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-temperature absorption and fluorescence
spectroscopy, time-resolved sub-picosecond spectroscopy, and
magnetic resonance spectroscopy. The charge separation process
showed bi-exponential kinetics at room temperature, with a main
time constant of 36 ps and an additional fast time constant of 5.1
ps. Temperature dependent fluorescence measurements predict that
the lifetime of P* becomes 4–5 times slower at cryogenic
temperatures. From EPR and absorbance-detected magnetic resonance
(ADMR, LD-ADMR) we conclude that the dimeric structure of P is not
significantly changed upon mutation. In contrast, the interaction
of the accessory bacteriochlorophyll BA with its environment
appears to be altered, possibly because of a change in its
position.