Ultrafast spectroscopy of the electron transfer in photosynthetic reaction centres: towards a better understanding of electron transfer in biological systems

Femtosecond spectroscopy shows that primary electron transfer in photosynthetic reaction centres is an ultrafast stepwise reaction where the electron is transferred via a chain of pigments. In a first reaction step, the electron is transferred from the special pair to the accessory bacteriochlorophyll with a time constant of ca. 3 ps. A second, subpicosecond reaction carries the electron to a bacteriopheophytin before it reaches the quinone QA within 200 ps. Experiments on native and mutated reaction centres yield information on energetics, reorganization energies and electronic coupling of the reaction centre. A consistent theoretical treatment of the data shows that standard non–adiabatic theory describes well the primary electron transfer process at room temperature. However, for a special fast reacting mutant at low temperatures, the limits of non–adiabatic theory may be reached.