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

[Image: see text] 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 (14)N and in (15)N uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 N(δ) and Ala-M260 peptide nitrogen (N(p)) 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 N(δ) and a((14)N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 N(p). Despite that His-M219 N(δ) is established as the stronger of the two H-bond donors, Ala-M260 N(p) 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 N(δ) 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–Dailey model, developed previously for copper complexes, to the semiquinones, we find the asymmetry parameter η to be a sensitive probe of the histidine N(δ)–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.