Electron spin-echo spectroscopic studies of Escherichia coli fumarate reductase.

Electron spin-echo envelope modulation (ESEEM) spectroscopy was applied to the study of reduced Centre 1 of Escherichia coli fumarate reductase (succinate:(acceptor) oxidoreductase, EC 1.3.99.1). The ESEEM spectrum derived from stimulated (3-pulse) echo envelopes obtained at 8.8 GHz contained lines at 0.9, 2.1, 3.0 and 4.2 MHz in the g = 1.94 region. When studied at 11.4 GHz, these low-frequency components scale with magnetic field in a manner indicating interaction between the unpaired electron spin of the Fe-S cluster and a weakly coupled 14N nucleus. Spectral simulations of these ESEEM data yield nuclear quadrupole interaction parameters indicative of peptide nitrogen. For oxidized protein, the magnetic-field dependence of the linear electric-field effect (LEFE) for Centre 3 was measured, and the results confirm the presence of a [3Fe-4S] cluster in the protein.