Session: Bioinorganic Chemistry:

The photochemical reduction of CO₂ is a promising route to producing renewable fuels using an abundant carbon feedstock. Enzymes are known for catalyzing multi-electron, multi-proton reactions with high selectivity in water using earth-abundant metals, making enzymes attractive systems to emulate when developing reactions requiring controlled proton and electron delivery. Herein, the activity of the synthetic metalloporphyrin-containing biocatalyst cobalt-mimochrome VI*a (CoMC6*a) for CO₂ reduction in a photocatalytic system using a quantum-dot (QD) photosensitizer is shown. CoMC6*a, a cobalt deuteroporphyrin sandwiched between two peptides, is shown to be highly active for reduction of CO₂ to CO, with turnover number (TON) ranging from 550 (48 hours) to 3420 (44 hours) with respect to catalyst, depending on conditions. Cyclic voltammetry of CoMC6*a indicates a peak anodic to the proton reduction peak when the system is purged with CO₂, proposed to represent CoMC6*a catalysis of CO₂ reduction. The potential of this peak supports the thermodynamic favorability of CO production over the competing proton reduction under electrocatalytic conditions. CoMC6*a is compared to cobalt microperoxidase-11 (CoMP11-Ac), a cobalt porphyrin with a single peptide bound to the proximal face, using photochemical and electrochemical techniques in water. CoMP11-Ac shows CO₂ reduction activity to CO but lacks the distinctive anodic peak produced by CoMC6*a; rather, a current increase is seen at the potential of the proton reduction peak when purged with CO₂. These results suggest that the CoMC6*a structure lowers the overpotential for CO₂ reduction. Consequences for selectivity for CO₂ reduction by CoMC6*a are under investigation.

Jose Alvarez-Hernandez

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