Metalloenzymes play
critical roles in the environment by catalyzing
redox reactions in biogeochemical cycles. The elucidation of structure–function
relationships in these biocatalysts has provided a foundation for
the development of bioinspired catalysts for fuel production and small-molecule
activation. In this Perspective, we highlight developments in engineered
biomolecular and bioinspired catalysts for the reduction of H+, O2, and CO2 and for the oxidation
of H2 and H2O. The roles of proton transfers
and second-sphere interactions in particular are highlighted.
Cobalt-mimochrome VI*a (CoMC6*a) is a synthetic mini-protein that catalyzes aqueous proton reduction to hydrogen (H 2). In buffered water, there are multiple possible proton donors, complicating the elucidation of mechanism. We have found that buffer pK a 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 H 2 evolution in the presence of buffer acids of pK a ranging from 5.8 to 11.6 was investigated, giving rise to a potential-pK a relationship that can be divided into two regions. For acids of pK a ≤ 8.7, the half-wave catalytic potential (E h) changes as a function of pK a with a slope of-128 mV/pK a unit, and for acids of pK a ≥ 8.7, E h changes as a function of pK a with a slope of-39 mV/pK a 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 pK a impacts activity. These results demonstrate that buffer acidity and structure are important considerations when optimizing and evaluating systems for proton-dependent catalysis in water.
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