A question of flexibility in cytochrome c oxidase models

In two cytochrome c oxidase models, the flexibility of the strapped porphyrin framework and the nature of the distal ligand determine the efficacy of both the oxygen binding and the four-electron reduction of O2. [Display omitted] •Vacant distal cavity allows reversible binding of exogenic guest (O2, CO).•Basicity and steric bulk of auxiliary ligands affect affinity for O2.•Efficient electrocatalyzed reduction of O2 depends on the scaffold's flexibility. The structure-property relationships were compared for the iron and iron-copper complexes of two functional cytochrome c oxidase models, 1 and 2, both constructed upon a phenanthroline-strapped porphyrin bearing respectively pyridyl or picolinyl built-in proximal and distal ligands. The behavior of these heme models in the absence and in the presence of copper was studied by 1H NMR, UV–visible absorption, EPR, Raman and FTIR spectroscopies, electrochemistry in solution and deposited on a rotating ring-disk graphite electrode. The distal binding site within the phenanthroline pocket of both 1 and 2 is available for the coordination of exogenic ligands, yet the oxygen binding affinity is higher for all complexes of 2 than for 1. Despite this difference, [1FeIICuI] more efficiently reproduced the electrocatalyzed reduction of oxygen to water than [2FeIICuI]. The oxygenated complexes of both iron(II)-copper(I) species mimic the ability of cytochrome c oxidase to reversibly bind O2, as shown by competitive binding studies in the presence of CO. Differences in the binding and electrocatalytic properties of these models stem from difference in rigidity of scaffolds upon binding of both the proximal and distal ligands, as well as from the bulkiness of the distal ligand.