Investigation into the source of electron transfer asymmetry in bacterial reaction centers.
We have investigated the primary photochemistry of two symmetry-related mutants of Rhodobacter sphaeroides in which the histidine residues associated with the central Mg2+ ions of the two bacteriochlorophylls of the dimeric primary electron donor (His-L173 and His-M202) have been changed to leucine, affording bacteriochlorophyll (BChl)/bacteriopheophytin (BPh) heterodimers. Reaction centers (RCs) from the two mutants, (L)H173L and (M)H202L, have remarkably similar spectral and kinetic properties, although they are quite different from those of wild-type RCs. In both mutants, as in wild-type RCs, electron transfer to BPhL and not to BPhM is observed. These results suggest that asymmetry in the charge distribution of the excited BChl dimer (P*) in wild-type RCs (due to differing contributions of the two opposing intradimer charge-transfer states) contributes only modestly to the directionality of electron transfer. The results also suggest that differential orbital overlap of the two BChls of P with the chromophores on the L and M polypeptides does not contribute substantially to preferential electron transfer to BPhL.
McDowell
L M
LM
Department of Chemistry, Washington University, St. Louis, Missouri 63130.
Gaul
D
D
Kirmaier
C
C
Holten
D
D
Schenck
C C
CC
D001429:Bacteriochlorophylls / Q000737:chemistry; D004579:Electron Transport; D045342:Light-Harvesting Protein Complexes; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000378:metabolism*; D013053:Spectrophotometry; D013237:Stereoisomerism
D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins