SPECTROSCOPIC CHARACTERIZATION OF REACTION CENTERS OF THE (M)Y210W MUTANT OF THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES

The tyrosine-(M)210 of the reaction center of RHodobacter sphaeroides 2.4.1 has been changed to a tryptophan using site-directed mutagenesis . The reaction center of this mutant has been characterized by low-tem perature absorption and fluorescence spectroscopy, time-resolved sub-p icosecond spectroscopy, and magnetic resonance spectroscopy. The charg e separation process showed bi-exponential kinetics at room temperatur e, with a main time constant of 36 ps and an additional fast time cons tant of 5.1 ps. Temperature dependent fluorescence measurements predic t that the lifetime of P- becomes 4-5 times slower at cryogenic tempe ratures. From EPR and absorbance-detected magnetic resonance (ADMR, LD -ADMR) we conclude that the dimeric structure of P is not significantl y changed upon mutation. In contrast, the interaction of the accessory bacteriochlorophyll B-A with its environment appears to be altered, p ossibly because of a change in its position.