New Method for the Spin Quantitation of [4Fe?4S] + Clusters with S = 3 / 2 . Application to the FS0 Center of the NarGHI Nitrate Reductase from Escherichia coli

Abstract : In conventional analyses of g ≈ 5 signals given by [4Fe−4S]+ clusters with S = 3/2, the effective g values that cannot be measured in the electron paramagnetic resonance (EPR) spectrum are deduced from rhombograms calculated by assuming that the g̃ matrix is isotropic with gx = gy = gz = 2.00. We have shown that when the two low-field peaks corresponding to the Kramers doublets are visible in the spectrum, a new, independent piece of information about the system can be obtained by studying the temperature dependence of the ratio of the area under these peaks. By applying this method to the g ≈ 5 signals displayed by NarGHI nitrate reductase, we were able to determine all the parameters of the spin Hamiltonian of FS0 centers with S = 3/2 and to measure accurately their number. Our results indicate that simple analyses based on the assumption of an isotropic g̃ matrix can give rise to very large errors.