Conformational analysis of quinone anion radicals in photosystem II and photosynthetic bacteria

Using density functional theory (DFT) techniques, we have investigated poss ible conformers of the radical anions of plastoquinone (psQ), ubiquinone (u bQ), and menaquinone (mnQ), which are formed in the reaction centers of pho tosynthetic bacteria, blue-green bacteria, and green plants. Replacing the hydrocarbon tail connected to the quinone ring by an ethyl group, we have c omputed the rotational potential energy surfaces for psQ(-) and ubQ(-). Our results show that in the absence of environmental effects both systems hav e global minima for near-perpendicular orientations of the gamma-carbon rel ative to the quinone ring. For psQ(-), however, a low-lying local minimum i s also observed for an in-plane arrangement with C gamma pointing away from the O4 oxygen. These differences in head-to-tail rotational energy surface s may explain the experimentally observed differences in beta-proton hyperf ine couplings of psQ(-) vs ubQ(-) and mnQ(-), and their corresponding model compounds. By replacing the C6 methyl group in ubQ and mnQ by hydrogen, or the C6 hydrogen in psQ by methyl, we show that the crucial factor determin ing the rotational arrangements of the quinones in biological systems (plan ar psQ in green plants; perpendicular ubQ and mnQ in bacteria) is the prese nce or absence of this methyl group. The computed barrier height to rotatio n in ubQ(-), ca. 6 kcal/mol, and the beta-proton hyperfine coupling constan ts for the planar vs perpendicular arrengements are in excellent accord wit h experimental data. Finally, we show that the methoxy group at the C2 posi tion in ubiquinone displays a conformational preference as a result of the electron addition process, which may effect the hydrogen bonding pattern an d hence promote the electron-transfer processes.