The Anionic States of Ubiquinone Characterized by Second-Order Approximate Coupled-Cluster Theory

Abstract

The reduction of ubiquinone is a critical step in cellular respiration thanks to its electron-accepting properties. Ubiquinone supports two distinct anionic states: a valence state and a non-valence, dipole-bound state. Dipole-bound states, where an excess electron is weakly bound by the molecular dipole, are of significant interest as potential doorway states for electron transfer processes. In this work, we employ the electron-attachment variant of the second-order approximate coupled-cluster (CC2) method to investigate the anionic states of ubiquinone analogues, Q₀ and Q₁. We characterize the conformational energy, molecular dipole, and dipole-bound state and valence state binding energies as a function of the two relevant coordinates of the system, which are the dihedral angles between the methoxy groups and the benzene ring. We find that the vertical electron affinity of the valence state varies by $\sim 0.2\text{eV}$ over thermally accessible regions, whereas the dipole-bound state appears discontinuously, governed by both dipole magnitude and orientation. Addition of a single isoprenoid unit to Q₀, resulting in Q₁, modestly reshapes the regions that support the dipole-bound state, but leaves the valence state surface essentially unchanged. Cluster scans with small molecules (${\text{H}}_2\text{O}$, HF, ${\text{NH}}_3$, and ${\text{CH}}_4$) placed along the quinone dipole axis show that intermolecular interactions can modulate the electron affinity of the valence state by up to $\sim 0.6\text{eV}$, far exceeding conformational tuning. The binding energy of the dipole-bound state is enhanced or quenched depending on dipole alignment. Larger cluster models focusing on bacterial reaction center ${\text{Q}}_{\text{A}}$ reproduce the experimental finding that mutating an aliphatic isoleucine near the quinone to polar side chains leads to $\sim 0.1\text{eV}$ lower electron affinity, while mutation to the also hydrophobic valine leaves the electron affinity unchanged. Our results demonstrate how the biological and chemical environments can modulate the electron-accepting properties of ubiquinone and contribute to the discussion about the possible existence of dipole-bound anions in biological environments. Our findings highlight the interplay between molecular conformation, intermolecular interactions, and electronic structure in determining the redox properties of biologically relevant molecules.