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 N-14 and in N-15 uniformly labeled photosynthetic reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 N-delta and Ala-M260 peptide nitrogen (N-p) were estimated through simultaneous simulation of the Q:band N-15 Davies ENDOR, X- and Q-band N-14,N-15 HYSCORE, and X-band N-14 three-pulse ESEEM spectra, with support from DFT calculations. The hyperfine coupling constants were found to be a(N-14) = 2.3 MHz, T = 0.3 MHz for His-M219 N-delta and a(N-14) = 2.6 MHz, T = 0.3 MHz for Ala-M260 N-p. Despite that His-M219 N-delta 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 e(2)Qq/4h = 0.38 MHz, eta = 0.97 and e(2)Qq/4h = 0.74 MHz, eta = 0.59 for His-M219 N-delta 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 eta to be a sensitive probe of the histidine N-delta-semiquinone hydrogen bond strength. This is supported by a strong correlation observed between eta and the isotropic coupling constant a(N-14) and is consistent with previous computational works and our own semiquinone-histidine model calculations. The empirical relationship presented here for a(N-14) and eta will provide an important structural characterization tool in future studies of semiquinone-binding proteins.