The anionic states of ubiquinone characterised by second order approximate coupled cluster theory

The reduction of ubiquinone (CoQ) is a critical step in cellular respiration. CoQ can support two distinct types of anionic states: a valence-bound state (VBS) and a non-valence, dipole-bound state (DBS). DBSs, 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 cost-effective electron- attachment equation-of-motion second-order approximate coupled-cluster method (EA- EOM-RICC2) to investigate the anionic states of CoQ analogs, Q0 and Q1. We charac- terise the conformational energy, molecular dipole, and DBS and VBS binding energies asafunctionofthetworelevantcoordinatesofthesystem: themethoxydihedralangles. The vertical electron affinity (VEA) of the VBS varies by only 0.2 eV over thermally ac- cessibleregions, whereastheDBSappearsdiscontinuouslyin“islands” governedbyboth dipole magnitude and orientation. Addition of a single isoprenoid unit (Q1) modestly reshapes DBS-supporting regions but leaves the VBS surface essentially unchanged. Cluster scans with small molecules (H2O, HF, NH3, CH4) placed along the quinone dipole axis show that intermolecular interactions can modulate VBS VEAs by up to 0.8-1.0 eV, far exceeding conformational tuning, and enhance or quench DBS binding depending on dipole alignment. Application to bacterial reaction center QA site mu- tants reproduces the experimentally observed 0.1 eV lowering of the quinone electron affinity upon replacing the hydrophobic residue isoleucine (Ile) with polar side chains (Thr, Ser, Asn) while remaining unchanged when mutated to the also hydrophobic valine (Val).