Synthesis of a [Fe₄S₄]−S−Ferriheme Bridged Assembly Containing an Isobacteriochlorin Component:  A Further Analogue of the Active Site of Sulfite Reductase

Abstract

The catalytic site of sulfite reductase consists of exchange-coupled cubane-type Fe 4 S 4 cluster and siroheme components bridged by a cysteinate sulfur atom in the Escherichia coli enzyme and, presumably, by sulfide in certain other bacterial enzymes. A synthetic analogue of the latter in the form of a sulfide-bridged assembly has been synthesized by the reaction of [Fe(OEiBC)Cl] and the site-differentiated cluster [Fe 4 S 4 (LS 3 )(SSiMe 3 )] 2-(4) in benzene/acetonitrile. Demonstration of the [Fe 4 S 4 (LS 3 )-S-Fe III (OEiBC)] 2-(5) formulation follows from spectroscopic evidence. The Mo ¨ssbauer spectrum proves the [Fe 4 S 4 ] 2+ core oxidation state. 1 H NMR spectra demonstrate a close juxtaposition of macrocycle and cluster owing to the appearance of doubled cluster resonances when two diastereomers of 5 are prepared from an isomeric mixture of [Fe(OEiBC)Cl]. Isotropic shifts of 5 are dominantly contact in origin and are 7-11 times larger than in 4, a typical [Fe 4 S 4 ] 2+ cluster, owing to spin delocalization involving Fe(III). These shifts exhibit a 1/T dependence whereas those of 4 are non-Curie. Spin delocalization requires the presence of a covalently bridged structure. Assembly 5 is a closer analogue of the native site than other bridged assemblies prepared in this laboratory because the OEiBC macrocycle, as that in siroheme, is at the isobacteriochlorin oxidation level. This work contributes to an experimental protocol for coupling cluster and heme components into bridged assemblies. (LS 3 ) trianion of 1,3,5-tris((4,6-dimethyl-3mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene; OEiBC ) dianion of octaethylisobacteriochlorin.)

The [MoFe3S4]2+ Oxidation State: Synthesis, Substitution Reactions, and Structures of Phosphine-Ligated Cubane-Type Clusters with the S = 2 Ground State

“…The scheme conveys the lability of the Me 3 Si-S bond previously exploited in Fe-S cluster formation. 23,32-35 Substitution reaction 5a has been independently verified. In reaction 5b, Me 3 SiS - acts as a nucleophile toward silicon in a second cluster with bridge formation and release of the silylsufide.…”

Developments in the Biomimetic Chemistry of Cubane-Type and Higher Nuclearity Iron–Sulfur Clusters

“…Analogue 3:1 clusters have been employed in a number of biologically relevant investigations, including ligand binding affinities at the unique site, 11 formation of sulfide-bridged double cubanes, 66,69 , binding of cluster and iron porphyrins through an Fe-S-Fe bridge, 69,70 construction of the H-cluster framework of [FeFe]-hydrogenases, 71 and reaction of thiolate at the unique site with sulfonium cations in relation to the reductive cleavage of 71 S- adenosyl -L- methionine by biotin synthase. 64 Other applications dependent upon regioselective reactivity will doubtless follow, particularly because of the availability of a succeeding generation of trithiols ( 23 ) more convenient synthetically than the first ( 21 ) and the emergence of clusters ( 24–28 ) not requiring multi-step ligand synthesis.…”

Tridentate Thiolate Ligands: Application to the Synthesis of the Site‐Differentiated [4Fe‐4S] Cluster having a Hydrosulfide Ligand at the Unique Iron Center

“…A common method to synthesize site-differentiated clusters is to introduce a tridentate thiolate ligand. [10][11][12][13] Holm and co-workers developed a tridentate ligand LS 3 3À that tightly chelates three iron centers of the [4Fe-4S] core, leaving one iron site for further functionalization, and they have reported the synthesis of various [3:1] site-differentiated [4Fe-4S] clusters [Fe 4 S 4 (L')A C H T U N G T R E N N U N G (LS 3 )] 2À (L(SH) 3 = 1,3,5-tris(4,6-dimethyl-3-mercaptophenylthio)-2,4,6-tris(p-tolylthio)benzene, L' = Cl, SR, SH, etc.). [10,11] Pohl and co-workers also reported a few clusters coordinated by the tridentate ligand TriS 3À (TriSH 3 = 1,3,5-triethyl-2,4,6-tris(3-sulfanylindolyl [1]methyl)benzene).…”

“…[10][11][12][13] Holm and co-workers developed a tridentate ligand LS 3 3À that tightly chelates three iron centers of the [4Fe-4S] core, leaving one iron site for further functionalization, and they have reported the synthesis of various [3:1] site-differentiated [4Fe-4S] clusters [Fe 4 S 4 (L')A C H T U N G T R E N N U N G (LS 3 )] 2À (L(SH) 3 = 1,3,5-tris(4,6-dimethyl-3-mercaptophenylthio)-2,4,6-tris(p-tolylthio)benzene, L' = Cl, SR, SH, etc.). [10,11] Pohl and co-workers also reported a few clusters coordinated by the tridentate ligand TriS 3À (TriSH 3 = 1,3,5-triethyl-2,4,6-tris(3-sulfanylindolyl [1]methyl)benzene). [12] As a part of our efforts to synthesize models of metalloenzyme active sites containing metal sulfide/thiolate complexes, [14] we have been interested in these site-differentiated [15] Along with the above approach, we extended the scope of the synthesis of site-differentiated [4Fe-4S] clusters by designing the new tripodal trithiols Temp(SH) 3 (1 a) and Tefp(SH) 3 (1 b; Scheme 1).…”