ON THE EFFICIENCY OF ENERGY-TRANSFER AND THE DIFFERENT PATHWAYS OF ELECTRON-TRANSFER IN MUTANT REACTION CENTERS OF RHODOBACTER-SPHAEROIDES

The efficiency of energy transfer from the monomeric pigments to the p rimary donor was determined from 77 K steady-state fluorescence excita tion spectra of three mutant reaction centers, YM210L, YM210F and LM16 0H / FM197H. For all three reaction centers this efficiency was not 10 0% and ranged between 55 and 70%. For the YM210L mutant it was shown u sing pump-probe spectroscopy with B band excitation at 798 nm that the excitations which are not transferred to P give rise to efficient cha rge separation. The results can be interpreted with a model in which e xcitation of the B absorbance band leads to direct formation of the ra dical pair state BA+HA- in addition to energy transfer to P. It is als o possible that some P+BA- is formed from B. In previous publications we have demonstrated the operation of such alternative pathways for t ransmembrane electron transfer in a YM210W mutant reaction center [van Brederode et al. (1996) The Reaction center of Photosynthetic Bacteri a, pp 225-238; (1997a,b) Chem Phys Lett 268: 143-149; Biochemistry 36: 6855-6861]. The results presented here demonstrate that these alterna tive mechanisms are not peculiar to the YM210W reaction center.