Nuclear Hyperfine and Quadrupole Tensor Characterization of the Nitrogen Hydrogen Bond Donors to the Semiquinone of the QB Site in Bacterial Reaction Centers: A Combined X- and S-Band 14,15N ESEEM and DFT Study

The secondary quinone anion radical QB(-) (SQB) in reaction centers of Rhodobacter sphaeroides interacts with Nδ of His-L190 and Np (peptide nitrogen) of Gly-L225 involved in hydrogen bonds to the QB carbonyls. In this work, S-band (∼3.6 GHz) ESEEM was used with the aim of obtaining a complete characterization of the nuclear quadrupole interaction (nqi) tensors for both nitrogens by approaching the cancelation condition between the isotropic hyperfine coupling and (14)N Zeeman frequency at lower microwave frequencies than traditional X-band (9.5 GHz). By performing measurements at S-band, we found a dominating contribution of Nδ in the form of a zero-field nqi triplet at 0.55, 0.92, and 1.47 MHz, defining the quadrupole coupling constant K = e(2)qQ/4h = 0.4 MHz and associated asymmetry parameter η = 0.69. Estimates of the hyperfine interaction (hfi) tensors for Nδ and Np were obtained from simulations of 1D and 2D (14,15)N X-band and three-pulse (14)N S-band spectra with all nuclear tensors defined in the SQB g-tensor coordinate system. From simulations, we conclude that the contribution of Np to the S-band spectrum is suppressed by its strong nqi and weak isotropic hfi comparable to the level of hyperfine anisotropy, despite the near-cancelation condition for Np at S-band. The excellent agreement between our EPR simulations and DFT calculations of the nitrogen hfi and nqi tensors to SQB is promising for the future application of powder ESEEM to full tensor characterizations.