Spin Dipolar Interactions of Radical Pairs in Photosynthetic Reaction Centers

Abstract

The optical detection of recombination dynamics and its dependence on external magnetic fields gives access to kinetic and structural features of short-lived radical pairs (RPs) In reaction centers (RCs) of purple bacteria this RP consists of the cation of the bacteriochlorophyll dimer, (BC)⁺2 and the anion of the bacteriopheophytin, BP⁻and is formed within a few picoseconds by electron-transfer from the singlet excited dimer state. In quinone-depleted RCs at room temperature the RP is stabilized for approximately 10 ns. The initially formed RP is singlet-phased as its precursor state and recombines via the rate k_s to form the ground state species. This is illustrated in the following kinetic scheme: Hyperfine interaction HFI of the two electron spins with their different nuclear spin environments can change the multiplicity of the RP spin state. This leads to a RP triplet state,thus providing a new channel of recombination forming the dimer triplet state with the rate k_T . Such singlet-triplet transitions,however, can be hindered by exchange and spin dipolar interaction (J and D) between the radical electrons as well as by the recombination rates, which are responsible for the lifetime broadening of the RP states.