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

The 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/QH(2)) redox couple were recognized. QH(2) bands are identified at 1491, 1470, 1433 and 1388-1375 cm(-1). The 1491 cm(-1) band, which is sensitive to (1)H/(2)H exchange, is assigned to a C-C ring mode coupled to a C-OH mode. A feature at approximately 1743/1720 cm(-1) is tentatively related to a perturbation of the carbonyl modes of phospholipid head groups induced by QH(2) 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.