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Hyperfine and nuclear quadrupole tensors of nitrogen donors in the Q(A) site of bacterial reaction centers: correlation of the histidine N(δ) tensors with hydrogen bond strength.

Taguchi AT, O'Malley PJ, Wraight CA, Dikanov SA - J Phys Chem B (2014)

Bottom Line: The hyperfine coupling constants were found to be a((14)N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a((14)N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np.Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a((14)N).An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals.

View Article: PubMed Central - PubMed

Affiliation: Center for Biophysics and Computational Biology, §Department of Biochemistry, and ∥Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.

ABSTRACT
X- and Q-band pulsed EPR spectroscopy was applied to study the interaction of the QA site semiquinone (SQA) with nitrogens from the local protein environment in natural abundance (14)N and in (15)N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 Nδ and Ala-M260 peptide nitrogen (Np) were estimated through simultaneous simulation of the Q-band (15)N Davies ENDOR, X- and Q-band (14,15)N HYSCORE, and X-band (14)N three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine coupling constants were found to be a((14)N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a((14)N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np. Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a((14)N). The nuclear quadrupole coupling constants were estimated as e(2)Qq/4h = 0.38 MHz, η = 0.97 and e(2)Qq/4h = 0.74 MHz, η = 0.59 for His-M219 Nδ and Ala-M260 Np, respectively. An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals. By applying the Townes-Dailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter η to be a sensitive probe of the histidine Nδ-semiquinone hydrogen bond strength. This is supported by a strong correlation observed between η and the isotropic coupling constant a((14)N) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a((14)N) and η will provide an important structural characterization tool in future studies of semiquinone-binding proteins.

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Interaction of SQA with His-M219 Nδ and Ala-M260 Np, which are H-bonded to carbonyl oxygensO4 and O1, respectively. The principal axesof the g-tensor are labeled as X, Y, and Z. The gX axis lies along the line connecting the two oxygenatoms, which carry about half of the spin density, the gZ axis is perpendicular to the molecularplane, and gY is perpendicularto both the other principal axes. The principal values of the SQAg-tensor are gX = 2.00649, gY = 2.00532, and gZ = 2.00210.9
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fig1: Interaction of SQA with His-M219 Nδ and Ala-M260 Np, which are H-bonded to carbonyl oxygensO4 and O1, respectively. The principal axesof the g-tensor are labeled as X, Y, and Z. The gX axis lies along the line connecting the two oxygenatoms, which carry about half of the spin density, the gZ axis is perpendicular to the molecularplane, and gY is perpendicularto both the other principal axes. The principal values of the SQAg-tensor are gX = 2.00649, gY = 2.00532, and gZ = 2.00210.9

Mentions: From the known structures of bacterial RCs,the C4 carbonylof QA is hydrogen-bonded to Nδ of His-M219(which is also a ligand of a high spin nonheme Fe2+), whereasthe backbone peptide nitrogen (Np) of Ala-M260 is the H-bonddonor to the C1 carbonyl (Figure 1).6,8 However, available X-ray structures do not providean unequivocal description of the quinone site, and the H-bond distancesand torsional angles of the two methoxy substituents on the ubiquinonerings are quite variable across different structures.7


Hyperfine and nuclear quadrupole tensors of nitrogen donors in the Q(A) site of bacterial reaction centers: correlation of the histidine N(δ) tensors with hydrogen bond strength.

Taguchi AT, O'Malley PJ, Wraight CA, Dikanov SA - J Phys Chem B (2014)

Interaction of SQA with His-M219 Nδ and Ala-M260 Np, which are H-bonded to carbonyl oxygensO4 and O1, respectively. The principal axesof the g-tensor are labeled as X, Y, and Z. The gX axis lies along the line connecting the two oxygenatoms, which carry about half of the spin density, the gZ axis is perpendicular to the molecularplane, and gY is perpendicularto both the other principal axes. The principal values of the SQAg-tensor are gX = 2.00649, gY = 2.00532, and gZ = 2.00210.9
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126732&req=5

fig1: Interaction of SQA with His-M219 Nδ and Ala-M260 Np, which are H-bonded to carbonyl oxygensO4 and O1, respectively. The principal axesof the g-tensor are labeled as X, Y, and Z. The gX axis lies along the line connecting the two oxygenatoms, which carry about half of the spin density, the gZ axis is perpendicular to the molecularplane, and gY is perpendicularto both the other principal axes. The principal values of the SQAg-tensor are gX = 2.00649, gY = 2.00532, and gZ = 2.00210.9
Mentions: From the known structures of bacterial RCs,the C4 carbonylof QA is hydrogen-bonded to Nδ of His-M219(which is also a ligand of a high spin nonheme Fe2+), whereasthe backbone peptide nitrogen (Np) of Ala-M260 is the H-bonddonor to the C1 carbonyl (Figure 1).6,8 However, available X-ray structures do not providean unequivocal description of the quinone site, and the H-bond distancesand torsional angles of the two methoxy substituents on the ubiquinonerings are quite variable across different structures.7

Bottom Line: The hyperfine coupling constants were found to be a((14)N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a((14)N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np.Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a((14)N).An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals.

View Article: PubMed Central - PubMed

Affiliation: Center for Biophysics and Computational Biology, §Department of Biochemistry, and ∥Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.

ABSTRACT
X- and Q-band pulsed EPR spectroscopy was applied to study the interaction of the QA site semiquinone (SQA) with nitrogens from the local protein environment in natural abundance (14)N and in (15)N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 Nδ and Ala-M260 peptide nitrogen (Np) were estimated through simultaneous simulation of the Q-band (15)N Davies ENDOR, X- and Q-band (14,15)N HYSCORE, and X-band (14)N three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine coupling constants were found to be a((14)N) = 2.3 MHz, T = 0.3 MHz for His-M219 Nδ and a((14)N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np. Despite that His-M219 Nδ is established as the stronger of the two H-bond donors, Ala-M260 Np is found to have the larger value of a((14)N). The nuclear quadrupole coupling constants were estimated as e(2)Qq/4h = 0.38 MHz, η = 0.97 and e(2)Qq/4h = 0.74 MHz, η = 0.59 for His-M219 Nδ and Ala-M260 Np, respectively. An analysis of the available data on nuclear quadrupole tensors for imidazole nitrogens found in semiquinone-binding proteins and copper complexes reveals these systems share similar electron occupancies of the protonated nitrogen orbitals. By applying the Townes-Dailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter η to be a sensitive probe of the histidine Nδ-semiquinone hydrogen bond strength. This is supported by a strong correlation observed between η and the isotropic coupling constant a((14)N) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a((14)N) and η will provide an important structural characterization tool in future studies of semiquinone-binding proteins.

Show MeSH
Related in: MedlinePlus