Effect of high pressure on the photochemical reaction center from Rhodobacter sphaeroides R26.1

High-pressure studies on the photochemical reaction center from the photosy nthetic bacterium Rhodobacter sphaeroides, strain R26.1, shows that, up to 9.6 GPa, this carotenoid-less membrane protein does not loose its three-dim ensional structure at room temperature. However, as evidenced by Fourier-tr ansform preresonance Raman and electronic absorption spectra, between the a tmospheric pressure and D.2 GPa, the structure of the bacterial reaction ce nter experiences a number of local reorganizations in the binding site of t he primary electron donor. Above that value, the apparent compressibility o f this membrane protein is inhomogeneous, being most noticeable in proximit y to the bacteriopheophytin molecules. In this elevated pressure range, no more structural reorganization of the primary electron donor binding site c an be observed, However, its electronic structure becomes dramatically pert urbed, and the oscillator strength of its Q(y) electronic transition drops by nearly one order of magnitude. This effect is likely due to very small, pressure-induced changes in its dimeric structure.