"Importance of the Conformation of Methoxy Groups on the Vibrational
and Electrochemical Properties of Ubiquinones" Burie, J.-R., Boullais, C., Nonella, M.,
Mioskowski, C., Nabedryk, E. and Breton, J.
J. Phys. Chem. 101B, 1997, 6607-6617
Abstract: On the basis of semiempirical calculations, the present study proposes a
comprehensive interpretation of the crystallographic, vibrational, and
electrochemical data on methoxy- substituted quinones, and in particular
for ubiquinones, in terms of the orientation of the methoxy groups
relative to the quinone ring plane. "Hindered" and "free"
methoxy groups are considered depending on the presence or absence on the
quinone ring of a bulky group in ortho position of the considered methoxy
group, respectively. The free methoxy groups have their O-CH3
bond in the quinone ring plane while the hindered methoxy groups cannot
adopt this conformation and have their methyl group tilted out of the
quinone ring plane. The electron donation of the methoxy is dependent on
the orientation of the O-CH3 bond and is maximum for a free
methoxy group. This effect is revealed by the analysis of both
electrochemical and IR data. An assignment of the n(C=O)
modes of the quinones bearing such groups is proposod. From
electrochemical data in literature, a new coefficient spara,
used in the Hammett equation, is determined for a hindered methoxy group
s =- 0.07 compared to -0.27 for a free methoxy
group). In the specific and biologically important case of the bulky group
being another methoxy group, such as in ubiquinones
(2,3-dimethoxy-substituted 1,4-benzoquinones), two types of conformation
have to be considered. In the first type (conformer A), one methoxy adopts
the conformation of a free methoxy group and the second that of a hindered
methoxy group. In the second type (conformer B), both methoxy groups adopt
the conformation of a hindered methoxy group. Both conformers appear
equiprobable within the precision of our semiempirical calculations and a
low rotational barrier, compared to kBT at room temperature,
is found between them. Only conformers A are encountered in crystals.
Using specific 13C labeling, IR data show that conformers A
are mostly encountered at room temperature in solution while a mixture of
both conformers is present at low temperature. On the other hand,
electrochemical data on these quinones are best interpreted as the
reduction of conformers B. This is explained by the higher electron
affinity of conformers B compared to conformers A and by the low
rotational barrier between the two conformers. Taking into account IR data
of ubiquinone in the bacterial photosynthetic reaction center of
Rhodobacter sphaeroides, the 70 mV difference found in the redox potential
of ubiquinone in the two quinone binding sites can be explained by a
difference of orientation of the methoxy groups imposed by the protein. By
selecting a different orientation of the methoxy groups in the two sites,
the protein would thus tune the redox potential of the quinone present in
each site.
BMO authors (in alphabetic order): Marco Nonella
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