Parent anion radical formation in coenzyme Q0: Breaking ubiquinone family rules

We report electron attachment (EA) measurements for the parent anion radical formation from coenzyme Q0 (CoQ0) at low electron energies (<2 eV) along with quantum chemical calculations. CoQ0 may be considered a prototype for the electron withdrawing properties of the larger CoQn molecules, in particular ubiquinone (CoQ10), an electron carrier in aerobic cell respiration. Herein, we show that the mechanisms for the parent anion radical formation of CoQ0 and CoQn (n = 1,2,4) are remarkably distinct. Reported EA data for CoQ1, CoQ2, CoQ4 and para-benzoquinone indicated stabilization of the parent anion radicals around 1.2–1.4 eV. In contrast, we observe for the yield of the parent anion radical of CoQ0 a sharp peak at ∼ 0 eV, a shoulder at 0.07 eV and a peak around 0.49 eV. Although the mechanisms for the latter feature remain unclear, our calculations suggest that a dipole bound state (DBS) would account for the lower energy signals. Additionally, the isoprenoid side chains in CoQn (n = 1,2,4) molecules seem to influence the DBS formation for these compounds. In contrast, the side chains enhance the parent anion radical stabilization around 1.4 eV. The absence of parent anion radical formation around 1.4 eV for CoQ0 can be attributed to the short auto-ionization lifetimes. The present results shed light on the underappreciated role played by the side chains in the stabilization of the parent anion radical. The isoprenoid tails should be viewed as co-responsible for the electron-accepting properties of ubiquinone, not mere spectators of electron transfer reactions.