Stark Effect in Wild-type and Heterodimer-containing Reaction Centers
from Rhodobacter capsulatus
DiMagno T.J., Bylina E.J., Angerhofer A., Youvan D.C., Norris J.R.
Chemistry Division, Argonne National Laboratory, Illinois 60439.
The effect of an external electric field on the optical absorption spectra of wild-type Rhodobacter
capsulatus and two Rb. capsulatus reaction centers that have been genetically
modified through site-directed mutagenesis (HisM200----LeuM200 and
HisM200----PheM200) was measured at 77 K. The two genetically
modified reaction centers replace histidine M200, the axial ligand to the M-side
bacteriochlorophyll of the special pair, with either leucine or phenylalanine. These
substitutions result in the replacement of the M-side bacteriochlorophyll with
bacteriopheophytin, forming a bacteriochlorophyll-bacteriopheophytin heterodimer. The
magnitude of the change in dipole moment from the ground to excited state (delta mu app)
and the angle delta between the Qy transition moment and the direction of delta
mu app were measured for the special pair absorption band for all three reaction centers.
The values for delta mu app and delta obtained for wild-type Rb. capsulatus (delta mu app
= 6.7 +/- 1.0 D, delta = 38 +/- 3°) were the same within experimental error as those of
Rhodobacter sphaeroides and Rhodopseudomonas viridis. The values for delta mu app and
delta obtained for the red-most Stark band of both heterodimers were the same, but delta
mu was substantially different from that of wild-type reaction centers (HisM200----LeuM200,
delta mu app greater than or equal to 14.1 D and delta = 33 +/- 3°; HisM200----PheM200,
delta mu app greater than or equal to 15.7 D and delta = 31 +/- 4° ).
The differences in the magnitude of delta mu app and the angle d between
wild-type and heterodimer reaction centers are consistent with increased charge transfer
state higher in energy than the excited singlet state in wild-type Rb. capsulatus RCs.