Photoreduction of the quinone pool in the bacterial photosynthetic membrane: identification of infrared marker bands for quinol formation

AbstractThe photoreduction of the quinone (Q) pool in the photosynthetic membrane of the purple bacterium Rhodobacter sphaeroides was investigated by steady-state and time-resolved Fourier transform infrared difference spectroscopy. The results are consistent with the existence of a homogeneous Q pool inside the chromatophore membrane, with a size of around 20 Q molecules per reaction center. IR marker bands for the quinone/quinol (Q/QH2) redox couple were recognized. QH2 bands are identified at 1491, 1470, 1433 and 1388–1375 cm−1. The 1491 cm−1 band, which is sensitive to 1H/2H exchange, is assigned to a C–C ring mode coupled to a C–OH mode. A feature at ∼1743/1720 cm−1 is tentatively related to a perturbation of the carbonyl modes of phospholipid head groups induced by QH2 formation. Complex conformational changes of the protein in the amide I and II spectral ranges are also apparent during reduction and reoxidation of the Q pool