Fig. 2: UV-Vis spectra of [4Fe4S]²⁺ clusters, with curve fitting based on ligand-metal charge-transfer excitations.

a Curve-fitting based on the method of Galambas et al. (see ‘Methods’) recorded over the 35,000–10,000 cm⁻¹(285–1000 nm) spectral range for 0.2 mM L-Cys, with Fe³⁺ and S²⁻ in a 5:1:1 ratio. We fitted the three lowest-energy ligand-to-metal charge transfers (LMCT) for [4Fe4S]²⁺: peak 3 (pink) is a high-energy LMCT band associated with thiolate S → Fe, while peaks 4 and 5 (green and blue) are intermediate energy LMCT bands associated with sulfides → Fe. b. Curve fitting for 5 mM L-Cys, with Fe³⁺ and S²⁻ in a 5:1:1 ratio. c Calculated concentration of [4Fe4S]²⁺ clusters (µM) versus cysteine concentration (mM) based on integrated curve area (black), compared with a three-point method (brown, Δ absorbance; see text). d Table of data including calculated uncertainties based on reference [4Fe4S]²⁺ concentrations from Galambas et al.³⁰. For panels a and b, each peak corresponds to a specific excitation (from left to right: peak 1 grey dashed line—peptide charge-transfer; peak 2 grey dashed line—thiolate RS⁻ ⟶ Fe; peak 3 pink line—thiolate RS⁻ ⟶ Fe; peak 4 green line—S²⁻ ⟶ Fe; peak 5 blue line—S²⁻ ⟶ Fe; peak 6 grey dashed line—ligand field). Source data are provided as a Source data file.