Figure 6.

EPR spectroscopic characterization of Fe^III-Ngb following the addition of Na₂S. Shown is the spectrum of wild-type (400 μm) Fe^III-Ngb in aerobic 100 mm HEPES buffer, pH 7.4, containing 20% glycerol (v/v) after the addition of Na₂S (10 mm) at 25 °C at the indicated times. Fe^III concentrations were quantified using 1 mm copper EDTA standard. The EPR settings are described under “Experimental procedures.” The signal due to adventitious ferric iron is labeled with an asterisk. R.U., relative units.