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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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Comparison of the experimental and simulatedQ-band 14N HYSCORE spectra of SQA at orientationsof the externalmagnetic field in-plane with the gX/gY axes (left)and along the gZ axis(right). Spectra are presented in stacked (top) and contour (bottom)modes. Simulations are shown in red (Tables 1 and 2). Experimental parameters: time betweenfirst and second pulses τ = 136 ns, microwave frequency = 34.112GHz, π/2-pulse length = 28 ns, temperature = 90 K.
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fig7: Comparison of the experimental and simulatedQ-band 14N HYSCORE spectra of SQA at orientationsof the externalmagnetic field in-plane with the gX/gY axes (left)and along the gZ axis(right). Spectra are presented in stacked (top) and contour (bottom)modes. Simulations are shown in red (Tables 1 and 2). Experimental parameters: time betweenfirst and second pulses τ = 136 ns, microwave frequency = 34.112GHz, π/2-pulse length = 28 ns, temperature = 90 K.

Mentions: The complexity of the X-band 14N HYSCORE spectrum islargely attributable to the cancellation condition being satisfiedand can be avoided by performing the experiment at a different microwavefrequency. The 14N HYSCORE spectra measured at Q-band arepresented in stacked and contour representations in Figure 7. The spectra are dramatically simplified in comparisonwith Figure 6B and have a similar appearanceto the X-band 14N HYSCORE spectrum of SQB wherethe nitrogen couplings are far from cancellation.29 Only two pairs of cross-peaks are observed, with maximaat (10.20, 5.01) MHz (1) and (10.88, 5.11) MHz (2) at gX/gY, and (9.95, 5.25) MHz (1) and (10.78, 5.20) MHz (2) at gZ. These coordinates correlate the double-quantumtransitions νdq+ and νdq– for each coupled nitrogen.


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)

Comparison of the experimental and simulatedQ-band 14N HYSCORE spectra of SQA at orientationsof the externalmagnetic field in-plane with the gX/gY axes (left)and along the gZ axis(right). Spectra are presented in stacked (top) and contour (bottom)modes. Simulations are shown in red (Tables 1 and 2). Experimental parameters: time betweenfirst and second pulses τ = 136 ns, microwave frequency = 34.112GHz, π/2-pulse length = 28 ns, temperature = 90 K.
© Copyright Policy
Related In: Results  -  Collection

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

fig7: Comparison of the experimental and simulatedQ-band 14N HYSCORE spectra of SQA at orientationsof the externalmagnetic field in-plane with the gX/gY axes (left)and along the gZ axis(right). Spectra are presented in stacked (top) and contour (bottom)modes. Simulations are shown in red (Tables 1 and 2). Experimental parameters: time betweenfirst and second pulses τ = 136 ns, microwave frequency = 34.112GHz, π/2-pulse length = 28 ns, temperature = 90 K.
Mentions: The complexity of the X-band 14N HYSCORE spectrum islargely attributable to the cancellation condition being satisfiedand can be avoided by performing the experiment at a different microwavefrequency. The 14N HYSCORE spectra measured at Q-band arepresented in stacked and contour representations in Figure 7. The spectra are dramatically simplified in comparisonwith Figure 6B and have a similar appearanceto the X-band 14N HYSCORE spectrum of SQB wherethe nitrogen couplings are far from cancellation.29 Only two pairs of cross-peaks are observed, with maximaat (10.20, 5.01) MHz (1) and (10.88, 5.11) MHz (2) at gX/gY, and (9.95, 5.25) MHz (1) and (10.78, 5.20) MHz (2) at gZ. These coordinates correlate the double-quantumtransitions νdq+ and νdq– for each coupled nitrogen.

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