Tuning Mechanism through Buffer Dependence of Hydrogen Evolution Catalyzed by a Cobalt Mini-enzyme

Cobalt-mimochrome VI*a (CoMC6*a) is a synthetic mini-protein that catalyzes aqueous proton reduction to hydrogen (H2). In buffered water, there are multiple possible proton donors, complicating the elucidation of mechanism. We have found that buffer pKa and sterics have significant effects on activity, evaluated through cyclic voltammetry (CV). Protonated buffer is proposed to act as the primary proton donor to the catalyst, specifically through the protonated amine of the buffers that were tested. At a constant pH of 6.5, catalytic H2 evolution in the presence of buffer acids of pKa ranging from 5.8 to 11.6 was investigated, giving rise to a potential-pKa relationship that can be divided into two regions. For acids of pKa ≤ 8.7, the half-wave catalytic potential (Eh) changes as a function of pKa with a slope of –128 mV/pKa unit, and for acids of pKa ≥ 8.7, Eh changes as a function of pKa with a slope of –39 mV/pKa unit. In addition, a series of buffer acids was synthesized to explore the influence of steric bulk around the acidic proton on catalysis. The catalytic current in CV shows a significant decrease in the presence of the sterically hindered buffer acids compared to their parent compounds, also consistent with the added buffer acid acting as the primary proton donor to the catalyst and showing that acid structure in addition to pKa impacts activity. These results demonstrate that buffer acidity and structure are important considerations when optimizing and evaluating systems for proton-dependent catalysis in water.