Electronic relaxation in P state of Rhodobacter sphaeroides reaction centers

Photosynthetic conversion of light energy to the free energy of charge separated states proceeds in the reaction centers (RCs)pigment protein complexes in the membrane. In the RCs of the purple bacterium Rhodobacter sphaeroides, primary charge separation consists in the electron transfer from the excited bac teriochlorophyll dimer P* to bacteriopheophytin HA within ~3 ps with the involvement of monomeric bacteriochlorophyll BA. Then, the electron is trans ferred to the primary quinone QA within ~200 ps. The quantum yield of this process is equal to unity. P dimer plays a key role in the charge separation as a primary electron donor. The spectroscopic features of P were studied in detail. Interestingly, the longwave Qy absorption band of P at room temperature has a maximum at 865 nm, which shifts to 895 nm at a cryogenic temperature, which can be explained by the interac tion of P* and the charge separated state. Using femtosecond IR spectroscopy, it was shown that the duration of the internal conversion of the excited state P* is 0.2 ps at 285 K. Resonance Raman spectros copy revealed a large number of vibrational modes in the range 301700 cm1, which belong to P, B, and H. Low frequency vibrational modes were found in the RCs of Rba. sphaeroides using coherent femto second spectroscopy.