Structures and properties of ubiquinone-1 and its radical anion from hybrid Hartree-Fock/density functional studies

Ubiquinones and their semiquinone radical anions-electron transfer cofactors vital for respiration and photosynthesis-are typically characterized in proteins by measuring their vibrational or electron spin resonance spectra. This contribution presents calculated geometries and spin properties for ubiquinone-1 (UQ(1)) and its radical anion (UQ(1)(.-)), as well as vibrational frequencies for a model with the methyl groups on the isoprenyl side chain of UQ(1) replaced by hydrogens (UQ and UQ(.)). Computations were done by using the hybrid Hartree-Fock/density functional B3LYP method with the 6-31G(d) and [632/41] basis sets. Bond distance changes upon reducing UQ(1) to UQ(1)(.-) are qualitatively consistent with results for other p-benzoquinones. Proton and C-13 hyperfine coupling constants calculated by using Chipman's [632/41] basis set agree exceptionally well with experimental measurements and reflect the distribution of unpaired spin. We suggest isotopic substitution experiments to resolve the nearly overlapping C=O and C=C stretching frequencies calculated for UQ and to shift methoxy and/or methyl CH bending modes away from one CO stretching band of UQ(.-).