Hyperfine and Nuclear Quadrupole Tensors of Nitrogen Donors in the QA Site of Bacterial Reaction Centers: Correlation of the Histidine N未 Tensors with Hydrogen Bond Strength

详细信息    查看全文

• 作者：Alexander T. Taguchi ; Patrick J. O鈥橫alley ; Colin A. Wraight ; Sergei A. Dikanov
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：August 7, 2014
• 年：2014
• 卷：118
• 期：31
• 页码：9225-9237
• 全文大小：508K
• ISSN：1520-5207

文摘

X- and Q-band pulsed EPR spectroscopy was applied to study the interaction of the Q_A site semiquinone (SQ_A) with nitrogens from the local protein environment in natural abundance ¹⁴N and in ¹⁵N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 N_未 and Ala-M260 peptide nitrogen (N_p) were estimated through simultaneous simulation of the Q-band ¹⁵N Davies ENDOR, X- and Q-band ^14,15N HYSCORE, and X-band ¹⁴N three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine coupling constants were found to be a(¹⁴N) = 2.3 MHz, T = 0.3 MHz for His-M219 N_未 and a(¹⁴N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 N_p. Despite that His-M219 N_未 is established as the stronger of the two H-bond donors, Ala-M260 N_p is found to have the larger value of a(¹⁴N). The nuclear quadrupole coupling constants were estimated as e²Qq/4h = 0.38 MHz, 畏 = 0.97 and e²Qq/4h = 0.74 MHz, 畏 = 0.59 for His-M219 N_未 and Ala-M260 N_p, 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鈥揇ailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter 畏 to be a sensitive probe of the histidine N_未鈥搒emiquinone hydrogen bond strength. This is supported by a strong correlation observed between 畏 and the isotropic coupling constant a(¹⁴N) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a(¹⁴N) and 畏 will provide an important structural characterization tool in future studies of semiquinone-binding proteins.