"Structures and vibrational frequencies of the quinones in
Rb. Sphaeroides derived by a combined
density functional / molecular mechanics approach."M. Nonella, G. Mathias, M. Eichinger, and P. Tavan
J. Phys. Chem. B 107: 316-322 (2003)
The structures of the ubiquinones QA and QB in the bacterial
photosynthetic reaction center of Rb. sphaeroides are
calculated by a QM/MM hybrid technique.
The orientations of their methoxy groups are found to differ by less than 30o.
Calculation of the vibrational spectra reveals
that one C=O stretching mode of QA is considerably shifted to lower
frequencies in agreement with the experimental data. The orientational differences
of the methoxy groups at QA and QB are too small as to provide
an explanation for this shift. Instead, it
must be due to a very special interaction of the protein with QA.
Artificial neutralization of the iron cofactor, while keeping all other
structural parameters of the RC fixed,
reveals that this interaction arises from the electrostatic field
of the Fe2+ ion, within which QA is oriented in a particular way.
BMO authors (in alphabetic order):
QM/MM hybrid descriptions of solutes in complex solvents
Vibrational Spectra of Quinones