57Fe enriched Mössbauer spectra (4.2 K, 45 mT, blue vertical bars) of (A) anaerobically purified Y463M BthA, (B) A exposed to O2 gas for 10 min. The black traces overlying the data are simulated sums using the listed doublet parameters: Fe(II) (blue fill), Fe(II)-CO (green fill), oxy-heme (orange fill), Fe(IV)=O (red fill). The broad magnetic HS species in B are fit with the same species shown in Fig. 3B (black dots and dashed).