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 Q(B)(-) (SQ(B)) in reaction centers of Rhodobacter sphaeroides interacts with N-delta of His-L190 and N-p (peptide nitrogen) of Gly-L225 involved in hydrogen bonds to the Q, carbonyls. In this work, S-band (similar to 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 N-14 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 N-delta 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 eta = 0.69. Estimates of the hyperfine interaction (hfi) tensors for N-delta and N-p were obtained from simulations of 1D and 2D N-14,N-15 X-band and three-pulse N-14 S-band spectra with all nuclear tensors defined in the SQB g-tensor coordinate system. From simulations, we conclude that the contribution of N-p 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 N-p at S-band. The excellent agreement between our EPR simulations and DFT calculations of the nitrogen hfi and nqi tensors to SQ(B) is promising for the future application of powder ESEEM to full tensor characterizations.