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 the mechanism. We have found that the buffer pKa and sterics have significant effects on activity, evaluated via 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 with pKa values 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 with pKa values of ≤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 with pKa of ≥8.7, Eh changes as a function of pKa with a slope of -39 mV/pKa unit. In addition, a series of buffer acids were 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 those of 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.