Figure 1.

The 4.2 K Mössbauer spectra of anaerobically isolated FNR (A), and FNR exposed to oxygen for 2 min (B) and 17 min (C). The solid line in A is a doublet for [4Fe-4S]²⁺ simulated with ΔE_Q = 1.22 mm/s and δ = 0.45 mm/s. The solid line in C is a simulated spectrum using one doublet with ΔE_Q = 0.58 mm/s and δ = 0.28 mm/s for the [2Fe-2S]²⁺ cluster (bracket). The solid line drawn through the data in B is a superposition of the theoretical curves for the [4Fe-4S]²⁺ cluster and the [2Fe-2S]²⁺ cluster, plus a contribution (arrows) from the Fe²⁺ component. The relative proportions of the various species are listed in Table 1.