Charge separation induces conformational changes in the photosynthetic reaction centre of purple bacteria

X-ray structures of the wild-type reaction centre from Rhodobacter sphaeroides have been determined to a resolution of 1.87 A in the neutral (dark) state and to 2.06 A in the charge-separated (light-excited) state. Whereas the overall protein structures of both states are rather similar, the domain around the secondary quinone shows significant shifts. The quinone molecule itself is observed at two different positions. In the neutral state, 55% of the quinone is located distally and 45% proximally to the cytoplasmic side. After excitation by light, however, at least 90% of the quinone is found at the proximal position. Results presented by Stowell et al. (1997) are confirmed, but the quality of crystallographic data has been improved. We compare the data with the structure of the mutant RC L 209 PY that keeps the Q(B) molecule in the proximal position even in the charge-neutral state.