Electron Spin−Lattice Relaxation Measurement of the 3Fe-4S (S-3) Cluster in Succinate:Ubiquinone Reductase from Paracoccus Denitrificans. A Detailed Analysis Based on a Dipole−Dipole Interaction Model

The electron spin−lattice relaxation for the 3Fe-4S (S-3) center in succinate:ubiquinone reductase has been examined using both inversion recovery and “picket-fence” pulse sequences at a temperature range of 4−8 K. The latter pulse sequence is used to eliminate the interference of spectral diffusion in frozen solids. An abnormally fast relaxation was observed for the S-3 center. We attribute this rapid relaxation to a magnetic dipolar interaction between the S-3 center and a nearby paramagnetic b -heme (cytochrome b ). A model has been developed to treat the interaction between two paramagnetic redox centers in a rigid lattice at a fixed distance apart but with random orientations in a magnetic field. Both the contribution to the spin−lattice relaxation rate from the dipolar interaction ( k 1 θ ), which is anisotropic, and the intrinsic electron spin relaxation, which is scalar ( k 1scalar ), have been deduced. We find that the contribution of exchange interaction to the anisotropic part of the relaxation rate ( k 1 θ ) is very small. Accordingly, we conclude that k 1scalar is dominated by the intrinsic electron spin−lattice relaxation. From k 1 θ, a lower limit ( r > 10 Å) has been deduced for the distance between the S-3 center and the b -heme.