Regeneration of the NADH Cofactor by a Rhodium Complex Immobilized on Multi-Walled Carbon Nanotubes

Abstract

The regeneration of the enzymatic cofactor nicotinamide adenine dinucleotide (NADH) by rhodium-based catalysts such as [Rh(Cp*)(bpy)Cl]+ (Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine) and derivatives have previously been studied extensively in solution. In this work, we report a synthetic route of a rhodium complex with a pyrene-substituted phenanthroline ligand (pyr-Rh). The immobilization of the pyr-Rh complex was accomplished on multi-walled carbon nanotubes (MWCNTs) via π-π stacking to obtain effective and durable indirect electrochemical regeneration of NADH. Cyclic voltammetry and amperometry were used to demonstrate the electrochemical activity of the surface-confined pyr-Rh complex. The loading quantity of the pyr-Rh complex was found to be 47 ± 2 nmol/mg of MWCNTs. The reusability of the electrodes modified with the pyr-Rh complex was investigated and an average turnover frequency of 3.6 ± 0.1 s−1 over ten cycles in the presence of 2 mM nicotinamide adenine dinucleotide (NAD+) was observed. Lastly, malate dehydrogenase (MDH), a NADH-dependent enzyme, was evaluated in the presence of the immobilized pyr-Rh complex to confirm the catalyst's capability to regenerate biologically active NADH for biocatalysis.