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

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 p-p 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. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.