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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 (blue)and simulated (red) X-band 14N three-pulse ESEEM spectraof SQA (Tables 1 and 2). τ-values chosenwere evenly spaced starting from 180 ns in the bottom trace and increasedin steps of 32 ns in subsequent traces. Experimental parameters: magneticfield = 345.7 mT, microwave frequency = 9.707 GHz, temperature = 80K.
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fig8: Comparison of the experimental (blue)and simulated (red) X-band 14N three-pulse ESEEM spectraof SQA (Tables 1 and 2). τ-values chosenwere evenly spaced starting from 180 ns in the bottom trace and increasedin steps of 32 ns in subsequent traces. Experimental parameters: magneticfield = 345.7 mT, microwave frequency = 9.707 GHz, temperature = 80K.

Mentions: A comparison of the experimental and simulated 14N three-pulseESEEM spectra is shown in Figure 8. The finalhfi and nqi parameters optimized by simultaneous simulation of the 14,15N ESEEM and 15N ENDOR spectra, together withour DFT calculated values for SQA (and SQB forcomparison), are summarized in Tables 1 and 2. A full 3D view of the hfi tensor alignments tothe molecular frame determined from DFT calculations is availablein the Supporting Information (Figure S7).The X-band 14N HYSCORE spectrum was not included in thesimulations due to complications arising from the many combinationlines, as well as the presence of intense diagonal peaks that cannotbe simulated with ideal pulses.


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 (blue)and simulated (red) X-band 14N three-pulse ESEEM spectraof SQA (Tables 1 and 2). τ-values chosenwere evenly spaced starting from 180 ns in the bottom trace and increasedin steps of 32 ns in subsequent traces. Experimental parameters: magneticfield = 345.7 mT, microwave frequency = 9.707 GHz, temperature = 80K.
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Comparison of the experimental (blue)and simulated (red) X-band 14N three-pulse ESEEM spectraof SQA (Tables 1 and 2). τ-values chosenwere evenly spaced starting from 180 ns in the bottom trace and increasedin steps of 32 ns in subsequent traces. Experimental parameters: magneticfield = 345.7 mT, microwave frequency = 9.707 GHz, temperature = 80K.
Mentions: A comparison of the experimental and simulated 14N three-pulseESEEM spectra is shown in Figure 8. The finalhfi and nqi parameters optimized by simultaneous simulation of the 14,15N ESEEM and 15N ENDOR spectra, together withour DFT calculated values for SQA (and SQB forcomparison), are summarized in Tables 1 and 2. A full 3D view of the hfi tensor alignments tothe molecular frame determined from DFT calculations is availablein the Supporting Information (Figure S7).The X-band 14N HYSCORE spectrum was not included in thesimulations due to complications arising from the many combinationlines, as well as the presence of intense diagonal peaks that cannotbe simulated with ideal pulses.

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