STARK-EFFECT IN WILD-TYPE AND HETERODIMER-CONTAINING REACTION CENTERS FROM RHODOBACTER-CAPSULATUS

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 (Δμapp) and the angle δ between the Qytransition moment and the direction of Δμappwere measured for the special pair absorption band for all three reaction centers. The values for Δμappana δ obtained for wild-type Rb. capsulatus (Δμapp= 6.7 ± 1.0 D, δ = 38 ± 3°) were the same within experimental error as those of Rhodobacter sphaeroides and Rhodopseudomonas viridis. The values for Δμappand δ obtained for the red-most Stark band of both heterodimers were the same, but Δμ was substantially different from that of wild-type reaction centers (HisM200 → LeuM200, Δμapp≥ 14.1 D and δ = 33 ± 3°; HisM200 → PheM200, Δμapp≥ 15.7 D and δ = 31 ± 4°). The differences in the magnitude of Δμappand the angle δ between wild-type and heterodimer reaction centers are consistent with increased charge transfer in the heterodimer special pair. These results support calculations that place the special pair charge-transfer state higher in energy than the excited singlet state in wild-type Rb. capsulatus RCs. © 1990, American Chemical Society. All rights reserved.