bi0c00060_si_001.pdf (6.23 MB)
Tuning Mechanism through Buffer Dependence of Hydrogen Evolution Catalyzed by a Cobalt Mini-enzyme
journal contribution
posted on 2020-03-19, 18:47 authored by Jennifer
M. Le, Georgios Alachouzos, Marco Chino, Alison J. Frontier, Angela Lombardi, Kara L. BrenCobalt-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.