzyme-catalyzed NADPH production in natural photosynthesis proceeds via a hydride transfer mechanism. Selective and effective regeneration of NAD(P)H from its oxidized form by artificial catalysts remains challenging due to the formation of byproducts. Herein, electrocatalytic NADH regeneration and the reaction mechanism on metal and carbon electrodes are studied. We find that the selectivity of bioactive 1,4-NADH is relatively high on Cu, Fe, and Co electrodes without forming commonly reported NAD2 byproducts. In contrast, more NAD2 side product is formed with the carbon electrode. ADP-ribose is confirmed to be a side product caused by the fragmentation reaction of NAD+. Based on H/D isotope effects and electron paramagnetic resonance analysis, it is proposed that the formation of NADH on these metal electrodes proceeds via a hydrogen atom-coupled electron transfer (HadCET) mechanism, in contrast to the direct electron-transfer and NAD˙ radical pathway on carbon electrodes, which leads to more by-product, NAD2. This work sheds light on the mechanism of electrocatalytic NADH regeneration, which is different from biocatalysis.

Redox Biocatalysis: Quantitative Comparisons of Nicotinamide Cofactor Regeneration Methods

Enzymatic processes, particularly those capable of performing redox reactions, have recently been of growing research interest. Substrate specificity, optimal activity at mild temperatures, high selectivity, and yield are among the desirable characteristics of these oxidoreductase catalyzed reactions. Nicotinamide adenine dinucleotide (phosphate) or NAD(P)H-dependent oxidoreductases have been extensively studied for their potential applications like biosynthesis of chiral organic compounds, construction of biosensors, and pollutant degradation. One of the main challenges associated with making these processes commercially viable is the regeneration of the expensive cofactors required by the enzymes. Numerous efforts have pursued enzymatic regeneration of NAD(P)H by coupling a substrate reduction with a complementary enzyme catalyzed oxidation of a co-substrate. While offering excellent selectivity and high total turnover numbers, such processes involve complicated downstream product separation of a primary product from the coproducts and impurities. Alternative methods comprising chemical, electrochemical, and photochemical regeneration have been developed with the goal of enhanced efficiency and operational simplicity compared to enzymatic regeneration. Despite the goal, however, the literature rarely offers a meaningful comparison of the total turnover numbers for various regeneration methodologies. This comprehensive Review systematically discusses various methods of NAD(P)H cofactor regeneration and quantitatively compares performance across the numerous methods. Further, fundamental barriers to enhanced cofactor regeneration in the various methods are identified, and future opportunities are highlighted for improving the efficiency and sustainability of commercially viable oxidoreductase processes for practical implementation.

Influence of electrode potential, pH and NAD+ concentration on the electrochemical NADH regeneration

Electrochemical NAD⁺ reduction is a promising method to regenerate NADH for enzymatic reactions. Many different electrocatalysts have been tested in the search for high yields of the 1,4-isomer of NADH, the active NADH, but aside from electrode material, other system parameters such as pH, electrode potential and educt concentration also play a role in NADH regeneration. The effect of these last three parameters and the mechanisms behind their influence on NADH regeneration was systematically studied and presented in this paper. With percentages of active NADH ranging from 10 to 70% and faradaic efficiencies between 1 and 30%, it is clear that all three system parameters drastically affect the reaction outcome. As a proof of principle, the NAD+ reduction in the presence of pyruvate and lactate dehydrogenase was performed. It could be shown that the electrochemical NADH regeneration can also be done successfully in parallel to enzymatically usage of the regenerated cofactor.

Electrocatalytic production of formaldehyde with formaldehyde dehydrogenase using a viologen redox mediator

The electrocatalytic reduction of formate to formaldehyde with formaldehyde dehydrogenase using methylviologen redox mediated was developed.

Electron flow between the worlds of Marcus and Warburg

Living organisms are characterized by the ability to process energy (all release heat). Redox reactions play a central role in biology, from energy transduction (photosynthesis, respiratory chains) to highly selective catalyzed transformations of complex molecules. Distance and scale are important: electrons transfer on a 1 nm scale, hydrogen nuclei transfer between molecules on a 0.1 nm scale, and extended catalytic processes (cascades) operate most efficiently when the different enzymes are under nanoconfinement (10 nm-100 nm scale). Dynamic electrochemistry experiments (defined broadly within the term "protein film electrochemistry," PFE) reveal details that are usually hidden in conventional kinetic experiments. In PFE, the enzyme is attached to an electrode, often in an innovative way, and electron-transfer reactions, individual or within steady-state catalytic flow, can be analyzed in terms of precise potentials, proton coupling, cooperativity, driving-force dependence of rates, and reversibility (a mark of efficiency). The electrochemical experiments reveal subtle factors that would have played an essential role in molecular evolution. This article describes how PFE is used to visualize and analyze different aspects of biological redox chemistry, from long-range directional electron transfer to electron/hydride (NADPH) interconversion by a flavoenzyme and finally to NADPH recycling in a nanoconfined enzyme cascade.

Recent Progress and Perspectives on Electrochemical Regeneration of Reduced Nicotinamide Adenine Dinucleotide (NADH)

NAD is a cofactor that maintains cellular redox homeostasis and has immense industrial and biological significance. It acts as an enzymatic mediator in several biocatalytic electrochemical reactions and undergoes oxidation/reduction to form NAD⁺ or NADH, respectively. The NAD redox couple (NAD⁺ /NADH) mostly exists in enzyme-assisted metabolic reactions as a coenzyme during which electrons and protons are transferred. NADH shuttles these charges between the enzyme and the substrate. In order to understand such complex metabolic reactions, it is vital to study the bio-electrochemistry of NADH. In addition, the regeneration of NADH in industries has attracted significant attention due to its vast usage and high cost. To make biocatalysis economically viable, primary methods of NADH regeneration including enzymatic, chemical, photochemical and electrochemical methods are widely used. This review is mainly focused on the electrochemical reduction of NAD⁺ to NADH with specific details on the mechanism and kinetics of the reaction. It provides emphasis on the different routes (direct and mediated) to electrochemically regenerate NADH from NAD⁺ highlighting the NAD dimer formation. Also, it describes the electrocatalysts developed until now and the scope for development in this area of research.

Electrocatalytic CO2 fixation by regenerating reduced cofactor NADH during Calvin Cycle using glassy carbon electrode

In this study, an enzymatic pathway has been developed to replicate the Calvin Cycle by creating the individual steps of the carbon cycle in a bioreactor. The technology known as “artificial photosynthesis” converts CO₂ emissions into a variety of intermediates that serve as precursors to high-value products. CO₂, light, water, and electricity were used as feedstock. An electrochemical reactor was also studied for the regeneration of active NADH operating at constant electrode potential. Initially, a batch electrochemical reactor containing 80 mL of 0.2 mM NAD⁺ in Tris-buffer (pH 7.40) was used to evaluate the electrode material operating at normal temperature and pressure. The results showed that the cathode is highly electrocatalytically efficient and selective to regenerate 97.45±0.8% of NADH from NAD⁺ at electrode potential of -2.3 V vs. mercury standard electrode (MSE). The NADH regeneration system was then integrated with ATP regeneration system and bioreactor containing Ribulose bisphosphate carboxylase/oxygenase (RuBisCO). NADH was regenerated successfully during the process electrochemically and then was used by the enzymatic reaction to produce triose phosphate and 3-Phosphoglycerate (3GPA).

Effective capture of circulating tumor cells from an S180-bearing mouse model using electrically charged magnetic nanoparticles

Background

Technology enabling the separation of rare circulating tumor cells (CTCs) provides the potential to enhance our knowledge of cancer metastasis and improve the care of cancer patients. Modern detection approaches commonly depend on tumor antigens or the physical size of CTCs. However, few studies report the detection of CTCs by the electrical differences between cancer cells and normal cells.

Results

In this study, we report a procedure for capturing CTCs from blood samples using electrically charged superparamagnetic nanoparticles (NPs). We found that only positively charged NPs attached to cancer cells, while negatively charged NPs did not. The capture method with positively charged NPs offered a sensitivity of down to 4 CTCs in 1 mL blood samples and achieved a superior capture yield (> 70%) for a high number of CTCs in blood samples (10³–10⁶ cells/mL). Following an in vitro evaluation, S180-bearing mice were employed as an in vivo model to assess the specificity and sensitivity of the capture procedure. The number of CTCs in blood from tumor-bearing mice was significantly higher than that in blood from healthy controls (on average, 75.8 ± 16.4 vs. zero CTCs/100 μL of blood, p < 0.0001), suggesting the high sensitivity and specificity of our method.

Conclusions

Positively charged NPs combined with an in vivo tumor model demonstrated that CTCs can be distinguished and isolated from other blood cells based on their electrical properties.

Electronic supplementary material

The online version of this article (10.1186/s12951-019-0491-1) contains supplementary material, which is available to authorized users.

Electrochemical detection of single cancer and healthy cell collisions on a microelectrode

The electrochemical detection of single cancer cells and healthy cells is reported. Detection was achieved by monitoring the consumption of a single cell's contents upon its collisions with a microelectrode in the presence of surfactant. The electrochemical response between acute lymphoblastic lymphoma T-cells and healthy thymocytes differed by two orders of magnitude.

Spatially Organized Enzymes Drive Cofactor-Coupled Cascade Reactions

We report the construction of an artificial enzyme cascade based on the xylose metabolic pathway. Two enzymes, xylose reductase and xylitol dehydrogenase, were assembled at specific locations on DNA origami by using DNA-binding protein adaptors with systematic variations in the interenzyme distances and defined numbers of enzyme molecules. The reaction system, which localized the two enzymes in close proximity to facilitate transport of reaction intermediates, resulted in significantly higher yields of the conversion of xylose into xylulose through the intermediate xylitol with recycling of the cofactor NADH. Analysis of the initial reaction rate, regenerated amount of NADH, and simulation of the intermediates' diffusion indicated that the intermediates diffused to the second enzyme by Brownian motion. The efficiency of the cascade reaction with the bimolecular transport of xylitol and NAD(+) likely depends more on the interenzyme distance than that of the cascade reaction with unimolecular transport between two enzymes.

Study on the effect of electrochemical dechlorination reduction of hexachlorobenzene using different cathodes

Hexachlorobenzene (HCB) is a persistent organic pollutant and poses great threat on ecosystem and human health. In order to investigate the degradation law of HCB, a RuO2/Ti material was used as the anode, meanwhile, zinc, stainless steel, graphite, and RuO2/Ti were used as the cathode, respectively. The gas chromatography (GC) was used to analyze the electrochemical products of HCB on different cathodes. The results showed that the cathode materials significantly affected the dechlorination efficiency of HCB, and the degradation of HCB was reductive dechlorination which occurred only on the cathode. During the reductive process, chlorine atoms were replaced one by one on various intermediates such as pentachlorobenzene, tetrachlorobenzene, and trichlorobenzene occurred; the trichlorobenzene was obtained when zinc was used as cathode. The rapid dechlorination of HCB suggested that the electrochemical method using zinc or stainless steel as cathode could be used for remediation of polychlorinated aromatic compounds in the environment. The dechlorination approach of HCB by stainless steel cathode could be proposed.

In situ regeneration of NADH via lipoamide dehydrogenase-catalyzed electron transfer reaction evidenced by spectroelectrochemistry

NAD/NADH is a coenzyme found in all living cells, carrying electrons from one reaction to another. We report on characterizations of in situ regeneration of NADH via lipoamide dehydrogenase (LD)-catalyzed electron transfer reaction to regenerate NADH using UV-vis spectroelectrochemistry. The Michaelis-Menten constant (K(m)) and maximum velocity (V(max)) of NADH regeneration were measured as 0.80±0.15 mM and 1.91±0.09 μM s(-1) in a 1-mm thin-layer spectroelectrochemical cell using gold gauze as the working electrode at the applied potential -0.75 V (vs. Ag/AgCl). The electrocatalytic reduction of the NAD system was further coupled with the enzymatic conversion of pyruvate to lactate by lactate dehydrogenase to examine the coenzymatic activity of the regenerated NADH. Although the reproducible electrocatalytic reduction of NAD into NADH is known to be difficult compared to the electrocatalytic oxidation of NADH, our spectroelectrochemical results indicate that the in situ regeneration of NADH via LD-catalyzed electron transfer reaction is fast and sustainable and can be potentially applied to many NAD/NADH-dependent enzyme systems.

Reversibility and efficiency in electrocatalytic energy conversion and lessons from enzymes

Enzymes are long established as extremely efficient catalysts. Here, we show that enzymes can also be extremely efficient electrocatalysts (catalysts of redox reactions at electrodes). Despite being large and electronically insulating through most of their volume, some enzymes, when attached to an electrode, catalyze electrochemical reactions that are otherwise extremely sluggish (even with the best synthetic catalysts) and require a large overpotential to achieve a useful rate. These enzymes produce high electrocatalytic currents, displayed in single bidirectional voltammetric waves that switch direction (between oxidation and reduction) sharply at the equilibrium potential for the substrate redox couple. Notoriously irreversible processes such as CO(2) reduction are thereby rendered electrochemically reversible--a consequence of molecular evolution responding to stringent biological drivers for thermodynamic efficiency. Enzymes thus set high standards for the catalysts of future energy technologies.

Synthesis and electrochemical behavior of triazole-containing nicotinamide adenine dinucleotide analogs

The coupling of 2′,3′-di-O-acetyl nicotinamide mononucleotide with 3-butyn-1-ol in the presence of 2,4,6-triisopropylbenzenesulfonyl chloride quantitatively afforded a terminal alkyne-containing intermediate. Furthermore, copper(I)-mediated Huisgen [3 + 2] cycloaddition with a series of azido compounds in a two-phase solvent system gave eight triazole-containing nicotinamide adenine dinucleotide analogs with yields over 88%. The cyclic voltammetric behaviors of these novel analogs were investigated with a glassy carbon electrode, and structural features of these analogs on their electrochemical properties were briefly discussed.

Interaction of flavin adenine dinucleotide (FAD) with a glassy carbon electrode surface

The interaction of flavin adenine dinucleotide (FAD) with a glassy carbon electrode (GCE) surface was investigated in terms of the FAD adsorption thermodynamics and kinetics, the subsequent electroreduction mechanism, and the corresponding electron-transfer rate. The kinetics of FAD electroreduction at the GCE was found to be an adsorption-controlled process. A set of electroreduction kinetic parameters was calculated: the true number of electrons involved in the FAD reduction, n=1.76, the apparent transfer coefficient, alpha(app)=0.41, and the apparent heterogeneous electron-transfer rate constant, k(app)=1.4 s(-1). The deviation of the number of exchanged electrons from the theoretical value for the complete reduction of FAD to FADH(2) (n=2) indicates that a small portion of FAD goes to a semiquinone state during the redox process. The FAD adsorption was well described by the Langmuir adsorption isotherm. The large negative apparent Gibbs energy of adsorption (DeltaG(ads)=-39.7 +/-0.4 kJ mol(-1)) indicated a highly spontaneous and strong adsorption of FAD on the GCE. The energetics of the adsorption process was found to be independent of the electrode surface charge in the electrochemical double-layer region. The kinetics of FAD adsorption was modeled using a pseudo-first-order kinetic model.

Adsorption behavior of dinucleotides on bare and ru-modified glassy carbon electrode surfaces

The interactive behavior of flavin adenine dinucleotide (FAD) with a bare glassy carbon electrode (GCE) and a Ru-modified GCE was investigated. The reduction of FAD at a GCE/ruthenium-modified GCE surface is a quasi-reversible, surface-controlled process, and our data implied that the attachment of FAD onto the surface is caused by nonspecific adsorption instead of covalent linkage, in which the adenine ring of FAD adopts a flat orientation on the GCE surface in neutral and dilute solutions in order to maximize the pi-pi stacking with the carbon surface and reorients to a perpendicular orientation as the surface gets more crowded. FAD desorption during the exchange with nicotinamide adenine dinucleotide (NAD+) is one order of magnitude slower than desorption in the absence of NAD+, which indicates a strong interaction between FAD and NAD+. General knowledge of the interactive behavior of NAD+ on a FAD-adsorbed GCE provides useful information for the design of a modified electrode surface for the generation of NADH from NAD+.

An electrochemical immunosensor using p-aminophenol redox cycling by NADH on a self-assembled monolayer and ferrocene-modified Au electrodes

Redox cycling of enzymatically amplified electroactive species has been widely employed for high signal amplification in electrochemical biosensors. However, gold (Au) electrodes are not generally suitable for redox cycling using a reducing (or oxidizing) agent because of the high background current caused by the redox reaction of the agent at highly electrocatalytic Au electrodes. Here we report a new redox cycling scheme, using nicotinamide adenine dinucleotide (NADH), which can be applied to Au electrodes. Importantly, p-aminophenol (AP) redox cycling by NADH is achieved in the absence of diaphorase enzyme. The Au electrodes are modified with a mixed self-assembled monolayer of mercaptododecanoic acid and mercaptoundecanol, and a partially ferrocenyl-tethered dendrimer layer. The self-assembled monolayer of long thiol molecules significantly decreases the background current of the modified Au electrodes, and the ferrocene modification facilitates easy oxidation of AP. The low amount of ferrocene on the Au electrodes minimizes ferrocene-mediated oxidation of NADH. In sandwich-type electrochemical immunosensors for mouse immunoglobulin G (IgG), an alkaline phosphatase label converts p-aminophenylphosphate (APP) into electroactive AP. The amplified AP is oxidized to p-quinoneimine (QI) by electrochemically generated ferrocenium ion. NADH reduces QI back to AP, which can be re-oxidized. This redox cycling enables a low detection limit for mouse IgG (1 pg mL(-1)) to be obtained.

Interactive adsorption behavior of NAD+ at a gold electrode surface

The adsorption of an oxidized form of nicotinamide adenine dinucleotide, NAD+, on a polycrystalline gold electrode surface and the subsequent surface conformation of the molecule were investigated over a wide temperature and potential range, using electrochemical differential capacitance and PM-IRRAS techniques. The adsorption process was described by the Langmuir adsorption isotherm. The corresponding thermodynamic parameters were determined: the Gibbs energy, enthalpy, and entropy of adsorption. The large negative Gibbs energy of adsorption (-43 +/- 4 kJ mol-1 and -39 +/- 2 kJ mol-1 on a positively and negatively charged surface, respectively) confirms that the NAD+ adsorption process is highly spontaneous, while the large entropy gain (285 J K-1 mol-1 and 127 J K-1 mol-1 on a positively and negatively charged surface, respectively) was found to represent the adsorption driving force. It was demonstrated that the energetics of the adsorption process is surface-charge controlled, while its kinetics is both mass-transport and surface-charge controlled. A surface-charge dependent conformation model for the adsorbed NAD+ molecule is proposed. These findings suggest that the origin of the NAD+ reduction overpotential is related to the surface conformation of the adsorbed NAD+ molecule, rather than to the electrode Fermi level position.