IMPORTANCE OF THE CONFORMATION OF METHOXY GROUPS ON THE VIBRATIONAL AND ELECTROCHEMICAL PROPERTIES OF UBIQUINONES

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 parti cular for ubiquinones, in terms of the orientation of the methoxy grou ps relative to the quinone ring plane. ''Hindered'' and ''free'' metho xy groups are considered depending on the presence or absence on the q uinone ring of a bulky group in ortho position of the considered metho xy group, respectively. The free methoxy groups have their O-CH3 bond in the quinone ring plane while the hindered methoxy groups cannot ado pt this conformation and have their methyl group tilted out of the qui none 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 gr oup. This effect is revealed by the analysis of both electrochemical a nd IR data. An assignment of the nu(C=O) modes of the quinones bearing such groups is proposed. From electrochemical data in literature, a n ew coefficient sigma(para), used in the Hammett equation, is determine d for a hindered methoxy group (sigma(para) = -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 ubiqui nones (2,3-dimethoxy-substituted 1,4-benzoquinones), two types of conf ormation have to be considered. In the first type (conformer A), one m ethoxy adopts the conformation of a free methoxy group and the second that of a hindered methoxy group. In the second type (conformer B), bo th methoxy groups adopt the conformation of a hindered methoxy group; Both conformers appear equiprobable within the precision of our semiem pirical calculations and a low rotational barrier, compared to k(B)T a t room temperature, is found between them. Only conformers A are encou ntered in crystals. Using specific C-13 labeling, IR data show that co nformers A are mostly encountered at room temperature in solution whil e a mixture of both conformers is present at low temperature. On the o ther hand, electrochemical data on these quinones are best interpreted as the reduction of conformers B. This is explained by the higher ele ctron 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 pote ntial of ubiquinone in the two quinone binding sites can be explained by a difference of orientation of the methoxy groups imposed by the pr otein. By selecting a different orientation of the methoxy groups in t he two sites, the protein would thus tune the redox potential of the q uinone present in each site.