EPR analysis of multiple forms of [4Fe–4S] 3+ clusters in HiPIPs

The electron paramagnetic resonance (EPR) spectrum from the [4Fe–4S]3+ cluster in several high-potential iron–sulfur proteins (HiPIPs) is complex: it is not the pattern of a single, isolated S=1/2 system. Multifrequency EPR from 9 to 130 GHz reveals that the apparent peak positions (g values) are frequency-independent: the spectrum is dominated by the Zeeman interaction plus g-strain broadening. The spectra taken at frequencies above the X-band are increasingly sensitive to rapid-passage effects; therefore, the X-band data, which are slightly additionally broadened by dipolar interaction, were used for quantitative spectral analysis. For a single geometrical [4Fe–4S]3+ structure the (Fe–Fe)5+ mixed-valence dimer can be assigned in six different ways to a pair of iron ions, and this defines six valence isomers. Systematic multicomponent g-strain simulation shows that the [4Fe–4S]3+ paramagnets in seven HiPIPs from different bacteria each consist of three to four discernible species, and these are assigned to valence isomers of the clusters. This interpretation builds on previous EPR analyzes of [4Fe–4S]3+ model compounds, and it constitutes a high-resolution extension of the current literature model, proposed from paramagnetic NMR studies.