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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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(A) Stacked representationof the X-band 14N three-pulseESEEM spectra for SQA with nqi triplets corresponding toHis-M219 Nδ (red) and Ala-M260 Np (blue)marked. The time between the first and second pulses τ was incrementedfrom 100 to 564 ns in steps of 16 ns in successive traces. Experimentalparameters: magnetic field = 345.7 mT, microwave frequency = 9.707GHz, temperature = 80 K. (B) Contour representation of the X-band 14N HYSCORE spectrum of SQA with diagonal and cross-peakfeatures marked for His-M219 Nδ (red) and Ala-M260Np (blue). A full 3D view of the spectrum is availablein Supporting Information (Figure S4).Experimental parameters: magnetic field = 345.7 mT, time between firstand second pulses τ = 136 ns, microwave frequency = 9.704 GHz,temperature = 80 K.
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fig6: (A) Stacked representationof the X-band 14N three-pulseESEEM spectra for SQA with nqi triplets corresponding toHis-M219 Nδ (red) and Ala-M260 Np (blue)marked. The time between the first and second pulses τ was incrementedfrom 100 to 564 ns in steps of 16 ns in successive traces. Experimentalparameters: magnetic field = 345.7 mT, microwave frequency = 9.707GHz, temperature = 80 K. (B) Contour representation of the X-band 14N HYSCORE spectrum of SQA with diagonal and cross-peakfeatures marked for His-M219 Nδ (red) and Ala-M260Np (blue). A full 3D view of the spectrum is availablein Supporting Information (Figure S4).Experimental parameters: magnetic field = 345.7 mT, time between firstand second pulses τ = 136 ns, microwave frequency = 9.704 GHz,temperature = 80 K.

Mentions: The X-band 14N three-pulse ESEEM spectraof SQA measured overa wide range of τ-values are shown in stacked presentation inFigure 6A. Two sets of 14N peakssatisfying the cancellation condition (ν14N ≈ /A(14N)//2) are present in thespectrum. The most intense set of features (∼0.75MHz, 1.5 MHz) can be assigned to a nqi triplet, as in eq 2, under the assumption that the low frequency peak is a superpositionof ν– and ν0. When thesetwo lines overlap, the nqi tensor is fully rhombic (principal values:−2K, 0, 2K) with an asymmetryparameter η ≈ 1. Under this condition the qcc followsthe relationship ν+ = 4K = e2qQ/h = 1.5MHz, or K = 0.38 MHz.


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)

(A) Stacked representationof the X-band 14N three-pulseESEEM spectra for SQA with nqi triplets corresponding toHis-M219 Nδ (red) and Ala-M260 Np (blue)marked. The time between the first and second pulses τ was incrementedfrom 100 to 564 ns in steps of 16 ns in successive traces. Experimentalparameters: magnetic field = 345.7 mT, microwave frequency = 9.707GHz, temperature = 80 K. (B) Contour representation of the X-band 14N HYSCORE spectrum of SQA with diagonal and cross-peakfeatures marked for His-M219 Nδ (red) and Ala-M260Np (blue). A full 3D view of the spectrum is availablein Supporting Information (Figure S4).Experimental parameters: magnetic field = 345.7 mT, time between firstand second pulses τ = 136 ns, microwave frequency = 9.704 GHz,temperature = 80 K.
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Related In: Results  -  Collection

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fig6: (A) Stacked representationof the X-band 14N three-pulseESEEM spectra for SQA with nqi triplets corresponding toHis-M219 Nδ (red) and Ala-M260 Np (blue)marked. The time between the first and second pulses τ was incrementedfrom 100 to 564 ns in steps of 16 ns in successive traces. Experimentalparameters: magnetic field = 345.7 mT, microwave frequency = 9.707GHz, temperature = 80 K. (B) Contour representation of the X-band 14N HYSCORE spectrum of SQA with diagonal and cross-peakfeatures marked for His-M219 Nδ (red) and Ala-M260Np (blue). A full 3D view of the spectrum is availablein Supporting Information (Figure S4).Experimental parameters: magnetic field = 345.7 mT, time between firstand second pulses τ = 136 ns, microwave frequency = 9.704 GHz,temperature = 80 K.
Mentions: The X-band 14N three-pulse ESEEM spectraof SQA measured overa wide range of τ-values are shown in stacked presentation inFigure 6A. Two sets of 14N peakssatisfying the cancellation condition (ν14N ≈ /A(14N)//2) are present in thespectrum. The most intense set of features (∼0.75MHz, 1.5 MHz) can be assigned to a nqi triplet, as in eq 2, under the assumption that the low frequency peak is a superpositionof ν– and ν0. When thesetwo lines overlap, the nqi tensor is fully rhombic (principal values:−2K, 0, 2K) with an asymmetryparameter η ≈ 1. Under this condition the qcc followsthe relationship ν+ = 4K = e2qQ/h = 1.5MHz, or K = 0.38 MHz.

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