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 few-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.