Electronic Coupling in a Synthetic Model of the Cu–Cofactor Unit of the PM State in Cytochrome c Oxidase

Abstract

Enzymes that couple electron transfer to proton translocation play a central role in biological energy conversion. Cytochrome c Oxidase, the terminal enzyme in the respiratory chain, catalyses the four-electron reduction of dioxygen to water while actively pumping protons across a membrane. At the heart of this process lies a covalent His–Tyr crosslink near the Cu_B centre, proposed to coordinate redox chemistry and proton movement. Yet its mechanistic role—especially in transient intermediates such as the P_M state—has yet to be fully elucidated. We report a hydroquinone–imidazole–Cu(II) complex that reproduces key structural elements of the His–Tys–Cu motif and provides a subunit-level analogue relevant to the P_M state. Electron paramagnetic resonance spectroscopy under laser excitation reveals a persistent semiquinone-radical bound to Cu(II). Density functional theory calculations show weak magnetic exchange due to orthogonal spin alignment between the redox-active ligand and the metal centre. This electronically decoupled configuration builds on prior phenol-based analogues, which have been limited by radical instability and challenges in accessing proton-coupled electron transfer intermediates. These results identify dihedral angle and orbital orientation as structural controls over electronic coupling. This work provides a platform for probing redox-linked proton transfer in Cytochrome c Oxidase and establishes design principles for constructing responsive cofactors in synthetic energy conversion systems.