The electronic structure of the cation radical of the primary electron donor was investigated in genetically modified reaction centers of Rhodobacter sphaeroides. The site-directed mutations were designed to add or remove hydrogen bonds between the conjugated carbonyl groups of the primary donor, a bacteriochlorophyll dimer, and histidine residues of the protein and were introduced at the symmetryrelated sites L168 His→Phe, HF(L168), and M197 Phe→His, FH(M197), near the 2-acetyl groups of the dimer and at sites M160 Leu→His, LH(M160), and L131 Leu→His, LH(L131), in the vicinity of the 9-keto carbonyls of the dimer. The single mutants and a complete set of double mutants were studied using EPR, ENDOR, and TRIPLE resonance spectroscopy. The changes in the hydrogen bond situation of the primary donor were accompanied by changes in the dimer oxidation midpoint potential, ranging from 410 to 710 mV in the investigated mutants [Lin, X., Murchison, H. A., Nagarajan, V., Parson, W. W., Williams, J. C., & Allen, J. P. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 10265-10269], It was found that the addition or removal of a hydrogen bond causes large shifts of the spin density between the two halves of the dimer. Measurements on double mutants showed that the unpaired electron can be gradually shifted from a localization on the L-half of the dimer to a localization on the M-half, depending on the hydrogen bond situation. As a control, the effects of the different hydrogen bonds on P.+ in the mutant HL(M202), which contains a BChlL-BPheM heterodimer as the primary donor with localized spin on the BChl ah [Bylina, E. J., & Youvan, D. C. (1988) Proc. Natl. Acad. Sci. U.SA. 85, 7226-7230; Schenck, C. C., Gaul, D., Steffen M., Boxer S. G., McDowell L., Kirmaier C., & Holten D. (1990) in Reaction Centers of Photosynthetic Bacteria (Michel-Beyerie M. E., Ed.) pp 229-238, Springer, Berlin] were studied. In this mutant only small local changes of the spin densities (≤10%) in the vicinity of the hydrogen bonds were observed. The effects of the introduced hydrogen bonds on the spin density distribution of the dimer in the mutants are discussed in terms of different orbital energies of the two BChl a moieties which are directly influenced by hydrogen bond formation. The observed changes of the spin density distribution for the double mutants are additive with respect to the single mutations. This indicates that no major structural changes occur due to the replacement of the amino acid residues. The asymmetric spin density distributions of P+ were compared with electron transfer rates. A pronounced influence was observed on the P.+ reduction rate by cytochrome C2.
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