PRIMARY ELECTRON-TRANSFER DYNAMICS IN MODIFIED BACTERIAL REACTION CENTERS CONTAINING PHEOPHYTIN-A INSTEAD OF BACTERIOPHEOPHYTIN-A

Primary electron transfer was studied in modified reaction centers of Rhodobacter sphaeroides R26.1, in which both bacteriopheophytin-a molecules were replaced by pheophytin-a to more than 90%. The absorption spectrum of the modified reaction centers shows a new Q(y), band at 674 nm, while the Q(y) absorption of bacteriopheophytin-a at 760 nm in native reaction centers is not present. The bands assigned to the monomeric bacteriochlorophylls show a slight blue shift (3 nm) and small but distinct changes in the circular dichroism spectra. Subpicosecond absorption spectroscopy reveals that the electron transfer kinetics in modified reaction centers are different from the kinetics in native samples. The accessory bacteriochlorophyll anion can be directly observed in the spectral region around 1000 nm. A detailed analysis of the data suggests that the modification raised the energy of the radical pair state P+HA- leading to a long-lived P+BA- population of 30%. Relative to P* the free energy levels of the intermediates are -450 cm(-1) (P+BA-) and -630 cm(-1) (P+HA-) in the modified reaction centers. Based on these calculations, a simplified electron transfer model is presented.