<i>In situ</i> electrochemical regeneration of active 1,4-NADH for enzymatic lactic acid formation <i>via</i> concerted functions on Pt-modified TiO₂/Ti.

Abstract

Nicotinamide adenine dinucleotide (NAD⁺) and its reduced form (NADH) are key cofactors serving as essential hydrogen acceptors and donors to facilitate energy and material conversions under mild conditions. We demonstrate direct electrochemical conversion to achieve highly efficient regeneration of enzymatically active 1,4-NADH using a Pt-modified TiO₂ catalyst grown directly on a Ti mesh electrode (Pt-TOT). Spectral analyses revealed that defects formed by the inclusion of Pt species in the lattice of TiO₂ play a critical role in the regeneration process. In particular, Pt-TOT containing approximately 3 atom% of Pt exhibited unprecedented efficiency in the electrochemical reduction of NAD⁺ at the lowest overpotential to date. This exceptional performance led to the production of active 1,4-NADH with a significantly high yield of 86 ± 3% at -0.6 V <i>vs.</i> Ag/AgCl (-0.06 V <i>vs.</i> RHE) and an even higher yield of 99.5 ± 0.4% at a slightly elevated negative potential of -0.8 V <i>vs.</i> Ag/AgCl (-0.2 V <i>vs.</i> RHE). Furthermore, the electrochemically generated NADH was directly applied in the enzymatic conversion of pyruvic acid to lactic acid using lactate dehydrogenase.