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

Journal Article · Fri May 01 00:00:00 EDT 1981 · Proc. Natl. Acad. Sci. U.S.A.; (United States)

DOI:https://doi.org/10.1073/pnas.78.5.2652· OSTI ID:6201711

Chang, C K ^[1]; Hanson, L K; Richardson, P F; Young, R; Fajer, J

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1. Michigan State Univ., East Lansing

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/sub 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 Hueckel 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.

• DOE Contract Number: AC02-76CH00016
• OSTI ID: 6201711
• Journal Information: Proc. Natl. Acad. Sci. U.S.A.; (United States), Vol. 78:5
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
HEME
ELECTRON SPIN RESONANCE
IRON COMPLEXES
REDOX REACTIONS
PORPHYRINS
ENZYME ACTIVITY
MOLECULAR ORBITAL METHOD
MOLECULAR STRUCTURE
NITRITES
OPTICAL PROPERTIES
OXIDOREDUCTASES
REACTION INTERMEDIATES
SULFITES
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
COMPLEXES
ENZYMES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
MAGNETIC RESONANCE
NITROGEN COMPOUNDS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
PIGMENTS
RESONANCE
SULFUR COMPOUNDS
TRANSITION ELEMENT COMPLEXES
550200* - Biochemistry