Coherent nuclear dynamics at room temperature in bacterial reaction centers

A room-temperature study is reported of the femtosecond spectral evolution of the stimulated emission band of the primary electron-transfer precursor P* in bacterial photosynthesis. The study was performed with membranes of the antenna-deficient RCO1 mutant of Rhodobacter sphaeroides. A time- dependent red shift, reflecting nuclear motion out of the Franck-Condon region of the excited state, is resolved. Analysis of oscillatory features persisting for >1 ps in the kinetics revealed main frequencies of the activated motions at 30, 84, 145, and 192 cm^-1. The oscillations occur on the time scale of primary electron transfer. Our results set a lower limit for the vibrational dephasing time in P* that is not compatible with the usual assumption in theoretical treatments of complete vibrational relaxation prior to electron transfer, even at room temperature.