Hydrogen bonds between nitrogen donors and the semiquinone in the Q B site of bacterial reaction centers

Erik Martin, Rimma I. Samoilova, Kupala V. Narasimhulu, Colin A. Wraight, Sergei A. Dikanov

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Photosynthetic reaction centers from Rhodobacter sphaeroides have identical ubiquinone-10 molecules functioning as primary (QA) and secondary (QB) electron acceptors. X-band 2D pulsed EPR spectroscopy, called HYSCORE, was applied to study the interaction of the QB site semiquinone with nitrogens from the local protein environment in natural and 15N uniformly labeled reactions centers. 14N and 15N HYSCORE spectra of the QB semiquinone show the interaction with two nitrogens carrying transferred unpaired spin density. Quadrupole coupling constants estimated from 14N HYSCORE spectra indicate them to be a protonated nitrogen of an imidazole residue and amide nitrogen of a peptide group. 15N HYSCORE spectra allowed estimation of the isotropic and anisotropic couplings with these nitrogens. From these data, we calculated the unpaired spin density transferred onto 2s and 2p orbitals of nitrogen and analyzed the contribution of different factors to the anisotropic hyperfine tensors. The hyperfine coupling of other protein nitrogens with the semiquinone is weak (<0.1 MHz). These results clearly indicate that the QB semiquinone forms hydrogen bonds with two nitrogens and provide quantitative characteristics of the hyperfine couplings with these nitrogens, which can be used in theoretical modeling of the QB site. On the basis of the quadrupole coupling constant, one nitrogen can only be assigned to Nδ of His-L190, consistent with all existing structures. However, we cannot specify between two candidates the residue corresponding to the second nitrogen. Further work employing multifrequency spectroscopic approaches or selective isotope labeling would be desirable for unambiguous assignment of this nitrogen.

Original languageEnglish (US)
Pages (from-to)11671-11677
Number of pages7
JournalJournal of the American Chemical Society
Issue number33
StatePublished - Aug 25 2010

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry


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