ENDOR Spectroscopic and Molecular Orbital Study of the Dynamical Properties of the Side Chain in Radical Anions of Ubiquinones Q-1, Q-2, Q-6, and Q-10

The dynamics of the side chain of the radical anions of ubiquinones Q-1 (2,3-dimethoxy-5-methyl-6-[3-methyl-2-butenyl]-1,4-benzoquinone), Q-2, Q-6, and Q-10 have been investigated using electron nuclear double-resonance (ENDOR) spectroscopy. When radicals are produced in the liquid phase, secondary radicals are also formed. The EPR spectra of these additional radicals overlap with the radical of interest. ENDOR spectroscopy was found to be capable for studying the dynamical properties of such conditions. The temperature dependence of the isotropic hyperfine coupling constants of the β- and γ-protons of the side chain was measured. The activation energy of the rotation and other dynamical properties of the side chain were calculated assuming that rotation can be modeled by the classical two-jump model. The rotation energy barrier for Q-1 was also determined by the hybrid Hartree–Fock/density functional method UB3LYP with the 6-31G(d) basis set. Calculated results were in good agreement with the experimental results. Despite the numerous parameters affecting the ENDOR linewidth ENDOR spectroscopy was shown to be a potential method for studying the dynamical properties of the mixtures of the radicals. Prominent forbidden transitions appear in the ENDOR spectra when alkali ions are present in the sample. From these transitions measured ENDOR-induced EPR spectra showed an additional doublet and phase transition in electron Zeeman frequency.