Photoelectrochemical NADH Regeneration using Pt‐Modified p‐GaAs Semiconductor Electrodes

Abstract

Cofactor regeneration in enzymatic reductions is crucial for the application of enzymes to both biological and energy-related catalysis. Specifically, regenerating NADH from NAD + is of great interest, and using electrochemistry to achieve this end is considered a promising option. Here, we report the first example of photoelectrochemical NADH regeneration at the illuminated (l > 600 nm), metal-modified, p-type semiconductor electrode Pt/p-GaAs. Although bare p-GaAs electrodes produce only enzymatically inactive NAD 2 , NADH was produced at the illuminated Pt-modified p-GaAs surface. At low overpotential (À0.75 V vs. Ag/AgCl), Pt/p-GaAs exhibited a seven-fold greater faradaic efficiency for the formation of NADH than Pt alone, with reduced competition from the hydrogen evolution reaction. Improved faradaic efficiency and low overpotential suggest the possible utility of Pt/p-GaAs in energy-related NADHdependent enzymatic processes. Scheme 1. Structure of NAD + (nicotinamide) and two-electron redox conversion between NAD + and NADH (1,4-dihydronicotinamide), which occurs in enzymatic processes (ADPR = adenosine diphosphoribose).

Copper oxide-based cathode for direct NADPH regeneration

“…The NADPH product electrochemically regenerated from NADP + using this nanostructured heterolayer cathode is found to be free of any dimers. This finding contrasts with previous studies that employed more expensive electrode materials 13 , 15 , 38 , 39 or even a Cu mesh electrode with the same sputtered Ni overcoat (this work).…”

“…The latter assay, together with mass spectrometry or NMR data, is important in confirming the bona fides of the end product; to establish the purity, we sought to not rely solely on electrochemical voltammetry and UV–vis absorption measurements since reaction products may contain the dimer in addition to inactive isomer 16 , 17 . Finally, unlike previous attempts that demonstrated photoelectrochemical regeneration of NADH 15 , we provide evidence here that illumination and electrochemistry need not be concurrent, implying that direct electrochemical regeneration can proceed without the undesirable production of the inactive dimer. Our facile, cheap, and direct electrochemical regeneration of NADPH, especially without any inactive dimer, is a useful advance in the regeneration of cofactors 18 , 19 .…”

“…Copper oxide-based electrodes have since been used for hydrogen production 20,25,26 , photoelectrocatalytic conversion of CO to liquid fuels (ethanol, acetate, and n-propanol) 25 , and solar energy conversion [26][27][28][29][30] . The Becquerel effect has been used in photoelectrocatalytic regeneration of the cofactor NADH, but with exotic and expensive materials (Pt-modified p-GaAs semiconductor electrodes) and requiring the presence of both illumination and applied bias 15 . Here, we use photoelectrochemical surface modification of the Ni-Cu 2 O-Cu electrode to directly electrochemically regenerate NADPH from NADP + using inexpensive materials (Ni and Cu) and, importantly, without the need for the simultaneous presence of illumination.…”

“…Bare metallic materials favor production of the inactive (NADP) 2 dimer during direct electrochemical regeneration of NAD(P)H and precious metals such as Pt, Ir, and Ru can increase selectivity toward 1,4 NAD(P)H 13 . NADH has also been regenerated using photocatalytic and photoelectrocatalytic approaches 14 , 15 . Recently, a photoelectrochemical process requiring collimated focused 625-nm radiation and expensive electrode materials (platinum and p-type GaAs), was used to regenerate NADH 15 ; without the platinization of the p-type GaAs surface, only the inactive dimer (NAD) 2 was produced 15 .…”

Improving Photocatalytic Energy Conversion via NAD(P)H

“…7 A successful in-situ coupling experiment with enzyme catalysis does not confirm the concentration or selectivity in the regeneration step alone. Any so-called NAD(P)H yields calculated using the molar absorption coefficient of 1,4-NAD(P)H when other molecules are produced (highly likely 3,5,6,8,9 ), will give erroneous data. Such unvalidated data (denoted as Y340 nm) should be read with extra caution as they reveal little useful information.…”