Naslov
Reduction of NAD^+ on a Gold Electrode

Autori
Damian, Alexis ; Omanović, Saša

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Skup
207th Meeting Electrochemical Society, 2005

Mjesto i datum
Quebec, Kanada, 15.05.2005. - 20.05.2005

Vrsta sudjelovanja
Pozvano predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
NAD(H); Au-RDE; Electrochemical methods; PM-IRRAS

Sažetak
Nicotinamide adenine dinucleotide NAD(H) is a cofactor that plays the role of electron and hydrogen shuttle in most of the biochemical reactions catalyzed by redox enzymes (dehydrogenases or oxidoreductases). In its reduced and enzymatically active form (1, 4-NADH), the molecule transfers two electrons and a proton to a substrate in the presence of a suitable enzyme to form NAD+. The high cost of NAD(H) is one of the major limitations for its large-scale industrial use. Therefore, it is of great importance to develop methods that could regenerate NADH in-situ and allow its catalytic quantities to be used. Electrochemistry offers a number of advantages compared to (bio)chemical processes for NADH regeneration. However, the use of unmodified metal electrode surfaces results in the formation of large quantities of an inactive dimer (NAD2), rather than the active form, 1, 4-NADH. The research in this area performed in our laboratory has focused on the development of direct [1, 2] and enzyme-mediated electrode systems for NADH regeneration. We have shown that the modification of GC by a sub-monolayer of Ru can provide an electrode surface capable of reducing NAD^+ directly to NADH at a high yield of enzymatically active 1, 4-NADH (96%). On the other side, the electro-enzymatic NADH regeneration resulted in a low conversion of NAD^+ to NADH and low yield of enzymatically active 1, 4-NADH. In addition, electro-enzymatic systems lack simplicity and long-term stability. Therefore, the major direction of our research in this field has been on the design of ‘ all-solid’ surfaces that would allow for high yield of the active 1, 4-NADH to be produced. The present work describes the initial results on the interaction of NAD^+g with a gold surface using a range of electrochemical techniques. The purpose of the study has been to get better insight into fundamental electro(chemical) and physical processes that are involved in the reduction of NAD^+ at a solid surface, which will serve as a basis for the further design of multicomponent nano-patterned catalysts for the regeneration of 1, 4-NADH. All the measurements have been done in an oxygen-free 0.1 M perchlorate electrolyte, using a standard two-compartment electrochemical cell containing either stationary or rotating-disc gold working electrode, platinum counter electrode and saturated calomel reference electrode. The techniques used in research are cyclic voltammetry (CV), differential pulse voltammetry (DPV), ac voltammetry (ACV), chrono-potentiometry (CE) and -amperometry (CA), electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The initial results show that the reduction of NAD^+ on a gold electrode is a highly irreversible reaction, occuring at overpotential of more than -550 mV (Figures 1 and 2). It has been postulated that a large electron-tunnelling distance between the electrode and nicotinamide ring of NAD^+ molecule could be one of the reasons for the high energy requirement for the electron transfer (high overpotential). With the reorientation of the molecule at the electrode surface, as a result of the change in polarity of the electrode at potentials negative of the pzc, the electron-tunnelling path decreases, which also contributes to the faster electron-transfer kinetic and the reduction of NAD^+ at significant current densities. It appears that the reaction is mass-transport controlled (inset to Fig. 1). A low value of diffusion coefficient D=(2.5 0.3) 10^-8 cm^2s^-1 indicates that the diffusion of electroactive species occurs on the electrode surface. It has been determined that the NAD^+ reduction reaction is of first order with respect to NAD^+. The kinetic analysis of CV and DPV measurements has indicated that at high NAD^+ surface concentrations the formation of both NADH and NAD_2 occurs, while at low surface concentrations, a two-electron process is enhanced, thus resulting in the formation of NADH. This has been explained on the basis of the molecular collision probability. Figure 1. Normalized CVs of an Au electrode in 0.1 M perchlorate solution containing 7.5 mM of NAD^+ recorded at various scan rates. The scan rate increases in the direction of the peak increase as 2, 10, 30, 50, 70, 100, 200, 300, 400, 500, 700 and 1000 mVs-1. Temperature, T=295 K. Inset: Dependence of the peak current on the square root of scan rate. Figure 2: Normalized DPVs for reduction of various concentrations of NAD^+ on an Au electrode. The NAD^+ concentrations increases in the direction of the peak increase as 1, 2, 3, 4, 5, 6, and 7.5 mM. Modulation time: 70 ms ; modulation amplitude: 50 mV ; interval time: 0.2 s ; step potential: 1.95 mV. Scan rate, sr=9.75 mVs^-1. Temperature, T=295 K. Inset: Dependence of the peak current on NAD^+ concentration. 1.A.Azem, F.Man and S.Omanovic, J.Mol.Catal.A: Chemical, 219(2004)283. 2.F.Man and S.Omanovic, J.Electroanal.Chem., 568C(2004)301.

Izvorni jezik
Engleski

Znanstvena područja
Kemija, Kemijsko inženjerstvo

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