Abstract
We have determined the first de novo position of the secondary quinone Q
B in the
Rhodobacter sphaeroides reaction center (RC) using phases derived by the single wavelength anomalous dispersion method from crystals with selenomethionine substitution. We found that in frozen RC crystals, Q
B occupies primarily the proximal binding site. In contrast, our room temperature structure showed that Q
B is largely in the distal position. Both data sets were collected in dark-adapted conditions. We estimate that the occupancy of the Q
B site is 80% with a proximal: distal ratio of 4:1 in frozen RC crystals. We could not separate the effect of freezing from the effect of the cryoprotectants ethylene glycol or glycerol. These results could have far-reaching implications in structure/function studies of electron transfer in the acceptor quinone complex because the above are the most commonly used cryoprotectants in spectroscopic experiments.