Electrochemical Reduction of Nicotinamide Adenine Dinucleotide in Dimethylsulfoxide: Ion‐Pair, Protonation, and Adsorption Effects

Abstract

The effects were examined of variations in background electrolyte tetraalkylammonium cation (R4N § and in proton availability upon the electrochemical reduction in DMSO of the coenzyme fl-nicotinamide adenine dinucleotide (NAD*), which has one available proton on the pyrophosphate group, and of its sodium salt [NAD+(Na)] in which the pyrophosphate proton has been replaced by Na(I). The proton reduction in solutions of NAD § HC104, H3PO4, and mixture of NAD+(Na) with HC104 and with H3PO4 was also investigated with reference to various possible equilibria involving Brhnsted acid in DMSO solutions, e.g., the relation of polarographic half-wave potential to pKa.In general, the size of R in R4N § (and its consequent adsorbability at the electrode/solution interface) does not appreciably affect the ease of reduction of the nicotinamide moiety but can exert a marked influence on proton reduction.Various aspects of the electrochemical behavior of the biologically important coenzyme, p-nicotinamide adenine dinucleotide (NAD+ ; Fig 1), have been investigated (1-5). In aqueous solution, NAD + is reduced enzymatically in a one-step process (Eq. [1]) and electrochemically in a two-step process (Eq. [2])There is one reported electrochemical study of NAD + in nonaqueous media (6); the reduction pattern involves an initial one-electron (le) uptake followed by rapid dimerization of the free radical product; no reduction of dimer or free radical occurs within the available potential rangeThe present study involves a detailed examination of the electrochemical reduction of NAD + in DMSO solution with particular attention to the effects of different supporting electrolytes involving tetraalkylammonium ions (R~N + ) and of varying proton availability. The reduction of protonated species was also examined from the viewpoint of Brhnsted acid equilibria in DMSO. ExperimentalChemicals.--Nicotinamide adenine dinucleotide (Chemalog) an,d nicotinamide adenine dinuc~eotide sodium salt (Sigma) (Fig. i) were the best grades available. Reagent-grade tetramethylammonium perchlorate (TMAP), tetraethylammonium perchlorate (TEA.P), tetra-n-butylammonium perchlorate (TBAP), and potassium perchlorate (PP) (G. Frederick Smith) were recrystallized four times from water and then dried in a vacuum at 60 ~ for 48 hr. Other chemicals used were analytical reagent grade or the equivalent. Aqueous 25% tetraethylammonium hydroxide (TEAH) solution was obtained from Eastman; 0.19M HC104 solution in DMSO was prepared from aqueous diluted 1:1 perchloric acid. DMSO (Fisher certified ACS), dried over Linde 5A molecular sieves, was purified by fractional crystallization, using a method identical to that described for purification of pyridine (7); the cooling bath was H20 at 12~176 Apparatus.--A Princeton Applied Research Model170 polarograph was used with a jacketed three-compartment cell thermostated at 25~ Rapid .cyclic voltammetric data were acquired on a Tektronix 5103N oscilloscope with 5A15N and 5A18N plug-ins using a C-5A camera. The hanging mercury drop elect...

Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate

“…The full conversion required a slight excess of supplied charge due to the competing proton reduction (Figure a). This experiment rules out the possibility that 2c is involved in any side reactions (e.g., dimerization of the forming radical) in the course of electrolysis under the given experimental conditions, an involvement which has often prevented the regeneration of previously studied NADH analogues. − …”