Magnetic interactions between a [4Fe–4S]1+ cluster and a flavin mononucleotide radical in the enzyme trimethylamine dehydrogenase: A high-field electron paramagnetic resonance study

Trimethylamine dehydrogenase is a bacterial enzyme which contains two redox centers: a flavin mononucleotide (FMN) group which constitutes the active site and a [4Fe–4S]1+,2+ cluster which transfers the electrons provided by the FMN to an electron-transferring flavoprotein. According to the x-ray crystal structure, the center-to-center distance is equal to 12 A and the nearest atoms of the two centers are separated by a 4 A gap. Although this arrangement does not appear especially favorable for mediating strong magnetic interactions, a triplet state electron paramagnetic resonance (EPR) spectrum arising from the intercenter magnetic coupling is observed at X band (9 GHz) when the enzyme is reduced by its substrate. In earlier work, the temperature dependence of this spectrum and its analysis based on a triplet state spin Hamiltonian were used to propose the range (0.8–100 cm−1) for the parameter J0 of the isotropic interaction J0SA.SB, but neither the magnitude of J0 nor its sign could be further specifie...