Viologen redox polymers for NADH-free alcohol dehydrogenase bioelectrocatalysis: Structure–activity relationships and mechanistic insight

Replacing NADH in dehydrogenase-catalyzed reactions remains challenging because an alternative must combine reducing power with productive recognition by the native cofactor-dependent enzyme machinery. Here, a library of viologen redox polymers was evaluated as artificial cofactors for alcohol dehydrogenase (ADH)-catalyzed reduction of formaldehyde to methanol. All polymers supported NADH-free bioelectrocatalytic methanol synthesis, but activity depended strongly on viologen substitution: charged and highly hydrated substituents generally outperformed neutral or hydrophobic analogues. Turnover showed no monotonic relationship with apparent redox potential or with the DPV-derived apparent electroactive-site coverage. Oxidized soluble methyl viologen altered the apparent formaldehyde dependence of the native NADH-dependent reaction, showing that it is not kinetically inert toward ADH, although the present data do not establish a specific inhibition mechanism. A composition-matched denatured-enzyme control produced no quantifiable methanol, and a cobaltocene-functionalized redox polymer was likewise inactive, arguing against direct non-enzymatic formaldehyde reduction or a generic low-potential electron-pool mechanism. Finally, deuterated-buffer experiments yielded deuterated methanol and decreased product formation, with an apparent solvent kinetic isotope effect of 2.2. Together, the results support a mechanism in which reduced viologen polymers deliver electrons while proton equivalents are supplied from solution. Successful NADH replacement therefore depends on scaffold-enzyme compatibility and coupled electron/proton delivery rather than reducing power or electroactive-site density alone.