Pulsed EPR/ENDOR Characterization of the Cu2+ Surface Site in Photosynthetic Bacterial Reaction Centers

X-band pulsed electron paramagnetic resonance was used to elucidate details of the Cu2+ surface binding site involved in QA -QB → QAQB - electron transfer in photosynthetic bacterial reaction centers (RCs). Electron spin−echo envelope modulation (ESEEM) spectroscopy was employed to measure the hyperfine and electric quadrupole interactions of weakly coupled 14N nuclei, and X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy was used to probe strongly coupled 14N nuclei. The 14N ESEEM observed for Cu2+ at the surface site of RCs results from a single histidine ligand. In addition, the ENDOR spectra reveal the presence of one more nitrogen ligand in the Cu2+ coordination sphere that we tentatively assign as a peptide backbone nitrogen. The similarity of the ESEEM and ENDOR spectra for RCs from Rb. sphaeroides, Rb. capsulatus, and Blc. viridis suggests a universal intrinsic structural site. These results indicate that Cu2+ has a different coordination environment from the crystallographically characterized Zn2+ and Cd2+ surface sites in Rb. sphaeroides RCs. These structural differences may be associated with observed metal ion specific differences in alteration of QA -QB → QAQB - electron transfer.