Electron transfer in bacterial reaction centers with an energetically raised primary acceptor: ultrafast spectroscopy and ENDOR/TRIPLE studies

Femtosecond time resolved absorbance changes in the near infrared spectral region were recorded in photosynthetic reaction centers (RCs) of Rhodobacter ( Rb .) sphaeroides and Chloroflexus aurantiacus . For Rb. sphaeroides , wild type and site directed mutant RCs, where the tyrosine at the M210 position was replaced by phenylalanine and leucine, were investigated. In the mutant RCs the decay of the stimulated emission signal was drastically slower and displayed strong multiexponentiality. In the spectral range between 1000 and 1100 nm, the existence of a fast kinetic component with a time constant of 1 ps could be demonstrated. Based on the spectral and dichroic characteristics of the data, we suggest a weakly populated P + B A - state as first electron transfer intermediate in these mutants. The results indicate that primary electron transfer follows a stepwise mechanism, even in the case of an energetically increased primary acceptor. Investigations of the electronic structure of the photooxidized primary donor using ENDOR/TRIPLE spectroscopy demonstrated that changes at the position M210 mainly affect B A and not P.