Investigation of the Nature of the Excited-State of the Primary Electron Donor in the Purple Bacterial Reaction Centers: Femtosecond Absorption Spectroscopy of a Cereibacter sphaeroides Mutant with Blocked Electron Transfer.

Using femtosecond transient absorption spectroscopy, we investigated dynamics of absorption changes in the mutant reaction centers (RCs) of Cereibacter (C.) sphaeroides harboring the amino acid substitutions M160LH+L131LH+M197FH and M210YL, a combination known to block photoinduced charge separation. In these mutant RCs, formation of a long-lived excited state of the primary electron donor (P*) was observed. This state is characterized by stimulated emission that is red-shifted by 20 nm relative to that in the wild-type RCs. We hypothesize that the excited special-pair dimer adopts a charge-transfer state, (PAδ+PBδ-)*, which becomes long-lived under conditions where electron transfer is blocked in the mutant RC. On the energy scale, our data place the (PAδ+PBδ-)* state approximately 20 meV below the excitonic P* level. Within this framework, in the native RCs, the (PAδ+PBδ-)* charge-transfer state is short-lived and participates in ultrafast coherent electron-transfer reactions to the BA acceptor.