SYNTHESIS OF A [FE4S4]-S-FERRIHEME BRIDGED ASSEMBLY CONTAINING AN ISOBACTERIOCHLORIN COMPONENT - A FURTHER ANALOG OF THE ACTIVE-SITE OF SULFITE REDUCTASE

The catalytic site of sulfite reductase consists of exchange-coupled c ubane-type Fe4S4 cluster and siroheme components bridged by a cysteina te sulfur atom in the Escherichia coli enzyme and, presumably, by sulf ide in certain other bacterial enzymes. A synthetic analogue of the la tter in the form of a sulfide-bridged assembly has been synthesized by the reaction of [Fe(OEiBC)CI] and the site-differentiated cluster [Fe 4S4(LS(3))(SSiMe(3))](2-) (4) in benzene/acetonitrile. Demonstration o f the [Fe4S4(LS(3))-S-Fe-III(OEiBC)](2-) (5) formulation follows from spectroscopic evidence. The Mossbauer spectrum proves the [Fe4S4](2+) core oxidation state. H-1 NMR spectra demonstrate a close juxtapositio n 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)CI]. Isotropic shifts of 5 are dominantly contact in origin and are 7-11 times larger than in 4, a typical [Fe4S4](2+) cluster, owing to spin delocalization involving Fe(III). These shifts exhibit a 1/T dependence whereas those of 4 are non-Curie. Spin deloca lization requires the presence of a covalently bridged structure. Asse mbly 5 is a closer analogue of the native site than other bridged asse mblies prepared in this laboratory because the OEiBC macrocycle, as th at in siroheme, is at the isobacteriochlorin oxidation level. This wor k contributes to an experimental protocol for coupling cluster and hem e components into bridged assemblies. (LS(3) = trianion of is((4,6-dim ethyl-3-mercaptophenyl)thio)-2,4,6-tris (p-tolylthio)benzene; OEiBC = dianion of octaethylisobacteriochlorin.)