Analytical Protocols for Quantitative Evaluation of [4Fe-4S]-maquette Spectra

A step-by-step tutorial is presented for the quantitative analysis of UV-vis spectra of <em>'as isolated'</em> [4Fe-4S]-maquettes that contain diamagnetic, [4Fe-4S]²⁺ clusters, as chromophores. The UV-vis model is justified by the schematic molecular orbital diagram of Fe-S(sulfide) and Fe-S(thiolate) bonding. The spectral analysis is based on integrated peak intensities, which is superior to using absorption maxima (\(\lambda _{max}\)), when the chemical composition of the chromophores are not identical, but similar. Three spectra (two model cluster complexes and a metalloprotein active site) provide the reference for converting spectral intensities to concentrations. The UV-vis modeling is demonstrated for a prototypical \([Fe_4S_4(CIACGAC)(\beta ME)]^{2-}\) complex (FdM-7 maquette) and the \([Fe_4S_4(\beta ME)_4]^{2-}\) complex formed under identical conditions, but without the presence of a peptide.