Figure 4.

Mössbauer spectra illustrating the different outcomes of the reaction of Mn^IV/Fe^III–β₂ with HU. (A) 4.2-K/53-mT Mössbauer spectra of samples in which Mn^IV/Fe^III–β₂ (800 µM β, 0.75 equiv Mn^IV/Fe^III cluster) was reacted with 20 mM HU at ambient temperature for 1 min (black hashed vertical bars) or 10 min (red solid line) in the presence of 1.2 mM α, 10 mM DTT, 1 mM ATP, and 10 mM CDP. The black solid line is the spectrum of the heterogeneous Mn^III/Fe^III-β₂ state (49). The black and red arrows indicate the conversion of the Mn^IV/Fe^III form to the new Fe^III-containing complex. The blue arrows indicate the outer lines of the magnetically split sub-spectrum of the Mn^III/Fe^III cluster (~45% of the total intensity). The amounts of the Mn^III/Fe^III cluster in the two samples are identical within the experimental uncertainty. (B) 4.2-K/zero-field Mössbauer spectra of samples prepared by treatment of Mn^IV/Fe^III-β₂ (top) with 20 mM HU for 30 min at ambient temperature (bottom). The solid lines on top are simulations as indicated in the text. The blue and red lines represent the contribution of the Mn^IV/Fe^III cluster and the new Fe^III-containing, diamagnetic species, respectively.