ELECTRON-TRANSFER IN BACTERIAL REACTION CENTERS WITH AN ENERGETICALLYRAISED PRIMARY ACCEPTOR - ULTRAFAST SPECTROSCOPY AND ENDOR TRIPLE STUDIES/

Femtosecond time resolved absorbance changes in the near infrared spec tral region were recorded in photosynthetic reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides and Chloroflexus aurantiacus. For Rb. sp haeroides, wild type and site directed mutant RCs, where the tyrosine at the M210 position was replaced by phenylalanine and leucine, were i nvestigated. In the mutant RCs the decay of the stimulated emission si gnal was drastically slower and displayed strong multiexponentiality. In the spectral range between 1000 and 1100 nm, the existence of a fas t kinetic component with a time constant of 1 ps could be demonstrated . Based on the spectral and dichroic characteristics of the data, we s uggest a weakly populated P+BA- state as first electron transfer inter mediate in these mutants. The results indicate that primary electron t ransfer follows a stepwise mechanism, even in the case of an energetic ally increased primary acceptor. Investigations of the electronic stru cture of the photooxidized primary donor using ENDOR/TRIPLE spectrosco py demonstrated that changes at the position M210 mainly affect B-A an d not P. (C) 1998 Elsevier Science B.V. All rights reserved.