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Structures and Vibrational Frequencies of the Quinones in Rb . sphaeroides Derived by a Combined Density Functional/Molecular Mechanics Approach

2002; American Chemical Society; Volume: 107; Issue: 1 Linguagem: Inglês

10.1021/jp026810+

1520-6106

Marco Nonella, Gerald Mathias, Markus Eichinger, Paul Tavan,

Advanced Chemical Physics Studies

The structures of the ubiquinones QA and QB in the bacterial photosynthetic reaction center of Rhodobacter sphaeroides are calculated by a quantum mechanical/molecular mechanics hybrid technique. The orientations of their methoxy groups are found to differ by less than 30°. Calculation of the vibrational spectra reveals that one CO 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 reaction center fixed, reveals that this interaction arises from the electrostatic field of the Fe2+ ion, within which QA is oriented in a particular way.