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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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Field-swept two-pulseecho of SQA at Q-band. gZ is resolved at the higherfield position ∼1221 mT, whereas gX and gY form an unresolved pair at ∼1219 mT. Circles mark the fieldpositions used for Davies ENDOR measurements. Triangles mark the orientationsof the external magnetic field in-plane with the gX/gY axes (1219.2 mT) and along the unique gZ axis (1221.2 mT) used for HYSCORE measurements.Experimental parameters: π/2-pulse length = 120 ns, time betweenthe first and second pulses τ = 500 ns, microwave frequency= 34.222 GHz, temperature = 80 K.
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fig3: Field-swept two-pulseecho of SQA at Q-band. gZ is resolved at the higherfield position ∼1221 mT, whereas gX and gY form an unresolved pair at ∼1219 mT. Circles mark the fieldpositions used for Davies ENDOR measurements. Triangles mark the orientationsof the external magnetic field in-plane with the gX/gY axes (1219.2 mT) and along the unique gZ axis (1221.2 mT) used for HYSCORE measurements.Experimental parameters: π/2-pulse length = 120 ns, time betweenthe first and second pulses τ = 500 ns, microwave frequency= 34.222 GHz, temperature = 80 K.

Mentions: Q-band Davies ENDOR wasacquired at eight evenly spaced field positions spanning gX, gY, and gZ of thefield-swept two-pulse echo (Figure 3). Theresulting orientation selective Davies ENDOR spectra in Figure 4 show only a single resolvable splitting with anaverage hyperfine coupling of ∼3.4 MHz. However, the Daviespulse sequence can lead to suppression of weaker couplings, so MimsENDOR (which is specifically sensitive to small couplings) was performedto ensure that no features were missed in the Davies ENDOR spectrum(Figure S2, Supporting Information). Noadditional peaks were observed in the Mims experiment.


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)

Field-swept two-pulseecho of SQA at Q-band. gZ is resolved at the higherfield position ∼1221 mT, whereas gX and gY form an unresolved pair at ∼1219 mT. Circles mark the fieldpositions used for Davies ENDOR measurements. Triangles mark the orientationsof the external magnetic field in-plane with the gX/gY axes (1219.2 mT) and along the unique gZ axis (1221.2 mT) used for HYSCORE measurements.Experimental parameters: π/2-pulse length = 120 ns, time betweenthe first and second pulses τ = 500 ns, microwave frequency= 34.222 GHz, temperature = 80 K.
© Copyright Policy
Related In: Results  -  Collection

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

fig3: Field-swept two-pulseecho of SQA at Q-band. gZ is resolved at the higherfield position ∼1221 mT, whereas gX and gY form an unresolved pair at ∼1219 mT. Circles mark the fieldpositions used for Davies ENDOR measurements. Triangles mark the orientationsof the external magnetic field in-plane with the gX/gY axes (1219.2 mT) and along the unique gZ axis (1221.2 mT) used for HYSCORE measurements.Experimental parameters: π/2-pulse length = 120 ns, time betweenthe first and second pulses τ = 500 ns, microwave frequency= 34.222 GHz, temperature = 80 K.
Mentions: Q-band Davies ENDOR wasacquired at eight evenly spaced field positions spanning gX, gY, and gZ of thefield-swept two-pulse echo (Figure 3). Theresulting orientation selective Davies ENDOR spectra in Figure 4 show only a single resolvable splitting with anaverage hyperfine coupling of ∼3.4 MHz. However, the Daviespulse sequence can lead to suppression of weaker couplings, so MimsENDOR (which is specifically sensitive to small couplings) was performedto ensure that no features were missed in the Davies ENDOR spectrum(Figure S2, Supporting Information). Noadditional peaks were observed in the Mims experiment.

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