Theoretical Insights into the Unique Ligation of [Fe₄S₄] Iron–Sulfur Clusters

Abstract

Fe 4 S 4 ] iron-sulfur clusters are typically anchored to the protein scaffold via four cysteine residues, but in a number of proteins one of the Cys is replaced with another ligand. The biological role of such a replacement is unknown although essential to maintain catalytic activity. Here, we explore the geometries and electronic structures of 3:1 site differentiated model compounds [Fe 4 S 4 ][S(CH 3 )] 3 (L) using quantum chemical methods. The unique ligands L were chosen to represent biologically relevant molecules: methanethiol (represents cysteine), acetic acid (asparagine), 4-methylimidazole (histidine), and water/ hydroxyl anion. Each ligand was considered as deprotonated anion or protonated neutral molecule. We found the replacements do not influence structure of the cluster significantly. On [a]

Characterization of charge transfer excited states in [2Fe–2S] iron–sulfur clusters using conventional configuration interaction techniques

“…The ground electronic state has antiferromagnetic character (total spin of 0 with locally highspin 3d 5 iron atoms) and it was simulated with the broken symmetry approach. From a technical point of view, orbital preparation is a three-step procedure that uses localized natural orbitals: [4,32] (1) first the high spin natural orbitals (NOs) state (multiplicity 11) orbitals are obtained, then (2) the singly occupied NOs are localized and duplicated into the alpha and beta sets providing the Fe-localized 3d orbitals of different iron atoms are placed in different sets so that (3) subsequent low spin (multiplicity 1) unrestricted calculations converge smoothly to the BS state. The optimized geometry was subject to second derivative calculations and all normal modes were positive (local energy minimum).…”

“…The clusters feature a core assembly of iron and sulfur atoms typically denoted as [Fe x S y ] z , where x and y > 1 and z denotes the charge of the cluster, that is anchored to the protein scaffold with cysteine residues. Other linkers, such as histidine or aspartate, are also possible and were recently shown both experimentally [3] and theoretically [4] to influence the electronic properties of the clusters.…”

“…Authors computed first 10 excited states of [Fe 2 S 2 ] and [Fe 4 S 4 ] clusters with CH 3 S − ligands at various oxidation and spin states and found high density of low lying excited states. More recent work covered 20 excited states of the [Fe 2 S 2 ](SMe) 4 3−/2− system [19]. Interestingly, no ligand-tometal CT states were found up to about 400 nm.…”

“…Similarly to DMRG work [18], orbitals optimized at high-spin state were used to lower the computational cost. The main focus of the study was on the spin-state ladder and, interestingly, the ground spin state of [Fe 2 S 2 ](SMe) 4 3− was found to be high-spin.…”

“…In any case, the states reported to date for the simplest multimetallic iron-sulfur cluster model [Fe 2 S 2 ] (SMe) 4 3−/2− cover excited states up to 25,000 cm −1 (3.1 eV, 400 nm). Some number of higher energy states were computed with RAS-based approach [22] but their nature was not analyzed.…”