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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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Comparisonof the experimental and simulated X-band 15N HYSCORE spectrumof SQA. The spectrum is presented instacked (left) and contour (right) modes. The dashed lines in thecontour representation are defined by /ν1 ±ν2/ = 2(ν15N). Simulationof feature 1 is shown in red (Table 1). Experimental parameters: magnetic field = 345.9 mT, timebetween first and second pulses τ = 136 ns, microwave frequency= 9.702 GHz, temperature = 80 K.
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fig2: Comparisonof the experimental and simulated X-band 15N HYSCORE spectrumof SQA. The spectrum is presented instacked (left) and contour (right) modes. The dashed lines in thecontour representation are defined by /ν1 ±ν2/ = 2(ν15N). Simulationof feature 1 is shown in red (Table 1). Experimental parameters: magnetic field = 345.9 mT, timebetween first and second pulses τ = 136 ns, microwave frequency= 9.702 GHz, temperature = 80 K.

Mentions: The X-band 15N HYSCORE spectrumfor SQA is shown in Figure 2. Thespectrum contains features 1 and 2, locatedin the (+/−) and (+/+) quadrants, whichappear to be two well-resolved nitrogen couplings that satisfy thecancellation condition (ν15N ≈/A(15N)//2). From the locations of thecross-peak maxima (−3.23, +0.37) MHz (1) and (+2.74,+0.33) MHz (2) the first-order estimates of the hyperfinecouplings are 3.60 and 2.41 MHz, respectively (2.57 and 1.72 MHz whenscaled to 14N). However, the reliability of this simpleanalysis is called into question by the unusual features of the spectrum.The cross-peak arcs are close to the coordinates (0, 2ν15N) and (2ν15N, 0), which, in regard to theHYSCORE intensity, correspond to a singularity for the perpendiculardirection or a simple maximum for the parallel orientation of thehfi tensor.28 Peaks in this region of theHYSCORE spectrum (/ν1(2)/ < 0.2 MHz) are alsosubject to strong suppression and broadening effects, which complicatethe analysis. In Figure 2, the peaks exhibita curvature not in accordance with the theoretically predicted lineshape for a single I = 1/2 anisotropic hfi.28 Overlap from multiple 15N couplingsmay explain these distortions, as simulations of features 1 and 2 as separate nitrogens failed to reproduce therelative peak intensities (as discussed in more detail later).


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)

Comparisonof the experimental and simulated X-band 15N HYSCORE spectrumof SQA. The spectrum is presented instacked (left) and contour (right) modes. The dashed lines in thecontour representation are defined by /ν1 ±ν2/ = 2(ν15N). Simulationof feature 1 is shown in red (Table 1). Experimental parameters: magnetic field = 345.9 mT, timebetween first and second pulses τ = 136 ns, microwave frequency= 9.702 GHz, temperature = 80 K.
© Copyright Policy
Related In: Results  -  Collection

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

fig2: Comparisonof the experimental and simulated X-band 15N HYSCORE spectrumof SQA. The spectrum is presented instacked (left) and contour (right) modes. The dashed lines in thecontour representation are defined by /ν1 ±ν2/ = 2(ν15N). Simulationof feature 1 is shown in red (Table 1). Experimental parameters: magnetic field = 345.9 mT, timebetween first and second pulses τ = 136 ns, microwave frequency= 9.702 GHz, temperature = 80 K.
Mentions: The X-band 15N HYSCORE spectrumfor SQA is shown in Figure 2. Thespectrum contains features 1 and 2, locatedin the (+/−) and (+/+) quadrants, whichappear to be two well-resolved nitrogen couplings that satisfy thecancellation condition (ν15N ≈/A(15N)//2). From the locations of thecross-peak maxima (−3.23, +0.37) MHz (1) and (+2.74,+0.33) MHz (2) the first-order estimates of the hyperfinecouplings are 3.60 and 2.41 MHz, respectively (2.57 and 1.72 MHz whenscaled to 14N). However, the reliability of this simpleanalysis is called into question by the unusual features of the spectrum.The cross-peak arcs are close to the coordinates (0, 2ν15N) and (2ν15N, 0), which, in regard to theHYSCORE intensity, correspond to a singularity for the perpendiculardirection or a simple maximum for the parallel orientation of thehfi tensor.28 Peaks in this region of theHYSCORE spectrum (/ν1(2)/ < 0.2 MHz) are alsosubject to strong suppression and broadening effects, which complicatethe analysis. In Figure 2, the peaks exhibita curvature not in accordance with the theoretically predicted lineshape for a single I = 1/2 anisotropic hfi.28 Overlap from multiple 15N couplingsmay explain these distortions, as simulations of features 1 and 2 as separate nitrogens failed to reproduce therelative peak intensities (as discussed in more detail later).

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