Figure 4.

Mössbauer spectroscopic characterization of the monothiolated intermediate in Tt LS. 4.2-K/53-mT Mössbauer spectrum of Tt LS (A, vertical bars) and simulation thereof with parameters quoted in the text (A, solid line). 4.2 K Mössbauer spectra of the Tt LS/peptide 2 complex reacted with SAM and dithionite recorded in externally applied magnetic fields of 0 mT (B, vertical bars) and 53 mT oriented parallel to the γ beam (C, vertical bars). The solid black line in B corresponds to the superposition of the features associated with the 3Fe-cluster (40%, red line), [4Fe–4S]²⁺ cluster (12%, green line), and site-differentiated [4Fe–4S]²⁺ cluster (48%, purple line, see below). The [0–53 mT] difference spectrum (C, vertical bars) is similar to that observed for Ec LS (C, solid line is experimental spectrum shown in Figure 3E scaled by 0.75). The reference spectrum of the site-differentiated [4Fe–4S]²⁺ cluster in Tt LS (E, vertical bars) is obtained by subtracting the contribution of the other components (see text) and can be simulated with three quadrupole doublets representing the valence-delocalized Fe₂^2.5 pair (green line; parameters given in the text) and the two partially localized sites (orange and blue lines; parameters given in Table 2) in a 2:1:1 intensity ratio. The black line in E is the added contribution of the three quadrupole doublets.