Identification of hydrogen bonds to the Rieske cluster through the weakly coupled nitrogens detected by electron spin echo envelope modulation spectroscopy

The interaction of the reduced [2Fe-2S] cluster of isolated Rieske fragment from the bc₁ complex of Rhodobacter sphaeroides with nitrogens (¹⁴N and ¹⁵N) from the local protein environment has been studied by X- and S-band pulsed EPR spectroscopy. The twodimensional electron spin echo envelope modulation spectra of uniformly ¹⁵N-labeled protein show two well resolved cross-peaks with weak couplings of ∼0.3-0.4 and 1.1 MHz in addition to couplings in the range of 6-8 MHz from two coordinating N_δ of histidine ligands. The quadrupole coupling constants for weakly coupled nitrogens determined from S-band electron spin echo envelope modulation spectra identify them as N_ε of histidine ligands and peptide nitrogen (N_p), respectively. Analysis of the line intensities in orientation-selected S-band spectra indicated that N_p is the backbone N-atom of Leu-132 residue. The hyperfine couplings from N _ε and N_p demonstrate the predominantly isotropic character resulting from the transfer of unpaired spin density onto the 2s orbitals of the nitrogens. Spectra also show that other peptide nitrogens in the protein environment must carry a 5-10 times smaller amount of spin density than the N_p of Leu-132 residue. The appearance of the excess unpaired spin density on the N_p of Leu-132 residue indicates its involvement in hydrogen bond formation with the bridging sulfur of the Rieske cluster. The configuration of the hydrogen bond therefore provides a preferred path for spin density transfer. Observation of similar splittings in the ¹⁵N spectra of other Rieske-type proteins and [2Fe-2S] ferredoxins suggests that a hydrogen bond between the bridging sulfur and peptide nitrogen is a common structural feature of [2Fe-2S] clusters.