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

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 Mössbauer 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-3-mercaptophenyl)thio)-2,4,6-tris( p -tolylthio)benzene; OEiBC = dianion of octaethylisobacteriochlorin.)