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 succina te: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 frown solids. An abnormally fast relaxation was observed for the S-3 ce nter. We attribute this rapid relaxation to a magnetic dipolar interaction between the S-3 center and a nearby paramagnetic b-heme (cytochrome b). A m odel has been developed to treat the interaction between two paramagnetic r edox centers in a rigid lattice at a fixed distance apart but with random o rientations in a magnetic field. Both the contribution to the spin-lattice relaxation rate from the dipolar interaction (k(l theta)), which is anisotr opic, and the intrinsic electron spin relaxation, which is scalar (k(l) (sc alar)), have been deduced. We find that the contribution of exchange intera ction to the anisotropic part of the relaxation rate (k(l theta)) is very s mall. Accordingly, we conclude that k(i scalar) is dominated by the intrins ic electron spin-lattice relaxation. From k(l theta), a lower limit (r > 10 Angstrom) has been deduced for the distance between the S-3 center and the b-heme.