Coherent Dynamics during the Primary Electron-Transfer Reaction in Membrane-Bound Reaction Centers of Rhodobacter sphaeroides

The temporal evolution of the near-infrared stimulated emission band of the special pair excited state (P*) in the reaction center of Rhodobacter sphaeroides has been studied in intracytoplasmic membranes of the antenna-deficient RCO1 mutant at 10 K with a resolution of 30 fs. On the 100-fs time scale the emission band gradually shifts to longer wavelengths. After 150 fs the band shifts back to shorter wavelengths and continues to develop on the picosecond time scale in a damped oscillatory manner (most prominent fundamental frequencies around 15 cm⁻¹ and at 92, 122, and 153 cm⁻¹). These phenomena are shown to be due to low-frequency vibrational motions in the P* excited state that conserve their phase on the time scale of electron transfer. These results imply that the vibrational manifold of P* is not thermalized during the electron-transfer reaction in functional reaction centers. The initial Stokes shift dynamics are largely determined by the modes in the 90–160-cm⁻¹ frequency range, which probably involve motions of several chromophores, including the bacteriopheophytin electron acceptor H_L.