The role of flexibility in a strapped porphyrin hemoprotein model

The role played by the distal coordination site’s flexibility on oxygen reduction was investigated by rotating ring-disk electrode experiments for a bipyridine-strapped iron(III) porphyrin mimic of cytochrome [Formula: see text] oxidase. Compared to a phenanthroline counterpart, the more flexible bipyridine strap improved by up to 15% the selectivity of the four-electron reduction of oxygen to water over the two-electron reduction in the presence of copper(I) coordinated to the distal bipyridine and [Formula: see text]-methylimidazole bound at the proximal site. The more modest gain in overpotential observed for the bipyridine-strapped porphyrin than for the phenanthroline derivative was attributed to a difference in stability of the iron(III) porphyrins that affects reduction to iron(II) and oxygen binding. Overall, the rotational flexibility imparted by the bipyridine strap allows the ferric porphyrin to remain relatively flat, thus improving its electrocatalytic performance.