pi cation radicals of ferrous and free base isobacteriochlorins: Models for siroheme and sirohydrochlorin

Theoretical and experimental optical, redox, and paramagnetic results are presented for models of siroheme, the iron isobacteriochlorin prosthetic group of nitrite and sulfite reductases, and of sirohydrochlorin, the metal-free siroheme that is an intermediate in the biosynthetic pathway to vitamin B(12). The facile oxidation of many isobacteriochlorins, which distinguishes them from porphyrins and chlorins, suggests that the siroheme macrocycle itself may undergo oxidation in the multi-electron enzymatic cycles that reduce nitrite to ammonia and sulfite to hydrogen sulfide. Extended Hückel MO calculations (i) help rationalize the redox properties of isobacteriochlorins compared with those of porphyrins and chlorins; (ii) indicate that Fe(II) pyridine carbonyl[(py) (CO)] complexes of isobacteriochlorins, unlike those of porphyrins and chlorins, should undergo oxidation from the macrocycle rather than the metal to yield pi cation radicals; (iii) suggest that, in hexacoordinated Fe(II) isobacteriochlorin complexes, the site of oxidation-i.e., the metal or the macrocycle-will depend on the ligand field induced by the axial ligands; and (iv) predict similar unpaired spin density profiles for metal-free and (py) (CO)Fe(II) isobacteriochlorin radicals. Experimental data for three isomeric free-base and (py) (CO)Fe(II) complexes of dimethyloctaethylisobacteriochlorins support the theoretical calculations and establish the existence of Fe(II) isobacteriochlorin pi cations in vitro.