ESEEM studies of succinate : ubiquinone reductase from Paracoccus denitrificans

Electron spin-echo envelope modulation (ESEEM) spectroscopy has been perfor med in order to obtain structural information about the environment of the reduced [2Fe-2S] cluster (S-l center), the oxidized [3Fe-4S] cluster (S-3 c enter), 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 para meters that are indicative of peptide nitrogens. We also observed a weak in teraction between the oxidized [3Fe-4S] cluster and a peptide N-14. There w as no evidence for coordination of any of the Fe atoms to N-14 atoms of imi dazole rings. The ESEEM data from the flavin semiquinone radical were more complicated. Here, evidence was obtained for interactions between the unpai red electron and only the two nitrogen atoms in the flavin ring.