Probing the secondary quinone (QB) environment in photosynthetic bacterial reaction centers by light‐induced FTIR difference spectroscopy

Abstract
The photoreduction of the secondary electron acceptor, QB, has been characterized by light‐induced Fourier transform infrared difference spectroscopy of Rb. sphaeroides and Rp. viridis reaction centers. The reaction centers were supplemented with ubiquinone (UQ10 or UQ0). The QB state was generated either by continuous illumination at very low intensity or by single flash in the presence of redox compounds which rapidly reduce the photooxidized primary electron donor P+. This approach yields spectra free from P and P+ contributions making possible the study of the microenvironment of QB and QB. Assignments are proposed for the C O vibration of QB and tentatively for the C = O and C = C vibrations of QB.

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