Spectrally silent light induced conformation change in photosynthetic reaction centers

Spectrally silent conformation change after photoexcitation of photosynthetic reaction centers isolated from Rhodobacter sphaeroides R-26 was observed by the optical heterodyne transient grating technique. The signal showed spectrally silent structural change in photosynthetic reaction centers followed by the primary P+BPh− charge separation and this change remains even after the charge recombination. Without bound quinone to the RC, the conformation change relaxes with about 28 μs lifetime. The presence of quinone at the primary quinone (QA) site may suppress this conformation change. However, a weak relaxation with 30–40 μs lifetime is still observed under the presence of QA, which increases up to 40 μs as a function of the occupancy of the secondary quinone (QB) site.