F. Himo et al., Conformational analysis of quinone anion radicals in photosystem II and photosynthetic bacteria, J PHYS CH A, 103(19), 1999, pp. 3745-3749
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.