Direct electron transfer from modified glassy carbon electrodes carrying covalently immobilised mediators to a dissolved viologen accepting pyridine nucleotide oxidoreductase and dihydrolipoamide dehydrogenase

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.

Dependence of the Properties of Cobalt(III) Cage Complex as a Function of the Derivatization of Amine Substituents

Control of redox properties of cobalt macrobicyclic hexaamine (cage) complexes by substituent modification is important for their use as electron-transfer agents, and the resultant derivatives can also change the lipophilicity of the complexes for a variety of biological and other applications. Such derivatization is also important for incorporating cage complexes into a range of redoxactive conjugates. Here, the derivatization of the amine groups in the 1 and 8 positions of [Co(sar)]3+ (sar = sarcophagine = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) are reported. The synthesis and properties of methylamide (from the reactions with acetic anhydride), arylimine (from Schiff base reactions), benzylamine, phthalimido, and tosylate derivatives are described. These reactions provide synthons that have the potential to act as precursors for building a range of conjugates containing metal cage complexes, including dimers. The effects of the substituents on the ligand conformations, which affect other chemical and physical properties of the cage complexes, are discussed.

Electro-oxidation of ascorbic acid catalyzed on cobalt hydroxide-modified glassy carbon electrode

The electrochemical behavior of ascorbic acid on a cobalt hydroxide modified glassy carbon (CHM-GC) electrode in alkaline solution was investigated. The process of the involved oxidation and its kinetics were established using the cyclic voltammetry, chronoamperometry techniques, as well as by steady state polarization measurements. The results revealed that cobalt hydroxide promotes the rate of oxidation by increasing the peak current; hence ascorbic acid is oxidized at lower potentials, which is thermodynamically more favorable. The cyclic voltammograms and chronoamperometry indicate a catalytic EC mechanism is operative with the electrogeneration of Co(IV) as the electrochemical process. Also, the process is diffusion-controlled and the current- time responses follow Cottrellian behavior. This result was confirmed by steady state measurements. The rate constants of the catalytic oxidation of ascorbic acid and the electron-transfer coefficient are reported.