Parallel proton transfer pathways in a metallopeptide artificial hydrogenase

Increasing our understanding of proton transfer processes in catalysis is needed to develop efficient catalysts and catalytic systems for proton-requiring reactions. Here, the effects of varying both the outer coordination sphere and exogenous acid/base species on hydrogen evolution activity of cobalt-peptide catalysts in water are reported. The catalysts are cobalt complexes of the tripeptides glycyl-glycyl-histidine (CoGGH), lysyl-lysyl-histidine (CoKKH), and trimethyllysyl-trimethyllysyl-histidine (CoTmlTmlH). In water without added acid or base, CoKKH exhibits a higher catalytic current and rate than the other derivatives, implicating proton transfer facilitated by Lys side chains in enhancing activity. The CV of CoKKH furthermore lacks pH dependence up to pH 11 in the absence of added buffering agents, supporting a role for the Lys side chains in determining local proton availability. Addition of weak acids/bases in the form of buffering agents enhances catalytic current at a less cathodic potential for all derivatives, consistent with catalysis facilitated by proton delivery by these species. The cyclic voltammogram of CoKKH maintains its cathodic feature associated with a buffer-independent hydrogen production pathway even in the presence of added buffer, indicating the presence of potential-dependent parallel proton delivery processes. A proton inventory study confirms that the presence vs. the absence of buffer species yields distinct proton transfer pathways for all catalyst derivatives, with the presence of buffer resulting in multiple protonatable sites playing a role in proton transfer in catalysis. Results indicate the existence of multiple proton transfer pathways supporting catalysis by CoKKH.