Stark effect in wild-type and heterodimer-containing reaction centers from Rhodobacter capsulatus
- 30 January 1990
- journal article
- research article
- Published by American Chemical Society (ACS) in Biochemistry
- Vol. 29 (4) , 899-907
- https://doi.org/10.1021/bi00456a008
Abstract
The effect of an external 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 (HisM20P0 .fwdarw. Leu M200 PheM200) was measured at 77 K. The two genetically modified reaction center 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-bacteriopheophtin 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.degree.) were the same within experimental error as those of Rhodobacter sphaeroides and Rhodopseudomonas virdis. 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 .fwdarw. LeuM200, .DELTA. .mu.app .gtoreq. 14.1 D and .delta. = 33 .+-. 3.degree.; HisM200 .fwdarw. PheM200, .DELTA. .mu.app .gtoreq. 15.7 D and .delta. = 31 .+-. 4.degree.). The differences in the magnitude of .delta. .mu.app and the angle .delta. 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.This publication has 12 references indexed in Scilit:
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