ESEEM studies of succinate:ubiquinone reductase from Paracoccus denitrificans.

Electron spin-echo envelope modulation (ESEEM) spectroscopy has been performed in order to obtain structural information about the environment of the reduced [2Fe-2S] cluster (S-1 center), the oxidized [3Fe-4S] cluster (S-3 center), and the flavin semiquinone radical in purified succinate:ubiquinone reductase from Paracoccus denitrificans. Spectral simulations of the ESEEM data from the reduced [2Fe-2S] yielded nuclear quadrupole interaction parameters that are indicative of peptide nitrogens. We also observed a weak interaction between the oxidized [3Fe-4S] cluster and a peptide 14N. There was no evidence for coordination of any of the Fe atoms to 14N atoms of imidazole rings. The ESEEM data from the flavin semiquinone radical were more complicated. Here, evidence was obtained for interactions between the unpaired electron and only the two nitrogen atoms in the flavin ring.